Difference between revisions of "Book - Buchanan's Manual of Anatomy including Embryology 15"

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The roof is formed by a delicate layer of epithelium which extends across between the upper margins of the lateral walls, and is continuous with the ependymal lining of the ventricle. Lying on this epithelial roof, and intimately connected with it, is the tela chorioidea, from the under surface of which the two choroid plexuses of the ventricle project downwards, one on either side of the middle line, each invaginating the epithelium of the roof. The epithelium of the roof is so intimately connected with the tela that, when the latter is removed, the epithelium comes away with it, and the cavity of the ventricle is exposed. Above the tela chorioidea is the ‘ body ’ of the fornix, and above this again is the ‘ body ' of the corpus callosum.
 
The roof is formed by a delicate layer of epithelium which extends across between the upper margins of the lateral walls, and is continuous with the ependymal lining of the ventricle. Lying on this epithelial roof, and intimately connected with it, is the tela chorioidea, from the under surface of which the two choroid plexuses of the ventricle project downwards, one on either side of the middle line, each invaginating the epithelium of the roof. The epithelium of the roof is so intimately connected with the tela that, when the latter is removed, the epithelium comes away with it, and the cavity of the ventricle is exposed. Above the tela chorioidea is the ‘ body ’ of the fornix, and above this again is the ‘ body ' of the corpus callosum.
  
Summary of the Roof.— To expose the ventricle from above, the following structures must be removed, in the order named: (1) the body of the corpus callosum; (2) the body of the fornix; and (3) the tela chorioidea, along with the
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Summary of the Roof.— To expose the ventricle from above, the following structures must be removed, in the order named: (1) the body of the corpus callosum; (2) the body of the fornix; and (3) the tela chorioidea, along with the epithelium of the roof.
  
epithelium of the roof.
 
  
 
The floor, which is sloped downwards and forwards (see Fig. 944 ). is formed by the structures which lie within the interpeduncular space at the base of the brain, from behind forwards: the locus perforatus posterior, the corpora mamillaria, and the tuber cmereum, with the upper end of the infundibulum. The tegmenta of the crura cerebri enter to a certain extent into the floor posteriorly, and the optic commissure lies across it anteriorly. Above the optic commissure the floor presents a depression, called the optic r«c«ss, and behind i there is another depression or diverticulum, called th e infundibular recess. The latter forms the upper part of the infundibulum, which leads to the posterior lobe of the hypophysis
 
The floor, which is sloped downwards and forwards (see Fig. 944 ). is formed by the structures which lie within the interpeduncular space at the base of the brain, from behind forwards: the locus perforatus posterior, the corpora mamillaria, and the tuber cmereum, with the upper end of the infundibulum. The tegmenta of the crura cerebri enter to a certain extent into the floor posteriorly, and the optic commissure lies across it anteriorly. Above the optic commissure the floor presents a depression, called the optic r«c«ss, and behind i there is another depression or diverticulum, called th e infundibular recess. The latter forms the upper part of the infundibulum, which leads to the posterior lobe of the hypophysis
  
The lateral wall is slightly convex and is formed for the m part by the inner surface of the thalamus, which has covering of grey matter. Towards its centre it presents a furrow, which leads from the interventricular foramen m a backward direction
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The lateral wall is slightly convex and is formed for the m part by the inner surface of the thalamus, which has covering of grey matter. Towards its centre it presents a furrow, which leads from the interventricular foramen m a backward direction towards the upper opening of the aqueduct T'^^ralwall Ihere ^s hypothalamic sulcus. At the upper part of the lateral wall there a delicate band of white fibres, called the stria thalami, which runs back toward the root of the pineal body, and passes to the anterior pillar of the fornix. Connecting the two lateral walls (the thalami), in front of the centre of the ventricle, there is a fragile band of grey matter, formerly called the middle or soft commissure, but now usually known as the massa intermedia or connexus thalami , since it is not really a commissure. At the anterior part of the lateral wall the corresponding anterior pillar of the fornix passes downwards and backwards.
  
towards the upper opening of the aqueduct T'^^ralwall Ihere ^s hypothalamic sulcus. At the upper part of the lateral wall there
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The anterior boundary is formed inferiorly by the lamina terminalis, which extends upwards from the optic commissure to the rostrum of the corpus callosum, and superiorly by the anterior pillars of the fornix and the central portion of the anterior commissure.
  
  
a delicate band of white fibres, called the stria thalami, which runs back toward the root of the pineal body, and passes to the anterior pillar of the fornix. Connecting the two lateral walls (the thalami), in front of the centre of the ventricle, there is a fragile band of grey matter, formerly called the middle or soft commissure, but now usually known as the massa intermedia or connexus thalami , since it is not really a commissure. At the anterior part of the lateral wall the corresponding anterior pillar of the fornix passes downwards and backwards.
 
  
The anterior boundary is formed inferiorly by the lamina terminalis, which extends upwards from the optic commissure to the rostrum of the corpus callosum, and superiorly by the anterior pillars of the fornix and the central portion of the anterior commissure.
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Fig. 943. — Drawing of a Metal Cast of the Ventricles of the Brain of an Adult (Right Lateral View) (Retzius).
  
 
 
Fig. 943. —Drawing of a Metal Cast of the Ventricles of the Brain of an Adult (Right Lateral View) (Retzius).
 
  
 
The posterior boundary is formed by the pineal body and the posterior commissure, and under cover of the latter is the upper opening of the aqueduct. The posterior boundary presents two recesses, pineal and suprapineal. The pineal recess passes backwards for a very short distance above the posterior commissure into the stalk of the pineal body, separating the stalk into two portions, dorsal and ventral. The suprapineal recess is connected with the back part of the epithelial roof of the ventricle, and passes backwards over the pineal body. The third ventricle has thus four diverticula—namely, the optic recess, the infundibular recess (both of which recesses are associated with the floor), the pineal recess, and the suprapineal recess. The cavity communicates with the fourth ventricle by means of the aqueduct of the mid-brain, and with the two lateral ventricles by means of the interventricular foramina.
 
The posterior boundary is formed by the pineal body and the posterior commissure, and under cover of the latter is the upper opening of the aqueduct. The posterior boundary presents two recesses, pineal and suprapineal. The pineal recess passes backwards for a very short distance above the posterior commissure into the stalk of the pineal body, separating the stalk into two portions, dorsal and ventral. The suprapineal recess is connected with the back part of the epithelial roof of the ventricle, and passes backwards over the pineal body. The third ventricle has thus four diverticula—namely, the optic recess, the infundibular recess (both of which recesses are associated with the floor), the pineal recess, and the suprapineal recess. The cavity communicates with the fourth ventricle by means of the aqueduct of the mid-brain, and with the two lateral ventricles by means of the interventricular foramina.
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The foramen of each side represents the original wide communication between the cavity of the cerebral vesicle and the cavity of the telencephalon.
 
The foramen of each side represents the original wide communication between the cavity of the cerebral vesicle and the cavity of the telencephalon.
  
Thalami.—The thalami (O.T. optic thalami) are two large ovoid masses of grey matter which lie obliquely, with their long axes directed backwards and outwards, for the most part on the sides of the third ventricle. Their anterior extremities are near each other, but their posterior extremities stand apart, the superior corpora quadrigemina being situated between them. Over their anterior two-thirds they are separated from each other by the third ventricle.
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Thalami. — The thalami (O.T. optic thalami) are two large ovoid masses of grey matter which lie obliquely, with their long axes directed backwards and outwards, for the most part on the sides of the third ventricle. Their anterior extremities are near each other, but their posterior extremities stand apart, the superior corpora quadrigemina being situated between them. Over their anterior two-thirds they are separated from each other by the third ventricle.
  
 
Each thalamus presents four surfaces—superior, inferior, lateral, and medial; and two extremities—anterior and posterior.
 
Each thalamus presents four surfaces—superior, inferior, lateral, and medial; and two extremities—anterior and posterior.
  
The superior surface is limited laterally by an oblique groove, which separates it from the nucleus caudatus, and contains the stria semicircularis, and anteriorly the vein of the corpus striatum. Medially it is bounded, from before backwards, by (1) the stria thalami, (2) the trigonum habenulae, and (3) the corpora quadrigemina. It is divided into two areas, lateral and medial, by a groove which is directed backwards and laterally from near the anterior extremity to the lateral end of the posterior extremity. This groove corresponds to the lateral margin of the body of the fornix. The lateral area enters descriptively into the body of the lateral ventricle, but is covered by the ependyma of that ventricle. The medial area is excluded from
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The superior surface is limited laterally by an oblique groove, which separates it from the nucleus caudatus, and contains the stria semicircularis, and anteriorly the vein of the corpus striatum. Medially it is bounded, from before backwards, by (1) the stria thalami, (2) the trigonum habenulae, and (3) the corpora quadrigemina. It is divided into two areas, lateral and medial, by a groove which is directed backwards and laterally from near the anterior extremity to the lateral end of the posterior extremity. This groove corresponds to the lateral margin of the body of the fornix. The lateral area enters descriptively into the body of the lateral ventricle, but is covered by the ependyma of that ventricle. The medial area is excluded from the lateral ventricle, and is covered by portions of the tela chorioidea and body of the fornix (see Fig. 933). The superior surface is covered by a thin layer of white fibres called the stratum zonale, these fibres being derived from the optic tract and optic radiation.
  
  
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Fig. 945 . — The Third Ventricle, Portions of the Lateral Ventricles, Pineal Body, and Corpora Quadrigemina (Superior View) (Henle).
 
Fig. 945 . — The Third Ventricle, Portions of the Lateral Ventricles, Pineal Body, and Corpora Quadrigemina (Superior View) (Henle).
  
The corpus callosum, fornix, and tela chorioidea have been removed.
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The corpus callosum, fornix, and tela chorioidea have been removed. A, surface covered by ependyma of lateral ventricle; B, groove caused by fornix.
  
the lateral ventricle, and is covered by portions of the tela chorioidea and body of the fornix (see Fig. 933). The superior surface is covered by a thin layer of white fibres called the stratum zonale, these fibres being derived from the optic tract and optic radiation.
 
  
The inferior surface lies posteriorly upon the upward prolongation
 
  
of the tegmental fibres of the crus cerebri, which constitutes the subthalamic tegmental region, but anteriorly it rests upon the corpus mamillare and a portion of the tuber cinereum.
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The inferior surface lies posteriorly upon the upward prolongation of the tegmental fibres of the crus cerebri, which constitutes the subthalamic tegmental region, but anteriorly it rests upon the corpus mamillare and a portion of the tuber cinereum.
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The lateral surface is directly related to the posterior limb of the internal capsule, which separates it from the nucleus lentiformis (see Fig. 940). Many fibres emerge from this surface, and enter the internal capsule on their way to the cerebral cortex, whilst others from the cerebral cortex enter the thalamus through this surface. These fibres constitute the thalamic radiation. On its surface the fibres form a well-marked reticular layer of white matter, which is called the external medullary lamina.
 
The lateral surface is directly related to the posterior limb of the internal capsule, which separates it from the nucleus lentiformis (see Fig. 940). Many fibres emerge from this surface, and enter the internal capsule on their way to the cerebral cortex, whilst others from the cerebral cortex enter the thalamus through this surface. These fibres constitute the thalamic radiation. On its surface the fibres form a well-marked reticular layer of white matter, which is called the external medullary lamina.
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A, surface covered by ependyma of lateral ventricle; B, groove caused by fornix.
 
  
  
 
The medial surface faces its fellow of the opposite side, with which it is connected by means of the connexus thalami. It forms the lateral wall of the third ventricle, and superiorly is limited by the stria thalami. It is covered by a thick layer of grey matter, which is continuous with that around the aqueduct of the mid-brain.
 
The medial surface faces its fellow of the opposite side, with which it is connected by means of the connexus thalami. It forms the lateral wall of the third ventricle, and superiorly is limited by the stria thalami. It is covered by a thick layer of grey matter, which is continuous with that around the aqueduct of the mid-brain.
  
The anterior extremity is marked by a prominence, called the anterior tubercle , which enters into the body of the lateral ventiicle, and forms the posterior boundary of the corresponding interventricular
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The anterior extremity is marked by a prominence, called the anterior tubercle , which enters into the body of the lateral ventiicle, and forms the posterior boundary of the corresponding interventricular foramen.  
 
 
foramen. . - ,
 
 
 
The posterior extremity presents at its inner end a well-marked
 
  
prominence, called the 'posterior tubercle or pulvinar . It lies over the
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The posterior extremity presents at its inner end a well-marked prominence, called the 'posterior tubercle or pulvinar . It lies over the brachia of the corpora quadrigemina, which it almost conceals. Below and external to the pulvinar there is an oval swelling, called the corpus geniculatum externum. Medial to this body is the brachium of the upper corpus quadrigeminum, and inferior to this is the corpus geniculatum internum (see Fig. 946).
  
  
brachia of the corpora quadrigemina, which it almost conceals. Below and external to the pulvinar there is an oval swelling, called the corpus geniculatum externum. Medial to this body is the brachium of the upper corpus quadrigeminum, and inferior to this is the corpus geniculatum internum (see Fig. 946).
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Metathalamus or Corpora Geniculata. — The corpora geniculata are external or lateral and internal or medial. They are associated with the posterior extremity of the thalamus, and the medial also with that portion of the mesencephalon which constitutes the corpora quadrigemina (see Fig. 946).
 
 
Metathalamus or Corpora Geniculata.—The corpora geniculata are external or lateral and internal or medial. They are associated with the posterior extremity of the thalamus, and the medial also with that portion of the mesencephalon which constitutes the corpora quadrigemina (see Fig. 946).
 
  
 
The corpus geniculatum laterale is an oval eminence situated on the posterior extremity of the thalamus below and lateral to the pulvinar. Internally it is connected with the upper quadrigeminal body by the superior brachium.
 
The corpus geniculatum laterale is an oval eminence situated on the posterior extremity of the thalamus below and lateral to the pulvinar. Internally it is connected with the upper quadrigeminal body by the superior brachium.
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The medial geniculate body contains many nerve-cells, the axons of which become corticipetal fibres, their destination being the cortex of the temporal region of the brain. By means of the lower quadrigeminal body and the inferior brachium this geniculate body receives fibres from the lateral or acoustic fillet , which terminate in arborizations around its cells. The axons of these cells become corticipetal fibres, the destination of which is the cortex of the temporal region of the brain. The medial geniculate body is associated with hearing.
 
The medial geniculate body contains many nerve-cells, the axons of which become corticipetal fibres, their destination being the cortex of the temporal region of the brain. By means of the lower quadrigeminal body and the inferior brachium this geniculate body receives fibres from the lateral or acoustic fillet , which terminate in arborizations around its cells. The axons of these cells become corticipetal fibres, the destination of which is the cortex of the temporal region of the brain. The medial geniculate body is associated with hearing.
  
Development. —The corpora geniculata appear as elevations on the lateral wall of the thalamencephalon or diencephalon.
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Development. — The corpora geniculata appear as elevations on the lateral wall of the thalamencephalon or {{diencephalon}}.
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Structure of the Thalamus. — The thalamus is composed chiefly of grey matter. Its superior surface is covered with a layer of white matter, known as the stratum zonale, and its lateral surface is covered with a reticular layer of white matter, called the external medullary lamina. The medial surface has a thick coating of grey matter, which is continuous with the grey matter around the aqueduct.
  
Structure of the Thalamus. —The thalamus is composed chiefly of grey matter. Its superior surface is covered with a layer of white matter, known as the stratum zonale, and its lateral surface is covered with a reticular layer of white matter, called the external medullary lamina. The medial surface has a thick coating of grey matter, which is continuous with the grey matter around the aqueduct.
 
  
 
The grey matter of the interior of the thalamus is traversed by a plate of white matter, called the internal medullary lamina, which divides it into tw r o nuclear areas—lateral and medial. The lateral nuclear area lies between the internal and external medullary laminae, and extends backwards as far as the pulvinar. The medial nuclear area lies between the internal medullary lamina and the thick layer of grey matter wdiich coats the medial surface of the thalamus. It extends backwards as far only as the habenular region, and anteriorly it is separated from the anterior tubercle by a lamina of white matter. The region of the anterior tubercle therefore constitutes a third or anterior nuclear area of grey matter. The grey nuclear areas are consequently three in number —lateral, medial, anddnterior.
 
The grey matter of the interior of the thalamus is traversed by a plate of white matter, called the internal medullary lamina, which divides it into tw r o nuclear areas—lateral and medial. The lateral nuclear area lies between the internal and external medullary laminae, and extends backwards as far as the pulvinar. The medial nuclear area lies between the internal medullary lamina and the thick layer of grey matter wdiich coats the medial surface of the thalamus. It extends backwards as far only as the habenular region, and anteriorly it is separated from the anterior tubercle by a lamina of white matter. The region of the anterior tubercle therefore constitutes a third or anterior nuclear area of grey matter. The grey nuclear areas are consequently three in number —lateral, medial, anddnterior.
  
  
Lateral Nuclear Area. —1 his area includes the pulvinar, the geniculate bodies, md the radiate nucleus. The pulvinar and geniculate bodies have just been described. Ihe radiate nucleus is associated with the fibres of the thalamic radiation, referred to later.
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Lateral Nuclear Area. — This area includes the pulvinar, the geniculate bodies, md the radiate nucleus. The pulvinar and geniculate bodies have just been described. Ihe radiate nucleus is associated with the fibres of the thalamic radiation, referred to later.
  
 
Anterior Nuclear Area. —This area includes the anterior tubercle, and is the chief sensory nucleus. It receives corticifugal fibres, and its cells furnish corticipetal fibres. It also receives many of the fibres of the lateral lemniscus as well as those of the superior cerebellar peduncle, and the fibres of the bundle of Vicq d’Azyr, the mamillo-thalamic tract.
 
Anterior Nuclear Area. —This area includes the anterior tubercle, and is the chief sensory nucleus. It receives corticifugal fibres, and its cells furnish corticipetal fibres. It also receives many of the fibres of the lateral lemniscus as well as those of the superior cerebellar peduncle, and the fibres of the bundle of Vicq d’Azyr, the mamillo-thalamic tract.
  
Medial Nuclear Area. —This area contains the ganglion habenulae, to be presently described.
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Medial Nuclear Area. — This area contains the ganglion habenulae, to be presently described.
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Connections of the Thalamus. —(1) Viewing the thalamus as an aggregation of ‘ cell-stations ' in the course of the centripetal fibres of the tegmentum of the crus cerebri, the tegmental fibres probably all terminate in the thalamic cells. (2) Through the lateral geniculate the thalamus is connected with the optic tract and optic radiation. (3) The cells of the anterior nucleus receive the fibres of the mamillo-thalamic tract, which are connected through the corpus mamillare with the fibres of the anterior pillar of the fornix. (4) Thalamic Radiation. —This is composed of thalamo-cortical fibres which arise within the thalamus as the axons of the thalamic cells. They issue from its lateral and inferior surfaces, and pass to all parts of the cerebral cortex. They are conveniently arranged in four groups or stalks —frontal, parietal, occipital, and inferior or ventral, (a) The fibres of the frontal stalk, having emerged from the front part of the external surface, traverse the lateral part of the anterior limb of the internal capsule, and most of them pass to the cortex of the frontal lobe. Some of these fibres are thalamo-caudate and thalamolenticular as regards their destination. ( b ) The parietal stalk, having issued from the thalamus, passes for the most part through the internal capsule, but also to a certain extent through the external capsule, to the cortex of the parietal lobe, and the central region of the frontal lobe, (c) The occipital stalk issues from the pulvinar, and, having traversed the postlenticular portion of the posterior limb of the internal capsule, it passes backwards and outwards lateral to the posterior horn of the lateral ventricle, and so reaches the cortex of the occipital lobe, (d) The inferior or ventral stalk emerges from the front part of the inferior surface of the thalamus, and its fibres arise as the axons of the cells of the lateral and medial nuclei. The most superficial of these fibres constitute a band, called the ansa lenticularis, which enters the nucleus lentiformis, where it terminates. The remaining fibres pass outwards beneath the nucleus to the cortex of the temporal lobe and insula.
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Connections of the Thalamus. — (1) Viewing the thalamus as an aggregation of ‘ cell-stations ' in the course of the centripetal fibres of the tegmentum of the crus cerebri, the tegmental fibres probably all terminate in the thalamic cells. (2) Through the lateral geniculate the thalamus is connected with the optic tract and optic radiation. (3) The cells of the anterior nucleus receive the fibres of the mamillo-thalamic tract, which are connected through the corpus mamillare with the fibres of the anterior pillar of the fornix. (4) Thalamic Radiation. —This is composed of thalamo-cortical fibres which arise within the thalamus as the axons of the thalamic cells. They issue from its lateral and inferior surfaces, and pass to all parts of the cerebral cortex. They are conveniently arranged in four groups or stalks —frontal, parietal, occipital, and inferior or ventral, (a) The fibres of the frontal stalk, having emerged from the front part of the external surface, traverse the lateral part of the anterior limb of the internal capsule, and most of them pass to the cortex of the frontal lobe. Some of these fibres are thalamo-caudate and thalamolenticular as regards their destination. ( b ) The parietal stalk, having issued from the thalamus, passes for the most part through the internal capsule, but also to a certain extent through the external capsule, to the cortex of the parietal lobe, and the central region of the frontal lobe, (c) The occipital stalk issues from the pulvinar, and, having traversed the postlenticular portion of the posterior limb of the internal capsule, it passes backwards and outwards lateral to the posterior horn of the lateral ventricle, and so reaches the cortex of the occipital lobe, (d) The inferior or ventral stalk emerges from the front part of the inferior surface of the thalamus, and its fibres arise as the axons of the cells of the lateral and medial nuclei. The most superficial of these fibres constitute a band, called the ansa lenticularis, which enters the nucleus lentiformis, where it terminates. The remaining fibres pass outwards beneath the nucleus to the cortex of the temporal lobe and insula.
  
 
Besides the thalamo-cortical fibres there are cortico-thalamic fibres, which pass from the various parts of the cerebral cortex into the thalamus, where they terminate in arborizations around the thalamic cells.
 
Besides the thalamo-cortical fibres there are cortico-thalamic fibres, which pass from the various parts of the cerebral cortex into the thalamus, where they terminate in arborizations around the thalamic cells.
  
Development. —The thalamus is developed as a thickening of the dorsal lamina of the thalamencephalon.
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Development. — The thalamus is developed as a thickening of the dorsal lamina of the thalamencephalon.
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Subthalamic Tegmental Region.—This region represents the upward prolongation of the tegmental fibres of the crus cerebri beneath the posterior portion of the thalamus. The parts to be noted are the upward prolongations of the red nucleus and substantia nigra of the tegmentum of the crus; the medial lemniscus; the fibres of the superior peduncle of the cerebellum; and the corpus subthalamicum (or nucleus of Luys). The red nucleus and the substantia nigra gradually disappear, and are no longer visible at the level of the corpus mamillare. The medial lemniscus lies on the superficial and lateral aspects of the red nucleus. The fibres of the superior peduncle of the cerebellum partly terminate in connection with the cells of the red nucleus, but many of them surround it in the form of a capsule. Beyond the red
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Subthalamic Tegmental Region. — This region represents the upward prolongation of the tegmental fibres of the crus cerebri beneath the posterior portion of the thalamus. The parts to be noted are the upward prolongations of the red nucleus and substantia nigra of the tegmentum of the crus; the medial lemniscus; the fibres of the superior peduncle of the cerebellum; and the corpus subthalamicum (or nucleus of Luys). The red nucleus and the substantia nigra gradually disappear, and are no longer visible at the level of the corpus mamillare. The medial lemniscus lies on the superficial and lateral aspects of the red nucleus. The fibres of the superior peduncle of the cerebellum partly terminate in connection with the cells of the red nucleus, but many of them surround it in the form of a capsule. Beyond the red nucleus the medial fillet, fibres of the superior cerebellar peduncle, and fibres which issue from the red nucleus enter the inferior surface of the thalamus, and terminate in connection with the thalamic cells. Some of these fibres may pass through the thalamus into the internal capsule, and thence to the cortex of the central (Rolandic) region of the cerebral hemisphere. The corpus or nucleus subthalamicum (or nucleus of Luys) is a small lenticular mass of grey matter, surrounded by white fibres, which lies above the substantia nigra.
  
nucleus the medial fillet, fibres of the superior cerebellar peduncle, and fibres which issue from the red nucleus enter the inferior surface of the thalamus, and terminate in connection with the thalamic cells. Some of these fibres may pass through the thalamus into the internal capsule, and thence to the cortex of the central (Rolandic) region of the cerebral hemisphere. The corpus or nucleus subthalamicum (or nucleus of Luys) is a small lenticular mass of grey matter, surrounded by white fibres, which lies above the substantia nigra.
 
  
Epithalamus.—The epithalamus includes the following parts:
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Epithalamus. — The epithalamus includes the following parts:
  
 
1. Pineal body. 3. Trigonum habenulae.
 
1. Pineal body. 3. Trigonum habenulae.
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bm. 947.— Section showing the Intermediate Subthalamic Area, where the Red Nucleus is appearing and the Subthalamic Nucleus has not yet Disappeared.
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Fig. 947. — Section showing the Intermediate Subthalamic Area, where the Red Nucleus is appearing and the Subthalamic Nucleus has not yet Disappeared.
 
 
  
Pineal Body, or Epiphysis Cerebri.—The pineal body resembles a small pine-cone. It is situated on the dorsal or superior surface of the mesencephalon, and occupies the depression between the upper quadrigeminal bodies. It is of small size, dark red in colour, and somewhat conical in shape. Superiorly it is intimately related to the pia mater as that membrane passes through the transverse cerebral fissure to form the tela chorioidea, and the splenium of the corpus callosum lies above it with the intervention of the pia mater. Inferiorly it is in contact with the depression between the upper quadrigeminal bodies. Its apex,, which is directed downwards and backwards, is free. Its base is directed upwards and forwards, and contains the pineal recess, which is continuous anteriorly with the cavity of the third ventricle. The portion of the base which lies below this recess is
 
  
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Pineal Body, or Epiphysis Cerebri. — The pineal body resembles a small pine-cone. It is situated on the dorsal or superior surface of the mesencephalon, and occupies the depression between the upper quadrigeminal bodies. It is of small size, dark red in colour, and somewhat conical in shape. Superiorly it is intimately related to the pia mater as that membrane passes through the transverse cerebral fissure to form the tela chorioidea, and the splenium of the corpus callosum lies above it with the intervention of the pia mater. Inferiorly it is in contact with the depression between the upper quadrigeminal bodies. Its apex,, which is directed downwards and backwards, is free. Its base is directed upwards and forwards, and contains the pineal recess, which is continuous anteriorly with the cavity of the third ventricle. The portion of the base which lies below this recess is connected with the posterior commissure, which separates it from the upper opening of the cerebral aqueduct. The portion above the recess contains the habenular commissure.
  
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Structure of the Pineal Body. — The pineal body is free from nervous constituents. It consists of a number of follicles lined with epithelial cells, and containing a variable amount of calcareous matter, called acervulus cerebri or brain-sand, which is composed of calcium phosphate, calcium carbonate, magnesium phosphate, and ammonium phosphate.
  
onnected with the posterior commissure, which separates it from the
 
jpper opening of the cerebral aqueduct. The portion above the recess
 
  
ontains the habenular commissure.
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Development. — The pineal body is developed as a diverticulum of the posterior part of the dorsal aspect of the thalamencephalon or diencephalon. This diverticulum for the most part becomes solid, but a portion of its cavity persists as the pineal recess of the third ventricle.
Structure of the Pineal Body. —The pineal body is free from nervous constituents. It consists of a number of follicles lined with epithelial cells, and containing a variable amount of calcareous matter, called acervulus cerebri or brain-sand, which is composed of calcium phosphate, calcium carbonate, magnesium phosphate, and ammonium phosphate.
 
  
Development. —The pineal body is developed as a diverticulum of the posterior part of the dorsal aspect of the thalamencephalon or diencephalon. This diverticulum for the most part becomes solid, but a portion of its cavity persists as the pineal recess of the third ventricle.
 
  
 
The pineal body is usually regarded as the representative of one of the stalks of the two median eyes of some of the higher arthropods, such as the king crab, among the Jnvertebrata, and is important in suggesting the possible line of evolution of the Vertebrata. In many of the reptiles the pineal eye as well as the eye-stalk is present, though it is never functional.
 
The pineal body is usually regarded as the representative of one of the stalks of the two median eyes of some of the higher arthropods, such as the king crab, among the Jnvertebrata, and is important in suggesting the possible line of evolution of the Vertebrata. In many of the reptiles the pineal eye as well as the eye-stalk is present, though it is never functional.
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Striae Thalami or Habenulae.—Each stria is a narrow strip of white longitudinal fibres lying along the upper part of the medial surface of the corresponding thalamus. It constitutes the habenula. Anteriorly most of its fibres are derived from the olfactory lobe, more particularly the olfactory bulb and anterior perforated substance. Some, however, may be derived from the anterior pillar of the fornix, and through the fornix from the cells of the hippocampus. Posteriorly the fibres are disposed in two ways: (i) The lateral fibres enter the ganglion habenulae, and terminate in connection with its cells. (2) The medial fibres curve inwards towards the base of the pineal body, in which they cross to the opposite side, lying above the pineal recess. As they cross the median line they decussate with the medial fibres of the opposite stria medullaris, and they terminate in the ganglion habenulae of the side to which they have crossed. Their decussation is known as the habenular commissure.
 
Striae Thalami or Habenulae.—Each stria is a narrow strip of white longitudinal fibres lying along the upper part of the medial surface of the corresponding thalamus. It constitutes the habenula. Anteriorly most of its fibres are derived from the olfactory lobe, more particularly the olfactory bulb and anterior perforated substance. Some, however, may be derived from the anterior pillar of the fornix, and through the fornix from the cells of the hippocampus. Posteriorly the fibres are disposed in two ways: (i) The lateral fibres enter the ganglion habenulae, and terminate in connection with its cells. (2) The medial fibres curve inwards towards the base of the pineal body, in which they cross to the opposite side, lying above the pineal recess. As they cross the median line they decussate with the medial fibres of the opposite stria medullaris, and they terminate in the ganglion habenulae of the side to which they have crossed. Their decussation is known as the habenular commissure.
  
Trigonum Habenulae.—This is a small triangular area (Fig. 948) which is bounded posteriorly by the upper quadrigeminal body, internally by the posterior part of the stria thalami, and laterally by the adjacent part of the thalamus. It contains an important group of multipolar nerve-cells, known as the ganglion habenulae. This ganglion belongs to the medial area of the thalamus. It receives some of the fibres of the stria, which come from the olfactory lobe, and, it may be, from the anterior pillar of the fornix. The axons of the ganglionic cells issue from the ventral surface of the ganglion and form a bundle, called the fasciculus retroflexus. This bundle passes downwards and forwards in the tegmentum of the crus cerebri, lying on the medial side of the red nucleus. Its fibres terminate in connection with the cells of the ganglion interpedunculare , which is situated in the lower part of the posterior perforated substance directly above the pons.
+
Trigonum Habenulae. — This is a small triangular area (Fig. 948) which is bounded posteriorly by the upper quadrigeminal body, internally by the posterior part of the stria thalami, and laterally by the adjacent part of the thalamus. It contains an important group of multipolar nerve-cells, known as the ganglion habenulae. This ganglion belongs to the medial area of the thalamus. It receives some of the fibres of the stria, which come from the olfactory lobe, and, it may be, from the anterior pillar of the fornix. The axons of the ganglionic cells issue from the ventral surface of the ganglion and form a bundle, called the fasciculus retroflexus. This bundle passes downwards and forwards in the tegmentum of the crus cerebri, lying on the medial side of the red nucleus. Its fibres terminate in connection with the cells of the ganglion interpedunculare , which is situated in the lower part of the posterior perforated substance directly above the pons.
  
 
The ganglia habenularum are connected with each other by fibres which constitute the habenular commissure or commissure of the habenular ganglia. These fibres cross in the dorsal part of the base of the pineal body, and are on a higher plane than the posterior commissure.
 
The ganglia habenularum are connected with each other by fibres which constitute the habenular commissure or commissure of the habenular ganglia. These fibres cross in the dorsal part of the base of the pineal body, and are on a higher plane than the posterior commissure.
Line 3,227: Line 3,214:
 
Shows trigonum and stria habenulae.
 
Shows trigonum and stria habenulae.
  
Posterior Commissure.—This is a band of white fibres which is situated at the back part of the third ventricle. It lies in the posterior wall of the ventricle directly above the upper opening of the aqueduct and underneath the base of the pineal body. Its fibres are regarded as arising from a nucleus in the grey matter of the lateral wall of the third ventricle near the upper opening of the aqueduct. Some of the fibres of either side, after crossing, may descend in the tegmentum of the crus cerebri as part of the medial longitudinal bundle of that side, and so reach the medulla oblongata.
+
Posterior Commissure. — This is a band of white fibres which is situated at the back part of the third ventricle. It lies in the posterior wall of the ventricle directly above the upper opening of the aqueduct and underneath the base of the pineal body. Its fibres are regarded as arising from a nucleus in the grey matter of the lateral wall of the third ventricle near the upper opening of the aqueduct. Some of the fibres of either side, after crossing, may descend in the tegmentum of the crus cerebri as part of the medial longitudinal bundle of that side, and so reach the medulla oblongata.
 +
 
 +
Hypothalamus. — The {{hypothalamus}} consists of two parts—mammillary and optic. The pars mamillaris hypothalami represents the two corpora mamillaria. The pars optica hypothalami includes the following structures:
  
Hypothalamus.—The hypothalamus consists of two parts—mammillary and optic. The pars mamillaris hypothalami represents the
+
# Tuber cinereum.
 +
# Infundibulum.
 +
# Posterior or cerebral lobe of the hypophysis.
 +
# Optic chiasma.
 +
# Lamina terminalis.
  
  
  
Fig. 949. —Diagram to show Position and Relations of Structures in
 
  
Tegmental Subthalamus.
+
Fig. 949. —Diagram to show Position and Relations of Structures in Tegmental Subthalamus.
  
 
Supposed to be viewed from the medial aspect. R, red nucleus. The subthalamic nucleus is shown antero-lateral to this. Dotted line shows course of fasciculus retroflexus from habenula to interpeduncular ganglion. Course of anterior pillar of fornix is indicated, also mamillo-thalamic tract (bundle of Vicq d’Azyr) passing up medial to front part of subthalamic nucleus. Substantia nigra is seen near pontine level, but passes upwards and laterally out of the section higher up.
 
Supposed to be viewed from the medial aspect. R, red nucleus. The subthalamic nucleus is shown antero-lateral to this. Dotted line shows course of fasciculus retroflexus from habenula to interpeduncular ganglion. Course of anterior pillar of fornix is indicated, also mamillo-thalamic tract (bundle of Vicq d’Azyr) passing up medial to front part of subthalamic nucleus. Substantia nigra is seen near pontine level, but passes upwards and laterally out of the section higher up.
  
 
two corpora mamillaria. The pars optica hypothalami includes the following structures:
 
  
 
1. Tuber cinereum.
 
 
2. Infundibulum.
 
 
3. Posterior or cerebral lobe of the hypophysis.
 
 
4. Optic chiasma.
 
 
5. Lamina terminalis.
 
  
  
Line 3,257: Line 3,237:
  
 
Development. — The corpora mamillaria are developed from the ventral aspect of the thalamencephalon or diencephalon. Up to the third month of intra-uterine life they are represented by a single corpus mamillare, but after that period this divides into two corpora.
 
Development. — The corpora mamillaria are developed from the ventral aspect of the thalamencephalon or diencephalon. Up to the third month of intra-uterine life they are represented by a single corpus mamillare, but after that period this divides into two corpora.
 +
  
 
The tuber cinereum is an elevated area of grey matter which lies in front of the corpora mamillaria and behind the optic commissure, the anterior portion of each optic tract being on either side. It is continuous anteriorly with the lamina terminalis, and on either side with the grey matter of the anterior perforated substance.
 
The tuber cinereum is an elevated area of grey matter which lies in front of the corpora mamillaria and behind the optic commissure, the anterior portion of each optic tract being on either side. It is continuous anteriorly with the lamina terminalis, and on either side with the grey matter of the anterior perforated substance.
  
In the lateral part of the tuber cinereum, in the vicinity of the optic tract, there is a collection of nerve-cells, which is variously spoken of as the basal ganglion of Meynert or the supra-optic nucleus of Cajal, and which is connected
+
In the lateral part of the tuber cinereum, in the vicinity of the optic tract, there is a collection of nerve-cells, which is variously spoken of as the basal ganglion of Meynert or the supra-optic nucleus of Cajal, and which is connected with the fibres of the commissure of Gudden.
  
with the fibres of the commissure of Gudden.
 
  
 
Behind the tuber cinereum, and in front of the corpora mamillaria, there is a small prominence, medially placed, called the eminentia saccularis of Retzius, who regards it as the homologue of the saccus vasculosus of some lower vertebrates— e.g., fishes.
 
Behind the tuber cinereum, and in front of the corpora mamillaria, there is a small prominence, medially placed, called the eminentia saccularis of Retzius, who regards it as the homologue of the saccus vasculosus of some lower vertebrates— e.g., fishes.
Line 3,270: Line 3,250:
 
Hypophysis (Pituitary Body).—As this structure is seldom removed in the course of dissection with the brain, it has already been described on p. 1171 with the pituitary fossa, in which it lies. It may be well, however, to repeat in this place the fact that the anterior lobe is a derivative of the ectodermal lining of the primitive mouth; that the posterior lobe, which is connected to the infundibulum, is a downgrowth from the brain (hypophysis cerebri); and that, between the two, lies the pars intermedia, which is only the posterior wall of the ectodermal pouch. The name (pituitary) was derived from the old belief that the gland secreted the pituita or mucus of the nose.
 
Hypophysis (Pituitary Body).—As this structure is seldom removed in the course of dissection with the brain, it has already been described on p. 1171 with the pituitary fossa, in which it lies. It may be well, however, to repeat in this place the fact that the anterior lobe is a derivative of the ectodermal lining of the primitive mouth; that the posterior lobe, which is connected to the infundibulum, is a downgrowth from the brain (hypophysis cerebri); and that, between the two, lies the pars intermedia, which is only the posterior wall of the ectodermal pouch. The name (pituitary) was derived from the old belief that the gland secreted the pituita or mucus of the nose.
  
Lamina Terminalis.— This is a thin plate of grey matter which extends between the upper surface of the optic commissure and the rostrum of the corpus callosum near the genu. On either side it is connected with the grey matter of the anterior perforated substance. It forms the lower part of the anterior wall of the third ventricle.
+
Lamina Terminalis. — This is a thin plate of grey matter which extends between the upper surface of the optic commissure and the rostrum of the corpus callosum near the genu. On either side it is connected with the grey matter of the anterior perforated substance. It forms the lower part of the anterior wall of the third ventricle.
  
 
Development. The lamina terminalis represents the terminal part of the ventral wall of the embryonic neural tube.
 
Development. The lamina terminalis represents the terminal part of the ventral wall of the embryonic neural tube.
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Occupying the back part of the commissure there are, as stated, iome fibres which have no connection with either retina.. These ibres constitute the commissure of Gudden. I hey lie behind the lecussating fibres, and represent the fibres of the medial root of the )ptic tract of each side. They form the innermost fibres of each >ptic tract, and connect one medial geniculate body with its fellow )f the opposite side.
 
Occupying the back part of the commissure there are, as stated, iome fibres which have no connection with either retina.. These ibres constitute the commissure of Gudden. I hey lie behind the lecussating fibres, and represent the fibres of the medial root of the )ptic tract of each side. They form the innermost fibres of each >ptic tract, and connect one medial geniculate body with its fellow )f the opposite side.
  
Summary. —The fibres which arise in the nasal half of one retina cross in fie optic commissure, and enter the optic tract of the opposite side. The fibres \fiich arise in the temporal half of one retina pass directly backwards into the )ptic tract of the same side. The fibres of the inner ropt of each optic tract
+
Summary. — The fibres which arise in the nasal half of one retina cross in fie optic commissure, and enter the optic tract of the opposite side. The fibres which arise in the temporal half of one retina pass directly backwards into the )ptic tract of the same side. The fibres of the inner ropt of each optic tract cross in the back part of the commissure, and form the commissure of Gudden,
 +
he fibres of which have no connection with the optic nerves, but connect the •wo medial geniculate bodies, right and left. The optic commissure therefore insists of the following groups of fibres: (1) The crossed fibres, which arise n the nasal portion of each retina; (2) the uncrossed fibres, which arise m the temporal portion of each retina, and occupy the outer part of the commissure; ind (3) the fibres of the commissure of Gudden, which occupy the back part of fie commissure.
  
ross in the back part of the commissure, and form the commissure of Gudden,
 
he fibres of which have no connection with the optic nerves, but connect the •wo medial geniculate bodies, right and left. The optic commissure therefore insists of the following groups of fibres: (1) The crossed fibres, which arise n the nasal portion of each retina; (2) the uncrossed fibres, which arise m the temporal portion of each retina, and occupy the outer part of the commissure; ind (3) the fibres of the commissure of Gudden, which occupy the back part of fie commissure.
 
  
 
The optic tract of each side is a flattened white band which passes backwards from the optic chiasma. It curves round the crus cerebri, ind in the region of the posterior extremity of the thalamus it divides into two roots, lateral and medial. Ihe lateral or visual foot is the larger of the two. It is chiefly composed of afferent fibres, which pass from the retina to the brain; but it also contains efferent ibres, which pass from the brain to the retina.. The efferent fibres ire derived from (1) the temporal half of the retma of the same side, and (2) the nasal half of the retina of the opposite side, the latter having crossed in the optic chiasma. The fibres of the lateral root terminate in the lateral geniculate body and the upper quadrigeminal body, reaching the last-named body through the superior brachium. They form arborizations around the cells of these bodies which constitute the terminal nuclei or lower visual centres of the
 
The optic tract of each side is a flattened white band which passes backwards from the optic chiasma. It curves round the crus cerebri, ind in the region of the posterior extremity of the thalamus it divides into two roots, lateral and medial. Ihe lateral or visual foot is the larger of the two. It is chiefly composed of afferent fibres, which pass from the retina to the brain; but it also contains efferent ibres, which pass from the brain to the retina.. The efferent fibres ire derived from (1) the temporal half of the retma of the same side, and (2) the nasal half of the retina of the opposite side, the latter having crossed in the optic chiasma. The fibres of the lateral root terminate in the lateral geniculate body and the upper quadrigeminal body, reaching the last-named body through the superior brachium. They form arborizations around the cells of these bodies which constitute the terminal nuclei or lower visual centres of the
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nerve. Having traversed the inner part of the optic tract, they cross in the back part of the optic commissure behind the decussating fibres, and are continuous with the corresponding fibres of the opposite side. These are the fibres which constitute the commissure of Gudden.
 
nerve. Having traversed the inner part of the optic tract, they cross in the back part of the optic commissure behind the decussating fibres, and are continuous with the corresponding fibres of the opposite side. These are the fibres which constitute the commissure of Gudden.
  
Optic Radiation. —The strand of fibres which forms the optic (or thalamo-occipital) radiation of either side establishes a connection between the lower visual centres (lateral geniculate body and upper quadrigeminal body) and the higher or cortical visual centre, which is situated (1) on the medial surface of the occipital lobe close to the calcarine fissure in the region of the cuneus and lingual gyrus, and (2) on the adjacent part of the postero-lateral surface of the occipital lobe. The strand passes through the post-lenticular part of the internal capsule, and then backwards in the medullary substance of the occipital lobe, lying on the lateral side of the posterior horn of the lateral ventricle. Thereafter the fibres pass in a radiating manner to the higher or cortical visual centre.
+
 
 +
Optic Radiation. — The strand of fibres which forms the optic (or thalamo-occipital) radiation of either side establishes a connection between the lower visual centres (lateral geniculate body and upper quadrigeminal body) and the higher or cortical visual centre, which is situated (1) on the medial surface of the occipital lobe close to the calcarine fissure in the region of the cuneus and lingual gyrus, and (2) on the adjacent part of the postero-lateral surface of the occipital lobe. The strand passes through the post-lenticular part of the internal capsule, and then backwards in the medullary substance of the occipital lobe, lying on the lateral side of the posterior horn of the lateral ventricle. Thereafter the fibres pass in a radiating manner to the higher or cortical visual centre.
  
 
This visual area in the neighbourhood of the calcarine fissure is distinguishable to the naked eye in a section of a fresh brain by the white band of Gennari which traverses it.
 
This visual area in the neighbourhood of the calcarine fissure is distinguishable to the naked eye in a section of a fresh brain by the white band of Gennari which traverses it.
  
The optic radiation consists of afferent or corticipetal and efferent or corticifugal fibres. The corticipetal fibres for the most part arise as the axons of the nerve-cells within the lateral geniculate body, which are terminal nuclei of the retinal nerve-fibres, and they end in the higher or cortical visual centre. Some corticipetal fibres arise in the higher or cortical visual centre of the opposite side and cioss in the splenium of the corpus callosum. These fibres are of a commissural character. The corticifugal fibres arise as the axons of the pyramidal cells of the cortex of the visual area of the occipital lobe, and they terminate in the pulvinar, geniculate, and upper quadrigeminal
+
The optic radiation consists of afferent or corticipetal and efferent or corticifugal fibres. The corticipetal fibres for the most part arise as the axons of the nerve-cells within the lateral geniculate body, which are terminal nuclei of the retinal nerve-fibres, and they end in the higher or cortical visual centre. Some corticipetal fibres arise in the higher or cortical visual centre of the opposite side and cioss in the splenium of the corpus callosum. These fibres are of a commissural character. The corticifugal fibres arise as the axons of the pyramidal cells of the cortex of the visual area of the occipital lobe, and they terminate in the pulvinar, geniculate, and upper quadrigeminal body.
 
 
body.
 
 
 
  
The lower visual centres are connected with the nuclei of origin of the nerves which supply the ocular muscles, probably through the medial longitudinal
 
  
bundle.
+
The lower visual centres are connected with the nuclei of origin of the nerves which supply the ocular muscles, probably through the medial longitudinal bundle.
  
  
Mesencephalon.
+
==Mesencephalon==
  
 
The mesencephalon is composed of the corpora quadngemma, which form its upper or dorsal portion; the crura cerebri, which form its lower or ventral portion; and the aqueduct, which passes through it from the fourth ventricle below to the third ventricle above.
 
The mesencephalon is composed of the corpora quadngemma, which form its upper or dorsal portion; the crura cerebri, which form its lower or ventral portion; and the aqueduct, which passes through it from the fourth ventricle below to the third ventricle above.
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Connexus Thalami
+
Fig. 951. — The Floor of the Fourth Ventricle and Adjacent Parts. I he pineal body has been removed to show the upper quadrigeminal bodies.
  
Pineal Peduncle ; Tllird Vtntricle
 
  
 +
The superior brachium is associated with the visual apparatus. The inferior brachium, though connected with the medial geniculate body, with which body the inner or commissural root of the optic tract (commissure of Gudden) is also connected, is associated with the acoustic apparatus.
  
 +
The superior brachium contains two sets of fibres—namely, retinal fibres, derived from the lateral root of the optic tract; and occipital fibres, from the cortex of the occipital lobe of the cerebrum.
  
Thalamus
+
The inferior brachium passes upwards from the lower quadrigeminal body to the under aspect of the medial geniculate body, which is a small oval mass on the lateral aspect of the mesencephalon, under cover of the pulvinar of the thalamus. Though the inner root of the optic tract is connected with this geniculate body, the inferior brachium passes clear of it, and most of its fibres are traceable to the thalamus through the tegmentum.
  
 +
Structure of Corpora Quadrigemina. — The lower quadrigeminal body {colliculus inferior ) is composed of the following parts:
  
..Upper Quadrigeminal Body W m( i!i'i(.( //).' VjV - Lower Quadi igeminal Body
+
# A central nucleus of grey matter.
 +
# A dorsal layer of white matter.
 +
# A ventral layer of white matter.
  
 +
The central grey nucleus consists of many multipolar cells and nerve-fibres. The axons of the cells pass partly to the dorsal and partly to the ventral layers of white matter. The nerve-fibres are derived from the lateral or acoustic lemniscus , and terminate in arborizations around the cells of the central nucleus.
  
Superior Fovea Auditory Striae
 
  
Inferior Fovea_
 
  
  
Cuneate Tubercle -
 
  
Basis Pedunculi
 
  
 +
Fig. 952.— Side View of the Mesencephalon.
  
Frenulum Veli Sup. Med. Velum ~-»»Supei ior Peduncle of Cerebellum
 
  
Mid. ped. (cut)
 
  
 +
The dorsal white layer derives its fibres from the lateral lemniscus and from the axons of the cells of the central grey nucleus. The fibres pass into the inferior brachium, by which they are conducted to the medial geniculate body.
  
"" Auditory Striae Vestibular Trigone
+
The ventral white layer also derives its fibres from the lateral lemniscus and from the axons of the cells of the central grey nucleus. This layer separates the central nucleus from the subjacent grey matter of the aqueduct. Some of the fibres cross the median plane, and decussate with corresponding fibres of the opposite side superficial to the roof of the aqueduct. Others enter the tegmentum of the crus of the same side and also of the opposite side, in which their course is downwards in the lateral lemniscus.
  
I dim J'-' Trigonum Hypoglossi
 
  
•^Trieonum Vagi
+
The lower quadrigeminal body ( colliculus inferior), which receives its fibres from the lateral or acoustic lemniscus, is associated with the acoustic apparatus The upper quadrigeminal body ( colliculus superior) is composed of the follow, ing layers :
  
( Spinal Tract of Fifth Nerve
+
1. Stratum zonale. 3. Stratum opticum.
  
tf# \ciava ppT-- Funiculus Gracilis
+
2. Stratum cinereum. 4. Stratum lemnisci.
  
Funiculus Cuneatus
+
The stratum zonale is the most superficial layer, and probably consists of retinal fibres which are derived from the outer root of the optic tract. Many of these fibres pass into the stratum cinereum and terminate in connection with its cells. Others cross the median plane and decussate with corresponding fibres from the opposite side superficial to the roof of the aqueduct.
  
Fig. 951. —The Floor of the Fourth Ventricle and Adjacent Parts. I he pineal body has been removed to show the upper quadrigeminal bodies.
+
The stratum cinereum, or second layer, lies beneath the stratum zonale, and consists of a crescentic layer of grey matter containing many nerve-cells. It represents the grey nucleus of the upper quadrigeminal body, and the axons of its cells pass to the more deeply seated strata.
  
  
The superior brachium is associated with the visual apparatus. The inferior brachium, though connected with the medial geniculate body, with which body the inner or commissural root of the optic tract (commissure of Gudden) is also connected, is associated with the acoustic apparatus.
 
  
The superior brachium contains two sets of fibres—namely, retinal fibres, derived from the lateral root of the optic tract; and occipital fibres, from the cortex of the occipital lobe of the cerebrum.
 
  
The inferior brachium passes upwards from the lower quadrigeminal body to the under aspect of the medial geniculate body, which is a small oval mass on the lateral aspect of the mesencephalon, under cover of the pulvinar of the thalamus. Though the inner root of the optic tract is connected with this geniculate body, the inferior brachium passes clear of it, and most of its fibres are traceable to the thalamus through the tegmentum.
 
  
Structure of Corpora Quadrigemina. —The lower quadrigeminal body {colliculus
+
Fig. 953. — The Medulla Oblongata, Pons, and Interpeduncular Region. C.C., crus cerebri; P., pyramid; O.B., olivary body.
  
inferior ) is composed of the following parts:
 
  
1. A central nucleus of grey matter.
+
The stratum opticum is the third layer, and consists of grey matter which contains numerous nerve-cells and nerve-fibres. The fibres are conducted to this stratum by the superior brachium, and are of two kinds: (1) Many are retinal fibres, and are derived from the outer root of the optic tract. (2) Others are corticifugal fibres, which come from the higher visual centre in the cortex of the occipital lobe, and form part of the optic radiation. The fibres pass into the stratum cinereum, and terminate in arborizations around its cells. The axons of the cells of the stratum opticum pass into the stratum lemnisci.
  
2. A dorsal layer of white matter.
+
The stratum lemnisci is the deepest layer. Like the stratum opticum, it consists of , grey matter, which contains numerous nerve-cells and nerve-fibres. The fibres are derived from the following sources: (1) Many are derived from the medial or main lemniscus; and (2) some are the axons of cells belonging to the stratum opticum and stratum lemnisci. The lemniscal fibres terminate in the stratum lemnisci. The fibres formed by the axons of the cells of the stratum opticum and stratum lemnisci cross the median plane, below the aqueduct, and decussate with the corresponding fibres of the opposite side. This decussation is known as the fountain decussation (of Meynert). The fibres, after crossing, form the tectospinal tract of that side, and this bundle or tract descends through the pons and medulla oblongata into the corresponding ventral or anterior column of the spinal cord.
  
3. A ventral layer of white matter.
+
The upper quadrigeminal body, by means of the superior brachium, is one of the lower visual centres, the other being the external geniculate body.
  
The central grey nucleus consists of many multipolar cells and nerve-fibres. The axons of the cells pass partly to the dorsal and partly to the ventral layers
+
Development of Corpora Quadrigemina. —The corpora quadrigemina are developed from the dorsal wall or roof of the mesencephalon. They are at first in the form of elongated paired swellings, later divided transversely.
  
  
Pulvinar
+
Dorsal Tegmental Decussation (or Commissure of Meynert). —This commissure or decussation consists of fibres which issue from each upper quadrigeminal body, and cross partly to the opposite posterior longitudinal bundle, but mostly form tecto-spinal tracts, in which they descend towards the pons.
  
Superior Quad. Body
+
Crura Cerebri. — The crura or pedunculi cerebri are two large strands which are situated above the pons. They lie at first near each other, being separated by the interpeduncular fossa, but afterwards diverge as they pass upwards and laterally to the cerebral hemisphere. The medial surface of each crus bounds the interpeduncular region, and has a furrow, the oculo-motor sulcus, through which the roots of the third nerve emerge near the pons.
  
Inferior Quad. Body • Fourth Nerve Lateral Fillet - ~ — Basis Pedunculi Superior Cerebellar Peduncle—
+
The lateral surface looks towards the temporal lobe of the brain, which to a large extent overlaps the crus, and this surface also has a furrow, the sulcus lateralis. The slender fourth cranial nerve lies upon this surface. Close to the cerebral hemisphere the ventral and lateral aspects of the crus are embraced by the optic tract of the corresponding side.
  
Inferior Cerebellar Peduncle. _
+
Each crus is composed of two parts—ventral and dorsal. The ventral part is the basis (or crusta), and the dorsal the tegmentum. The separation between these is indicated superficially by the sulcus lateralis and the oculo-motor sulcus. Within the crus the two parts are separated by a mass of dark grey matter, called the substantia
  
Middle Cerebellar Peduncle Restiform Body Eighth Nerve ^
 
  
Olive
 
  
 +
Fig. 954. — Topography of the Crus Cerebri (after Poirier).
  
Med. Geniculate Body
 
  
-1 Lat. Geniculate Body
 
  
.Optic Tract __ . Corpus Mam.
+
The basis pedunculi (crusta) is continuous superiorly with the internal capsule of the corpus striatum, and inferiorly its fibres enter the ventral part of the pons.
  
 +
Structure of the Basis. — The crusta, or basis, as seen m transverse section, presents a crescentic outline, the concavity of the crescent being occupied by the convexity of the substantia nigra. It consists of longitudinal corticifugal fibres which arise in the cells of the cerebral cortex. These fibres form two groups—pyramidal and cortico-pontine.
  
 +
The pyramidal fibres form the motor tract from the precentral motor region of the cortex of the frontal lobe, and they arise for the most part from the cells of that region, which control the voluntary muscles of the body.
  
Tuber Cinereum
+
The cortico-pontine fibres lie on each side of the pyramidal tract, those coming from the frontal region of the cortex being on the medial and those from the temporal region on the lateral side; the basis of each peduncle, therefore, is formed, from within outward, by frontopontine, pyramidal, and temporo-pontine tracts (see Fig. 958).
  
- Third Nerve
+
Tegmentum.— The tegmentum is continuous interiorly with the formatio reticularis of the dorsal portion of the pons, which in turn is continuous interiorly with the formatio reticularis of the medulla oblongata; the upward prolongation of the tegmentum makes the tegmental subthalamic region. The two tegmenta, right and left, are separated from each other by a median raphe, which is continuous with that of the pons. In the lower part of the mesencephalon this raphe is indistinct on account of the decussation which takes place across the median plane between the superior cerebellar peduncles, underneath the lower pair of quadrigeminal bodies.
  
  
Fifth Nerve
 
  
  
- - Seventh Nerve
+
Fig. 955 Section through Inferior Corpora Quadrigemina.
  
" * Sixth Nerve
 
  
  
Fig. 952.— Side View of the Mesencephalon.
+
The dorsal surface of each tegmentum extends on either side of the grey matter of the aqueduct, and becomes continuous with the basal parts of the upper and lower quadrigeminal bodies of the corresponding side, which constitute the tectum. The ventral surface is separated from the crusta by the substantia nigra.
  
 +
Structure of the Tegmentum. —Each tegmentum, besides being continuous interiorly with the formatio reticularis of the dorsal portion of the pons, consists of bundles of longitudinal and transverse fibres, the intervals between which are occupied by grey matter.
  
of white matter. The nerve-fibres are derived from the lateral or acoustic lemniscus , and terminate in arborizations around the cells of the central nucleus.
 
  
The dorsal white layer derives its fibres from the lateral lemniscus and from the axons of the cells of the central grey nucleus. The fibres pass into the inferior brachium, by which they are conducted to the medial geniculate body.
 
  
The ventral white layer also derives its fibres from the lateral lemniscus and from the axons of the cells of the central grey nucleus. This layer separates the central nucleus from the subjacent grey matter of the aqueduct. Some of the fibres cross the median plane, and decussate with corresponding fibres of the opposite side superficial to the roof of the aqueduct. Others enter the tegmentum of the crus of the same side and also of the opposite side, in which their course is downwards in the lateral lemniscus.
+
Grey Matter. — The grey matter of the tegmentum contains the red nucleus.
  
 +
The red nucleus (nucleus ruber) is a round reddish mass, which is situated in the centre of the upper part of the tegmentum, and lies in the path of the superior cerebellar peduncle of the opposite side. It is on the same level as the upper quadrigeminal body, and is prolonged upwards into the subthalamic tegmental region. Some of the fibres of the superior cerebellar peduncle of the opposite side surround the red nucleus in the form of a capsule on their way to the thalamus. Other fibres of that peduncle enter the red nucleus, and terminate in arborizations around its cells.
  
The lower quadrigeminal body ( colliculus inferior), which receives its fibres from the lateral or acoustic lemniscus, is associated with the acoustic apparatus The upper quadrigeminal body ( colliculus superior) is composed of the follow, ing layers :
+
The axons of the cells of the red nucleus form two sets of nervefibres—ascending and descending. The ascending fibres pass to the thalamus in company with those fibres of the superior cerebellar peduncle which encapsule the red nucleus. These ascending fibres form relays which carry on those fibres of the superior cerebellar peduncle which terminate within the red nucleus, that nucleus being a cell-station in their path. The descending fibres constitute the rubro-spinal tract (or bundle of Monakow). The fibres of this tract cross the median plane in the raphe, and by their decussation with those of the opposite side they constitute the ventral fountain decussation [of Foret), in contradistinction to the dorsal fountain decussation (of Meynert). The latter decussation is on a higher level, and involves the fibres of the ventral longitudinal bundles, or tecto-spinal tracts, which derive their fibres from the cells of the stratum opticum and stratum lemnisci of the upper quadrigeminal bodies. The rubrospinal tract of either side descends through the pons and medulla oblongata into the lateral column of the spinal cord, where each constitutes the prepyramidal tract, which lies on the ventro-lateral aspect of the lateral cortico-spinal tract. (The tecto-spinal tract, or ventral longitudinal bundle, on either side descends into the anterior column of the spinal cord.)
  
1. Stratum zonale. 3. Stratum opticum.
+
White Matter of the Tegmentum. — The principal tracts of the white matter on either side are as follows:
  
2. Stratum cinereum. 4. Stratum lemnisci.
+
1. Superior cerebellar peduncle.
  
The stratum zonale is the most superficial layer, and probably consists of retinal fibres which are derived from the outer root of the optic tract. Many of these fibres pass into the stratum cinereum and terminate in connection with its cells. Others cross the median plane and decussate with corresponding fibres from the opposite side superficial to the roof of the aqueduct.
+
2. Medial (posterior) longitudinal bundle.
  
The stratum cinereum, or second layer, lies beneath the stratum zonale, and consists of a crescentic layer of grey matter containing many nerve-cells. It represents the grey nucleus of the upper quadrigeminal body, and the axons of its cells pass to the more deeply seated strata.
+
3. Ventral longitudinal bundle, or tecto-spinal tract.
  
 +
4. Pallido-rubro-olivary tract.
  
Infundibulum Tuber Cinereum _
+
5. Rubro-spinal tract.
  
Mamillary Body Mesial Root of Optic Tract Lateral Root of Optic Tract /\
+
6. Medial lemniscus (chief sensory tract).
  
Lateral Geniculate Body b.
+
7. Lateral (acoustic) lemniscus.
  
Locus Perforatus Posterior
+
Superior Cerebellar Peduncle.— The fibres of this peduncle emerge for the most part through the hilum of the nucleus dentatus in the cerebellar hemisphere. The two peduncles, right and left, having emerged from the hemispheres, pass upwards on the lateral parts of the dorsal surface of the pons in a converging manner towards the lower pair of the quadrigeminal bodies, being connected by the superior medullary velum. On entering the mesencephalon, the two peduncles decussate across the raphe beneath the lower quadrigeminal bodies. This decussation extends as high as the upper quadrigeminal bodies, and it involves almost all the fibres of the two peduncles. Each peduncle, having gained the opposite side, ascends in the upper part of the tegmentum as a longitudinal tract,
  
 +
and soon comes into contact with the red nucleus. Many of its fibres enter this nucleus and terminate in arborizations around its cells. Other fibres of the peduncle encapsule the nucleus, and then ascend through the subthalamic tegmental region to the anterior part of the thalamus, within which they terminate in arborizations around the cells of the anterior nuclear area (chief sensory nucleus). From the cells of the red nucleus relays of fibres proceed upwards, which carry on those fibres of the peduncle which terminate within the nucleus, and these relays ascend with those fibres of the peduncle which encapsule the red nucleus to the thalamus.
  
Sixth Nerve Motor Root, Facial Nerve Sensory Root Auditory Nerve " Glosso-pharyngeal Nerve —3 Vagus Nerve
+
The superior cerebellar peduncle of one side connects the cerebellar hemisphere of that side with the postcentral in the Mid-brain as it re gion of the cerebral cortex of the opposite side, the red nucleus and the flbres su P enor peduncles thalamus being cell-stations in the path are shown decussating and reaching red nucleus, from which the rubro-spinal tracts emerge, decussate, and pass below the cerebellar fibres to enter the pons laterally. On the right the pallidorubro-olivary tract is shown in white, indicating its relation to the peduncular fibres; it sinks deeply in the pons, turning somewhat laterally. It is not shown in front of the red nucleus.
  
Superficial Arcuate Fibres Accessory Ner y e
 
  
First and Second Cervical Nerv
 
  
  
Optic Chiasma
 
  
3-)ptie Nerve
+
Fig. 956. — Plan of Relations of Certain Fibre-tracts
  
  
  
.Optic Tract
 
  
 +
A few of the fibres of each superior cerebellar peduncle do not take part in the decussation beneath the lower pair of quadrigeminal bodies, but ascend to the red nucleus of their own side.
  
Third Nerve
 
  
 +
Before the fibres decussate, or after the decussation has taken place, each of them furnishes a descending branch.
  
_Fourth Nerve
+
These descending branches form the descending cerebellar bundle (of Cajal), which traverses the dorsal part of the pons and the medulla oblongata, giving off collaterals to the motor nuclei of these parts. According to Cajal, the fibres of this bundle enter the anterior column of the spinal cord, and are connected with the cells of the ventral column of grey matter.
  
-Motor Root of Fifth Nerve
+
The superior cerebellar peduncle also contains the indirect or ventral spino-cerebellar tract (of Gowers).
  
-Sensory Root of Fifth Nerve
 
  
 +
Medial or Posterior Longitudinal Bundle. — This bundle occupies he dorsal part of the tegmentum, and is intimately related to the grey matter which forms the ventral wall or floor of the aqueduct, it lies close to the median raphe, as does its fellow of the opposite ide, and across the raphe an interchange of fibres takes place between he two bundles. In the spinal cord it is represented by the anterior ntersegmental fibres and the anterior marginal bundle (of Lowenthal). n the medulla oblongata it traverses the dorsal part of the pyramid, >eing separated from the pyramidal (motor) fibres by the medial emniscus or chief sensory tract. Thereafter it traverses the formatio eticularis of the dorsal part of the pons, and is continued upwards is one of the tracts of the tegmentum of the crus cerebri.
  
Middle Peduncle of Cerebellum
+
The fibres of the medial longitudinal bundle are regarded as being the axons of cells belonging to (1) the nucleus of Deiters, which is
 +
me of the terminal nuclei of the vestibular root of the auditory nerve,
  
 +
2) the formatio reticularis of the medulla oblongata and pons,
  
Restiform Body —.Hypoglossal Nerve ■ --Anterior Median Fissure
+
3) the formatio reticularis of the tegmentum, (4) the sensory nucleus )f the fifth cranial nerve, and (5) the nucleus of the longitudinal rundle. Inferiorly the fibres of the bundle ramify within the interior column of the spinal cord in connection with the motor cells )f the ventral horn of grey matter. Superiorly its fibres are intimately •elated to the following important nuclei—namely, (1) the oculonotor nucleus, or nucleus of the third cranial nerve; (2) the trochlear lucleus, or nucleus of the fourth cranial nerve; and (3) the abducent mcleus, or nucleus of the sixth cranial nerve, these being the nuclei which control the muscles of the eyeball and upper eyelid. The Dundle furnishes numerous collaterals to each of these nuclei, which
  
.Decussation of the Pyramids
+
erminate in arborizations around their cells. The bundle also estabishes connections with the motor nuclei in the pons and medulla
 +
iblongata.
  
 +
The medial longitudinal bundle extends as high as a special nucleus, called the nucleus of the posterior longitudinal bundle, which is situated n the grey matter of the ventro-lateral portion of the third ventricle rear the upper opening of the aqueduct, from the cells of which nucleus
  
Fig. 953. — The Medulla Oblongata, Pons, and Interpeduncular Region. C.C., crus cerebri; P., pyramid; O.B., olivary body.
+
some of its fibres arise.  
  
 +
The bundle consists of ascending and descending association fibres, which form connections between the important nuclei just referred to. Probably the chief use of the bundle is to maintain a functional association between these nuclei, and insure harmonious action of the muscles which are supplied by the nerves arising from them.
  
The stratum opticum is the third layer, and consists of grey matter which contains numerous nerve-cells and nerve-fibres. The fibres are conducted to this stratum by the superior brachium, and are of two kinds: (1) Many are retinal fibres, and are derived from the outer root of the optic tract. (2) Others are corticifugal fibres, which come from the higher visual centre in the cortex of the occipital lobe, and form part of the optic radiation. The fibres pass into the stratum cinereum, and terminate in arborizations around its cells. The axons of the cells of the stratum opticum pass into the stratum lemnisci.
+
Tecto-spinal Tract (Ventral Longitudinal Bundle). This bundle ties on the ventral aspect of the medial longitudinal bundle.. Its fibres are derived from the stratum opticum and stratum lemnisci of the upper quadrigeminal body of the opposite side. These fibres, as stated in connection with the upper quadrigeminal bodies, descend beside t e °Tey matter round the aqueduct, cross the median plane, and decussate with the corresponding fibres of the opposite side, the decussation being known as the dorsal fountain decussation (of Meynert). The fibres descend through the tegmentum, lying close to the red nucleus, to which they furnish collaterals. Thereafter they traverse the formatio reticularis of the pons and medulla oblongata, still lying on the ventral aspect of the medial longitudinal bundle. From the medulla oblongata the fibres pass into the anterior ground-bundle of the lateral column of the spinal cord, where they lie just in front of the rubro-spinal tract, and they form arborizations around the motor cells of the ventral horn of grey matter.
  
The stratum lemnisci is the deepest layer. Like the stratum opticum, it consists of , grey matter, which contains numerous nerve-cells and nerve-fibres. The fibres are derived from the following sources: (1) Many are derived from the medial or main lemniscus; and (2) some are the axons of cells belonging to the stratum opticum and stratum lemnisci. The lemniscal fibres terminate in the stratum lemnisci. The fibres formed by the axons of the cells of the stratum opticum and stratum lemnisci cross the median plane, below the aqueduct, and decussate with the corresponding fibres of the opposite side. This decussation is known as the fountain decussation (of Meynert). The fibres, after crossing, form the tectospinal tract of that side, and this bundle or tract descends through the pons and medulla oblongata into the corresponding ventral or anterior column of the spinal cord.
+
The pallido-rubro-olivary tract (Figs. 955 and 956) is a well-formed and marked bundle of fibres which can be found in sections through the mid-brain and pons. The fibres lie between the red nucleus and the olive, above the medial part of the red nucleus and the cerebellar peduncular fibres in the crura, within the concavity of these fibres as they pass between their decussation and the superior peduncle, more laterally in the middle and lower parts of the pontine tegmentum, and gain the inferior olive just below the lower border of the pons; they may be visible in part on the surface here (Fig. 883). The exact path of the tract between the red nucleus and the globus pallidus is not certainly known; the tract is probably in great part interrupted at the red nucleus, from which the rubro-olivary fibres take origin, but direct pallido-olivary fibres are known to be present also.
  
The upper quadrigeminal body, by means of the superior brachium, is one of the lower visual centres, the other being the external geniculate body.
+
This tract is essentially a structure belonging to the brains of the higher vertebrates, in which the inferior olive replaces or reinforces the primitive olivary formation; this is represented in the human brain by the medial and other accessory olives.
  
Development of Corpora Quadrigemina. —The corpora quadrigemina are developed from the dorsal wall or roof of the mesencephalon. They are at first in the form of elongated paired swellings, later divided transversely.
+
Rubro-spinal Tract (or Bundle of Monakow). —The fibres of this tract are derived, as previously stated, from the axons of the cells of the red nucleus. They cross the median plane, decussating with the corresponding fibres of the opposite side, and constituting the ventral fountain decussation [of Foret). The tract then descends through the pons and medulla oblongata into the lateral column of the spinal cord, in which it constitutes the prepyramidal tract on the ventrolateral aspect of the lateral cortico-spinal tract.
  
 +
Medial Lemniscus. —The medial or main lemniscus of either side begins in the lower part of the medulla oblongata. It is here the only lemniscus on either side, and its fibres are derived from the deep lemniscal arcuate fibres, which arise from the cells of the nucleus gracilis and nucleus cuneatus of the opposite side. The main lemniscus therefore represents the upward continuation of the posterior column of the spinal cord (gracile and cuneate fasciculi), and it is spoken of as the chief sensory tract. The deep lemniscal arcuate fibres cross the median plane directly above the decussation of the pyramids [motor decussation) , thus constituting the decussation of the lemnisci (main fillets or chief sensory tracts), or the sensory decussation. The fibres, after crossing the median plane, form the tract of the side to which they have crossed. In the medulla oblongata it lies close to the median raphe, and at first is in front of the medial longitudinal bundle, and directly behind the pyramid. The main lemniscus then ascends through
  
Dorsal Tegmental Decussation (or Commissure of Meynert). —This commissure or decussation consists of fibres which issue from each upper quadrigeminal body, and cross partly to the opposite posterior longitudinal bundle, but mostly form tecto-spinal tracts, in which they descend towards the pons.
+
he dorsal part of the pons, its relative position remaining unchanged,
 +
[n this situation the lateral lemniscus, to be presently described, takes jp its position on the outer or lateral aspect of the main or medial Dand. The main tract, on leaving the pons, enters the ventral part i)f the tegmentum, still having the lateral tract on its outer side. As t encounters the red nucleus it is displaced laterally and backwards, md then occupies the dorso-lateral part of the tegmentum, lying almost beneath the medial geniculate body.
  
Crura Cerebri. —The crura or pedunculi cerebri are two large strands which are situated above the pons. They lie at first near each other, being separated by the interpeduncular fossa, but afterwards diverge as they pass upwards and laterally to the cerebral hemisphere. The medial surface of each crus bounds the interpeduncular region, and has a furrow, the oculo-motor sulcus, through which the roots of the third nerve emerge near the pons.
+
The fibres of the main or medial lemniscus (chief sensory tract) terminate in two ways: (1) Some enter the upper quadrigeminal body, md these probably terminate in the stratum lemnisci; (2) others traverse the subthalamic tegmental region, and enter the anterior part of the thalamus, within which they terminate in arborizations around the cells of the ventro-lateral nuclear area (chief sensory nucleus). From these cells relays of thalamo-corticipetal fibres proceed to the cerebral cortex.
  
The lateral surface looks towards the temporal lobe of the brain, which to a large extent overlaps the crus, and this surface also has a furrow, the sulcus lateralis. The slender fourth cranial nerve lies upon this surface. Close to the cerebral hemisphere the ventral and lateral aspects of the crus are embraced by the optic tract of the corresponding side.
+
It is convenient to refer to the main or medial band as the sensory lemniscus.
  
Each crus is composed of two parts—ventral and dorsal. The ventral part is the basis (or crusta), and the dorsal the tegmentum. The separation between these is indicated superficially by the sulcus lateralis and the oculo-motor sulcus. Within the crus the two parts are separated by a mass of dark grey matter, called the substantia
+
Lateral Lemniscus. — The main or medial band being the sensory lemniscus, it is convenient to refer to the lateral one as the auditory fillet or lemniscus. The fibres of this fillet are derived from the following sources: (1) The corpus trapezoides, the fibres of which come from the ventral cochlear nucleus, the nucleus trapezoides, and the superior olive of the opposite side, as well as from the superior olive of the same side; (2) the auditory striae, which are derived from the lateral cochlear nucleus of the opposite side; and (3) the nucleus of the lateral lemniscus.
  
 +
The fibres of the right and left lateral lemnisci decussate across the median plane. Having crossed to the opposite side, the fibres become longitudinal and form a well-marked ascending tract in the dorsal part of the pons, which takes up a position on the lateral or outer side of the main or medial tract. In this part of its course the lateral lemniscus encounters a collection of grey matter, called its nucleus. Some of its fibres end in this nucleus. Others pursue their upward couise, and are reinforced by relays of fibres which arise from the nerve-cells of the nucleus On leaving the pons the lateral band enters the tegmentum, and its fibres terminate in (1) the nuclei of the lower quadrigeminal body, and (2) the cells of the medial geniculate body. The fibres destined for the lower quadrigeminal body, having curved round the lateral aspect of- the superior cerebellar peduncle, become superficial on the outer side of the tegmentum. The fibres destined for the medial geniculate body reach it through the inferior brachium partly directly and partly through the intervention of the lower quadrigeminal body. The axons of the cells of the geniculate body form corticipetal fibres which pass to the cortex of the first or superior
  
 +
temporal gyrus of the temporal lobe. .
  
Fig. 954. — Topography of the Crus Cerebri (after Poirier).
+
The lateral lemniscus, therefore, is associated with the auditory apparatus. It is chiefly composed of ascending fibres. There are, however, some descending fibres which are probably derived from the lower quadrigeminal bodies.
  
 +
In addition to the foregoing, there are other tracts.
  
 +
Fasciculus Retroflexus. —The fibres of this bundle, already described, arise from the cells of the ganglion habenulce. They descend in the upper part of the tegmentum internal to the red nucleus, and they terminate in arborizations around the cells of the interpeduncular ganglion.
  
The basis pedunculi (crusta) is continuous superiorly with the internal capsule of the corpus striatum, and inferiorly its fibres enter the ventral part of the pons.
+
Bundle of Munzer. —The fibres of this tract descend from the lower quadrigeminal body to the formatio reticularis of the lateral part of the pons.
  
Structure of the Basis.— The crusta, or basis, as seen m transverse section, presents a crescentic outline, the concavity of the crescent being occupied by the convexity of the substantia nigra. It consists of longitudinal corticifugal fibres which arise in the cells of the cerebral cortex. These fibres form two groups—pyramidal and cortico-pontine.
+
Spino-thalamic Tract. —The fibres of this tract, as stated in connection with the tracts of the spinal cord, arise from the cells of the dorsal grey column of the opposite side. Having crossed in the ventral or white commissure, they enter the antero-lateral or indirect cerebellar tract (tract of Gowers), in which they ascend through the medulla oblongata, pons, and tegmentum of the crus cerebri to the thalamus of the side to which they have crossed.
  
The pyramidal fibres form the motor tract from the precentral motor region of the cortex of the frontal lobe, and they arise for the most part from the cells of that region, which control the voluntary muscles of the body.
 
  
The cortico-pontine fibres lie on each side of the pyramidal tract, those coming from the frontal region of the cortex being on the medial and those from the temporal region on the lateral side; the basis of each peduncle, therefore, is formed, from within outward, by frontopontine, pyramidal, and temporo-pontine tracts (see Fig. 958).
 
  
Tegmentum.— The tegmentum is continuous interiorly with the formatio reticularis of the dorsal portion of the pons, which in turn is continuous interiorly with the formatio reticularis of the medulla oblongata; the upward prolongation of the tegmentum makes the tegmental subthalamic region. The two tegmenta, right and left, are separated from each other by a median raphe, which is continuous with
+
Fig. 957.—Diagram to show Position and Relations of Structures in Tegmental Subthalamus.
  
 +
Supposed to be viewed from the medial aspect. R, red nucleus. The subthalamic nucleus is shown antero-lateral to this. Dotted line shows course of fasciculus retroflexus from habenula to interpeduncular ganglion. Course of anterior pillar of fornix is indicated, also mamillo-thalamic tract (bundle of Vicq d’Azyr) passing up medial to front part of subthalamic nucleus. Substantia nigra is seen near pontine level, but passes upwards and laterally out of the section higher up.
  
 +
Subthalamic Tegmental Region. —This region represents the upward prolongation of the tegmentum of the crus cerebri beneath the posterior part of the inferior or ventral surface of the thalamus. The prolongation contains (1) an upward extension of the red nucleus of the tegmentum, (2) the fibres of the superior peduncle of the cerebellum, and (3) the main or medial lemniscus (chief sensory tract).
  
Fig. 955 Section through Inferior Corpora Quadrigemina.
+
The upward extension of the red nucleus ceases about the level of the corresponding corpus mamillare. Some of the fibres of the superior peduncle of the cerebellum terminate, as stated, in the red nucleus, and others encapsule it, as they do in the tegmentum. Many fibres issue from the cells of the red nucleus, and these, along with the investing fibres of the superior cerebellar peduncle, enter the inferior or ventral surface of the thalamus.
  
  
that of the pons. In the lower part of the mesencephalon this raphe is indistinct on account of the decussation which takes place across the median plane between the superior cerebellar peduncles, underneath the lower pair of quadrigeminal bodies.
 
  
The dorsal surface of each tegmentum extends on either side of the grey matter of the aqueduct, and becomes continuous with the basal parts of the upper and lower quadrigeminal bodies of the corresponding side, which constitute the tectum. The ventral surface is separated from the crusta by the substantia nigra.
+
The main lemniscus (chief sensory tract), which lies on the dorsolateral aspect of the red nucleus, also enters the inferior or ventral surface of the thalamus.
  
Structure of the Tegmentum. —Each tegmentum, besides being continuous interiorly with the formatio reticularis of the dorsal portion of the pons, consists of bundles of longitudinal and transverse fibres, the intervals between which are occupied by grey matter.
+
Development of the Crura Cerebri. —The crura cerebri are developed in the ventral wall of the mesencephalon.
  
 +
Basis Pedunculi (Crusta or Pes). —The basis is the ventral portion of the crus cerebri, and is separated from the tegmentum of the crus by a mass of dark grey matter, called the substantia nigra, which is situated in the interior. Externally the separation is indicated on the outer aspect by the lateral sulcus, and on the inner aspect by the oculomotor sulcus, through which the fasciculi of the oculo-motor nerve emerge. The basis is continuous with the internal capsule of the corpus striatum, and it consists of longitudinal centrifugal fibres, which arise in the cells of the cerebral cortex. These fibres are arranged in two sets, pyramidal and cortico-pontine. The pyramidal fibres form the motor tract from the precentral region of the cerebral cortex, and the cortico-pontine fibres are arranged in two strands—namely, frontopontine and temporo-pontine.
  
  
Grey Matter. —The grey matter of the tegmentum contains the red nucleus.
 
  
The red nucleus (nucleus ruber) is a round reddish mass, which is situated in the centre of the upper part of the tegmentum, and lies in the path of the superior cerebellar peduncle of the opposite side. It is on the same level as the upper quadrigeminal body, and is prolonged upwards into the subthalamic tegmental region. Some of the fibres of the superior cerebellar peduncle of the opposite side surround the red nucleus in the form of a capsule on their way to the thalamus. Other fibres of that peduncle enter the red nucleus, and terminate in arborizations around its cells.
 
  
The axons of the cells of the red nucleus form two sets of nervefibres—ascending and descending. The ascending fibres pass to the thalamus in company with those fibres of the superior cerebellar peduncle which encapsule the red nucleus. These ascending fibres form relays which carry on those fibres of the superior cerebellar peduncle which terminate within the red nucleus, that nucleus being a cell-station in their path. The descending fibres constitute the rubro-spinal tract (or bundle of Monakow). The fibres of this tract cross the median plane in the raphe, and by their decussation with those of the opposite side they constitute the ventral fountain decussation [of Foret), in contradistinction to the dorsal fountain decussation (of Meynert). The latter decussation is on a higher level, and involves the fibres of the ventral longitudinal bundles, or tecto-spinal tracts, which derive their fibres from the cells of the stratum opticum and stratum lemnisci of the upper quadrigeminal bodies. The rubrospinal tract of either side descends through the pons and medulla oblongata into the lateral column of the spinal cord, where each constitutes the prepyramidal tract, which lies on the ventro-lateral aspect of the lateral cortico-spinal tract. (The tecto-spinal tract, or ventral longitudinal bundle, on either side descends into the anterior column of the spinal cord.)
 
  
White Matter of the Tegmentum. —The principal tracts of the white matter on either side are as follows:
+
F IG . 958.—The Crura Cerebri and their Relations.
  
1. Superior cerebellar peduncle.
 
  
2. Medial (posterior) longitudinal bundle.
 
  
3. Ventral longitudinal bundle, or tecto-spinal tract.
+
The pyramidal fibres form the motor tract from the precentral region of the cerebral cortex. T hey traverse the lenticular portion of the posterior limb of the internal capsule, and then occupy the middle three-fifths of the crusta. Thereafter they descend through the ventral portion of the pons and the pyramid of the medulla oblongata. In the lower part of the pyramid they give rise to the crossed and direct pyramidal tracts. The crossed pyramidal tract, having taken part in the decussation of the pyramids, descends in the spinal cord as the lateral corticospinal tract, occupying the posterior part of the lateral column of the opposite side The direct pyramidal tract descends (anterior cortico-spmal tract) in the spinal cord, occupying the medial part of the anterior column of the same side. Its fibres however, cross at intervals to the opposite side.
  
4. Pallido-rubro-olivary tract.
 
  
5. Rubro-spinal tract.
 
  
6. Medial lemniscus (chief sensory tract).
+
As the pyramidal tract descends through the pons and medulla oblongata, some of its fibres pass to the motor nuclei of the cranial nerves in these regions.
  
7. Lateral (acoustic) lemniscus.
+
The cortico-pontine fibres are arranged in two strands, frontopontine and temporo-pontine. The fibres of the fronto-pontine strand arise from the cells of the cortex of the anterior part of the frontal lobe, and, having traversed the anterior limb of the internal capsule, they are regarded as occupying the medial fifth of the basis pedunculi. The fibres of the temporo-pontine strand arise from the cells of the cortex of the temporal lobe, and having traversed the postlenticular part of the internal capsule, they occupy the lateral fifth of the crusta. In the ventral part of the pons both the fronto-pontine and the temporo-pontine fibres terminate in arborizations around the cells of the nucleus pontis, whereas the pyramidal fibres pass uninterruptedly through the ventral part of the pons.
  
Superior Cerebellar Peduncle.— The fibres of this peduncle emerge for the most part through the hilum of the nucleus dentatus in the cerebellar hemisphere. The two peduncles, right and left, having emerged from the hemispheres, pass upwards on the lateral parts of the dorsal surface of the pons in a converging manner towards the lower pair of the quadrigeminal bodies, being connected by the superior medullary velum. On entering the mesencephalon, the two peduncles decussate across the raphe beneath the lower quadrigeminal bodies. This decussation extends as high as the upper quadrigeminal bodies, and it involves almost all the fibres of the two peduncles. Each peduncle, having gained the opposite side, ascends in the upper part of the tegmentum as a longitudinal tract,
+
Substantia Nigra. — t his is a mass of dark grey matter which is situated between the tegmentum and the basis of the crus cerebri. Like the basis, it is semilunar or crescentic, as seen in transverse section. It contains many multipolar nerve-cells, which are deeply pigmented, and it extends from the upper border of the pons into the subthalamic tegmental region. Laterally it reaches the lateral sulcus on the lateral aspect of the crus, where it is thin, and the oculo-motor sulcus on the medial aspect, where it is thick, and is traversed by the fasciculi of the third cranial or oculo-motor nerve. Its tegmental surface is concave, and the surface directed towards the basis is convex. From the latter surface prolongations extend into the basis.
  
and soon comes into contact with the red nucleus. Many of its fibres enter this nucleus and terminate in arborizations around its cells. Other fibres of the peduncle encapsule the nucleus, and then ascend through the subthalamic tegmental region to the anterior part of the thalamus, within which they terminate in arborizations around the cells of the anterior nuclear area (chief sensory nucleus). From the cells of the red nucleus relays of fibres proceed upwards, which carry on those fibres of the peduncle which terminate within the nucleus, and these relays ascend with those fibres of the peduncle which encapsule the red nucleus to the thalamus.
+
1 he substantia nigra does not acquire its pigment before the second or third year after birth.
  
The superior cerebellar peduncle of one side connects the cerebellar hemisphere of that side with the postcentral in the Mid-brain as it re gion of the cerebral cortex of the
+
Aqueduct of Mid-brain. —The aqueduct is the narrow passage which leads through the mesencephalon from the third to the fourth ventricle (iter a tertio ad quartum ventriculum ). It lies nearer the dorsal than the ventral aspect of the mesencephalon; its direction is from above downwards, and its length is rather more than J inch. Its upper opening is situated on the posterior boundary of the third ventricle immediately underneath the posterior commissure, and its lower opening occupies the superior median angle of the floor of the fourth ventricle. In transverse section the aqueduct is T-shaped in its upper part near the third ventricle, and triangular in its lower part near the fourth ventricle. The passage is lined with ciliated columnar epithelium, external to which there is a thick layer of grey matter, which is spoken of as the central (Sylvian) grey matter. This is continuous superiorly with the grey matter of the floor and lateral walls of the third ventricle, and inferiorly with that which covers the floor of the fourth ventricle. It contains numerous nerve-cells disposed in a scattered manner, but, in addition to these, there are certain definite cell-groups.. These groups constitute the nuclei of origin of the following cranial nerves: the third or oculo-motor, the fourth or trochlear, and the mesencephalic root of the fifth nerve. The oculo-motor nucleus is situated in the ventral portion of the grey matter underneath the upper quadrigeminal body, and it extends upwards into the layer of grey matter on the adjacent portion of the lateral wall of the third ventricle. The trochlear nucleus is also situated in the ventral portion of the grey matter, but at a lower level than the oculo-motor nucleus, being placed underneath the upper part of the lower quadrigeminal body. The nucleus of the mesencephalic root of the fifth nerve is extensive, and is situated in the lateral portion of the grey matter.
  
. opposite side, the red nucleus and the
+
Development. — The aqueduct is the persistent remains of the cavity of the {{mesencephalon}}.
  
lbres su P enor peduncles thalamus being cell-stations in the path are shown decussating and f gu & r
 
  
rprl thipIahc '-'■l tilt; IlDlCS.
+
Posterior Perforated Substance. — This area has been previously described in a general way in connection with the base of the encephalon. It will here be considered more fulfy. It lies at the bottom of a deep depression, called the interpeduncular fossa, which forms the back part of the interpeduncular space. The fossa is bounded posteriorly by the median portion of the upper border of the pons, and laterally by the crura cerebri. Anteriorly it is limited by the corpora mamillaria.
  
 +
The locus perforatus is a perforated lamina of grey matter which forms the floor of the interpeduncular fossa, the openings being for the passage of the postero-medial ganglionic branches of the posterior cerebral arteries. This grey lamina extends between the tegmenta of the crura cerebri.
  
 +
Ganglion Interpedunculare. —This is a collection of nerve-cells situated medially in the lower part of the grey lamina which constitutes the posterior perforated substance. On either side it receives the fibres of the fasciculus retroflexus , which are derived from the ganglion habenulae.
  
Fig. 956. — Plan of Relations of Certain Fibre-tracts
 
  
 +
==Structure of the Cerebral Hemispheres==
  
reaching red nucleus, from which the rubro-spinal tracts emerge, decussate, and pass below the cerebellar fibres to enter the pons laterally. On the right the pallidorubro-olivary tract is shown in white, indicating its relation to the peduncular fibres; it sinks deeply in the pons, turning somewhat laterally. It is not shown in front of the red nucleus.
 
  
 +
The cerebral hemisphere is composed of grey and white matter. The grey matter is disposed externally, and forms the cerebral cortex, which, with the exception of the rhinencephalon, is known as the neopallium. The white matter occupies the interior, and constitutes the medullary centre.
  
A few of the fibres of each superior cerebellar peduncle do not take part in the decussation beneath the lower pair of quadrigeminal bodies, but ascend to the red nucleus of their own side.
+
Cerebral Cortex. — The grey matter forms a continuous covering to the entire hemisphere, dipping into the sulci, so as to cover the opposed surfaces of the gyri, as well as the bottom of the sulci, is thicker over the superficial surfaces of the gyri than at the bottom of the sulci, and attains its greatest thickness over the upper portions of the precentral and postcentral gyri, whilst it is thinnest over the
  
  
Before the fibres decussate, or after the decussation has taken place, each of them furnishes a descending branch.
+
The cerebral cortex is indistinctly divided into strata by means of layers of a whitish substance. When examined m section it therefore presents a stratified appearance, and is seen to consist of successive grey and white layers alternating with each other. In most parts of the cerebral cortex there are four superimposed strata; but in certain situations— e.g., over the precentral gyrus—there are as many as six. These strata are as follows, from without inwards:
  
These descending branches form the descending cerebellar bundle (of Cajal), which traverses the dorsal part of the pons and the medulla oblongata, giving off collaterals to the motor nuclei of these parts. According to Cajal, the fibres of this bundle enter the anterior column of the spinal cord, and are connected with the cells of the ventral column of grey matter.
+
# Molecular layer, a superficial white layer (pale and narrow).
 +
# Superficial grey layer, the outer granular.
 +
# Outer white band of Baillarger.
 +
# Middle grey layer.
 +
# Inner white band of Baillarger.
 +
# Inner or deep grey layer, the polymorphous layer, subjacent to which is the white matter of the medullary centre.
  
The superior cerebellar peduncle also contains the indirect or ventral spino-cerebellar tract (of Gowers).
 
  
 +
Layers 3, 4, and 5 are included in the pyramidal layer, in which the cells tend to increase in size as they lie more deeply; the largest lie over the inner band of Baillarger.
  
Medial or Posterior Longitudinal Bundle. —This bundle occupies he dorsal part of the tegmentum, and is intimately related to the
 
  
rey matter which forms the ventral wall or floor of the aqueduct,
+
The medullated fibres of the medullary centre pass into the stratified grey cortex in a radiating manner, and within the cortex their course for the most part is perpendicular to the superficial surface, and between the component cells of the cortex.
t lies close to the median raphe, as does its fellow of the opposite ide, and across the raphe an interchange of fibres takes place between he two bundles. In the spinal cord it is represented by the anterior ntersegmental fibres and the anterior marginal bundle (of Lowenthal). n the medulla oblongata it traverses the dorsal part of the pyramid, >eing separated from the pyramidal (motor) fibres by the medial emniscus or chief sensory tract. Thereafter it traverses the formatio eticularis of the dorsal part of the pons, and is continued upwards is one of the tracts of the tegmentum of the crus cerebri.
 
  
The fibres of the medial longitudinal bundle are regarded as being
+
Minute Structure of the Cerebral Cortex. —The cortex is composed of nerve-cells and nerve-fibres.
  
he axons of cells belonging to (1) the nucleus of Deiters, which is
+
Nerve-cells. — These are arranged in four layers, which are, from without inwards: (1) the molecular layer; (2) the layer of small pyramidal cells; (3) the layer of large pyramidal cells; and (4) the layer of polymorphous cells.
me of the terminal nuclei of the vestibular root of the auditory nerve,
 
  
2) the formatio reticularis of the medulla oblongata and pons,
 
  
3) the formatio reticularis of the tegmentum, (4) the sensory nucleus )f the fifth cranial nerve, and (5) the nucleus of the longitudinal rundle. Inferiorly the fibres of the bundle ramify within the interior column of the spinal cord in connection with the motor cells )f the ventral horn of grey matter. Superiorly its fibres are intimately •elated to the following important nuclei—namely, (1) the oculonotor nucleus, or nucleus of the third cranial nerve; (2) the trochlear lucleus, or nucleus of the fourth cranial nerve; and (3) the abducent mcleus, or nucleus of the sixth cranial nerve, these being the nuclei which control the muscles of the eyeball and upper eyelid. The Dundle furnishes numerous collaterals to each of these nuclei, which
+
The molecular or plexiform layer, which is the most superficial, is thin, and consists of cells and fibres. Many of the cells are neurogliacells, the others being nerve-cells. These nerve-cells are for the most part fusiform, and are disposed horizontally. They are known as the horizontal cells of Cajal. Their dendrons and axons are long, the latter forming medullated fibres which are disposed horizontally or parallel to the surface. These furnish minute branches which pass vertically towards the surface. The horizontal cells, according to Cajal, receive impulses from the corticipetal fibres which extend from the thalamus to the cerebral cortex.
  
erminate in arborizations around their cells. The bundle also estabishes connections with the motor nuclei in the pons and medulla
+
In addition to these fibres there are many others which enter the molecular layer from deeper sources, and form a dense interlacement by their ramifications. The sources from which these extraneous fibres are derived are: (1) the terminal ramifications of the apical dendrons of the pyramidal cells (small and large); (2) the axons of the cells of Martinotti, which lie in the polymorphous layer; and (3) corticipetal fibres derived from the medullary centre of the gyrus.
iblongata.
 
  
The medial longitudinal bundle extends as high as a special nucleus, called the nucleus of the posterior longitudinal bundle, which is situated n the grey matter of the ventro-lateral portion of the third ventricle rear the upper opening of the aqueduct, from the cells of which nucleus
+
The pyramidal layers represent the second and third layers, and are composed of characteristic pyramidal cells which are peculiar to the cerebral cortex, those of the second layer being small, whilst those of the third layer are large. The layer of small pyramidal cells is narrow, but the layer of large pyramidal cells is of considerable thickness. The giant pyramidal cells of the motor cortex are known as cells of Betz. There is no well-marked line of demarcation between these two layers, the one passing imperceptibly into the other. They constitute the chief part of the cerebral cortex.
  
some of its fibres arise. . .
 
  
The bundle consists of ascending and descending association fibres, which form connections between the important nuclei just referred to. Probably the chief use of the bundle is to maintain a functional association between these nuclei, and insure harmonious action of the muscles which are supplied by the nerves arising from them.
 
  
Tecto-spinal Tract (Ventral Longitudinal Bundle). This bundle ties on the ventral aspect of the medial longitudinal bundle.. Its fibres are derived from the stratum opticum and stratum lemnisci of the upper quadrigeminal body of the opposite side. These fibres, as stated in connection with the upper quadrigeminal bodies, descend beside t e °Tey matter round the aqueduct, cross the median plane, and decussate with the corresponding fibres of the opposite side, the decussation being known as the dorsal fountain decussation (of Meynert). The fibres descend through the tegmentum, lying close to the red nucleus, to which they furnish collaterals. Thereafter they traverse the formatio reticularis of the pons and medulla oblongata, still lying on the ventral aspect of the medial longitudinal bundle. From the medulla oblongata the fibres pass into the anterior ground-bundle of the lateral column of the spinal cord, where they lie just in front of the rubro-spinal tract, and they form arborizations around the motor cells of the ventral horn of grey matter.
+
Fig , W -Diagram showing the Minute Structure of the Cerebral J Cortex (Poirier).
  
The pallido-rubro-olivary tract (Figs. 955 and 956) is a well-formed and marked bundle of fibres which can be found in sections through the mid-brain and pons. The fibres lie between the red nucleus and the olive, above the medial part of the red nucleus and the cerebellar peduncular fibres in the crura, within the concavity of these fibres as they pass between their decussation and the superior peduncle, more laterally in the middle and lower parts of the pontine tegmentum, and gain the inferior olive just below the lower border of the pons; they may be visible in part on the surface here (Fig. 883). The exact path of the tract between the red nucleus and the globus pallidus is not certainly known; the tract is probably in great part interrupted at the red nucleus, from which the rubro-olivary fibres take origin, but direct pallido-olivary fibres are known to be present also.
+
The fibres are shown on the right, and the cells on the left.
  
This tract is essentially a structure belonging to the brains of the higher vertebrates, in which the inferior olive replaces or reinforces the primitive olivary formation; this is represented in the human brain by the medial and other accessory olives.
 
  
Rubro-spinal Tract (or Bundle of Monakow). —The fibres of this tract are derived, as previously stated, from the axons of the cells of the red nucleus. They cross the median plane, decussating with the corresponding fibres of the opposite side, and constituting the ventral fountain decussation [of Foret). The tract then descends through the pons and medulla oblongata into the lateral column of the spinal cord, in which it constitutes the prepyramidal tract on the ventrolateral aspect of the lateral cortico-spinal tract.
+
The apex of each pyramidal cell is directed towards the surface of the evrus and is prolonged into a long tapering dendrite which (Fig. 960) passes into the molecular layer, giving off delicate collaterals in its course. Near the surface of the molecular layer it divides into terminal filaments, which are disposed horizontally and mingle with the tangential fibres. The base of the pyramidal cell is directed towards the medullary centre of the gyrus, and from its centre an axon is given off, which enters the medullary centre, giving off collaterals in its course. From each side of the body of the cell, as well as from each lateral angle of its base, dendrites are given off.
  
Medial Lemniscus. —The medial or main lemniscus of either side begins in the lower part of the medulla oblongata. It is here the only lemniscus on either side, and its fibres are derived from the deep lemniscal arcuate fibres, which arise from the cells of the nucleus gracilis and nucleus cuneatus of the opposite side. The main lemniscus therefore represents the upward continuation of the posterior column of the spinal cord (gracile and cuneate fasciculi), and it is spoken of as the chief sensory tract. The deep lemniscal arcuate fibres cross the median plane directly above the decussation of the pyramids [motor decussation) , thus constituting the decussation of the lemnisci (main fillets or chief sensory tracts), or the sensory decussation. The fibres, after crossing the median plane, form the tract of the side to which they have crossed. In the medulla oblongata it lies close to the median raphe, and at first is in front of the medial longitudinal bundle, and directly behind the pyramid. The main lemniscus then ascends through
 
  
he dorsal part of the pons, its relative position remaining unchanged,
 
[n this situation the lateral lemniscus, to be presently described, takes jp its position on the outer or lateral aspect of the main or medial Dand. The main tract, on leaving the pons, enters the ventral part i)f the tegmentum, still having the lateral tract on its outer side. As t encounters the red nucleus it is displaced laterally and backwards, md then occupies the dorso-lateral part of the tegmentum, lying almost beneath the medial geniculate body.
 
  
The fibres of the main or medial lemniscus (chief sensory tract) terminate in two ways: (1) Some enter the upper quadrigeminal body, md these probably terminate in the stratum lemnisci; (2) others traverse the subthalamic tegmental region, and enter the anterior part of the thalamus, within which they terminate in arborizations around the cells of the ventro-lateral nuclear area (chief sensory nucleus). From these cells relays of thalamo-corticipetal fibres proceed to the cerebral cortex.
 
  
It is convenient to refer to the main or medial band as the sensory lemniscus.
 
  
Lateral Lemniscus. —The main or medial band being the sensory lemniscus, it is convenient to refer to the lateral one as the auditory fillet or lemniscus. The fibres of this fillet are derived from the following sources: (1) The corpus trapezoides, the fibres of which come from the ventral cochlear nucleus, the nucleus trapezoides, and the superior olive of the opposite side, as well as from the superior olive of the same side; (2) the auditory striae, which are derived from the lateral cochlear nucleus of the opposite side; and (3) the nucleus of the lateral lemniscus.
 
  
The fibres of the right and left lateral lemnisci decussate across the median plane. Having crossed to the opposite side, the fibres become longitudinal and form a well-marked ascending tract in the dorsal part of the pons, which takes up a position on the lateral or outer side of the main or medial tract. In this part of its course the lateral lemniscus encounters a collection of grey matter, called its nucleus. Some of its fibres end in this nucleus. Others pursue their upward couise, and are reinforced by relays of fibres which arise from the nerve-cells of the nucleus On leaving the pons the lateral band enters the tegmentum, and its fibres terminate in (1) the nuclei of the lower quadrigeminal body, and (2) the cells of the medial geniculate body. The fibres destined for the lower quadrigeminal body, having curved round the lateral aspect of- the superior cerebellar peduncle, become superficial on the outer side of the tegmentum. The fibres destined for the medial geniculate body reach it through the inferior brachium partly directly and partly through the intervention of the lower quadrigeminal body. The axons of the cells of the geniculate body form corticipetal fibres which pass to the cortex of the first or superior
+
Terminal Ramifications
  
temporal gyrus of the temporal lobe. .
 
  
The lateral lemniscus, therefore, is associated with the auditory apparatus. It is chiefly composed of ascending fibres. There are, however, some descending fibres which are probably derived from the lower quadrigeminal bodies.
 
  
In addition to the foregoing, there are other tracts.
+
The polymorphous layer is the deepest layer, and is composed of cells which have different shapes. Each cell gives off several dendrites, which pass towards, but do not enter, the molecular layer. The axon of each cell enters the medullary centre as a nerve-fibre.
  
Fasciculus Retroflexus. —The fibres of this bundle, already described, arise from the cells of the ganglion habenulce. They descend in the upper part of the tegmentum internal to the red nucleus, and they terminate in arborizations around the cells of the interpeduncular ganglion.
+
In addition to the foregoing cells of the cerebral cortex, two other kinds of cells are met with amongst the pyramidal and polymorphous cells—namely, the cells of Golgi and the cells of Martinotti. The cells of Golgi are characterized by the fact that the axon of each almost immediately divides into several branches, which pass towards the surface, but soon terminate without entering the molecular layer. The cells of Martinotti are chiefly met with in the polymorphous layer. The axon of each cell passes towards the surface, and enters the molecular layer, where it divides into terminal branches, which form part of the tangential fibres of this layer.
  
Bundle of Munzer. —The fibres of this tract descend from the lower quadrigeminal body to the formatio reticularis of the lateral part of the pons.
 
  
Spino-thalamic Tract. —The fibres of this tract, as stated in connection with the tracts of the spinal cord, arise from the cells of the dorsal grey column of the opposite side. Having crossed in the ventral or white commissure, they enter the antero-lateral or indirect cerebellar tract (tract of Gowers), in which they ascend through the medulla oblongata, pons, and tegmentum of the crus cerebri to the thalamus of the side to which they have crossed.
 
  
  
 +
Human Cerebral Cortex (Ramon y Cajal).
  
Fig. 957.—Diagram to show Position and Relations of Structures in
 
  
Tegmental Subthalamus.
+
Nerve-fibres of the Cortex. — These are arranged in two groups — vertical and tangential.
  
Supposed to be viewed from the medial aspect. R, red nucleus. The subthalamic nucleus is shown antero-lateral to this. Dotted line shows course of fasciculus retroflexus from habenula to interpeduncular ganglion. Course of anterior pillar of fornix is indicated, also mamillo-thalamic tract (bundle of Vicq d’Azyr) passing up medial to front part of subthalamic nucleus. Substantia nigra is seen near pontine level, but passes upwards and laterally out of the section higher up.
+
The vertical (radial) fibres are disposed in radiating bundles, which issue from the medullary centre, and traverse the polymorphous and large pyramidaf layers, after which they become indistinguishable. The polymorphous and large pyramidal cells lie m the spaces between these bundles, and assume a columnar arrangement. The fibres of the radiating bundles gradually become less numerous, some of them becoming the axons of the polymorphous cells, but most of them becoming the axons of the large pyramidal cells. The radiating bundles contain centripetal cortical fibres, which pass into the molecular layer and end in terminal ramifications, forming part of its tangential fibres.
  
Subthalamic Tegmental Region. —This region represents the upward prolongation of the tegmentum of the crus cerebri beneath the posterior part of the inferior or ventral surface of the thalamus. The prolongation contains (1) an upward extension of the red nucleus of the tegmentum, (2) the fibres of the superior peduncle of the cerebellum, and (3) the main or medial lemniscus (chief sensory tract).
 
  
The upward extension of the red nucleus ceases about the level of the corresponding corpus mamillare. Some of the fibres of the superior peduncle of the cerebellum terminate, as stated, in the red nucleus, and others encapsule it, as they do in the tegmentum. Many fibres issue from the cells of the red nucleus, and these, along with the investing fibres of the superior cerebellar peduncle, enter the inferior or ventral surface of the thalamus.
+
The tangential fibres are disposed horizontally at different levels, and form the following strata: (1) the superficial tangential fibres (plexus of Exner), which occupy the superficial part of the molecular layer; (2) the band of Bechterew, which is situated in the superficial part of the small pyramidal layer; (3) the outer band of Baillarger (band of Vicq d’Azyr), which intersects the large pyramidal layer; (4) the inner band of Baillarger, which is situated between the large pyramidal and polymorphous layers; and (3) the deep tangential fibres (intracortical association fibres), which are situated in the deep part of the polymorphous layer.
  
 +
The tangential fibres are formed by (1) the collaterals of the polymorphous and pyramidal cells and of the cells of Martinotti; (2) the ramifications of the axons of the cells of Golgi; and (3) association fibres.
  
 +
Medullary Centre of Cerebral Hemisphere. —The white matter of the medullary centre consists of medullated nerve-fibres, which pursue different courses, and are arranged in three groups—namely, projection fibres, commissural fibres, and association fibres.
  
THE NERVOUS SYSTEM
+
The projection fibres connect the cerebral cortex with parts at a lower level, and they are of two kinds—corticipetal or afferent, and corticifugal or efferent. The commissural fibres pass from one hemisphere to the other, and connect portions of the cerebral cortex of opposite hemispheres. The association fibres are confined to one side of the median plane, and they bring different parts of the cerebral cortex of the same hemisphere into association with one another.
  
 +
Projection Fibres. —These fibres, as stated, are both corticipetal and corticifugal, and the majority of them constitute the internalcapsule of the corpus striatum, and the diverging arrangement of its fibres known as the corona radiata, which passes to all parts of the cerebral cortex. Some projection fibres, however, do not traverse the internal capsule and corona radiata—£.g., the fibres of the ansa peduncularis.
  
1559
+
Corticifugal Fibres.— The corticifugal or efferent fibres constitute the following tracts:
  
 +
1. Pyramidal or motor. 3 - Fronto-pontine.
  
The main lemniscus (chief sensory tract), which lies on the dorsolateral aspect of the red nucleus, also enters the inferior or ventral surface of the thalamus.
+
2. Cortico-thalamic. 4- Temporo-pontine.
  
Development of the Crura Cerebri. —The crura cerebri are developed in the ventral wall of the mesencephalon.
+
5. Optic radiation (portion).
  
Basis Pedunculi (Crusta or Pes). —The basis is the ventral portion of the crus cerebri, and is separated from the tegmentum of the crus by a mass of dark grey matter, called the substantia nigra, which is situated in the interior. Externally the separation is indicated on the outer aspect by the lateral sulcus, and on the inner aspect by the oculomotor sulcus, through which the fasciculi of the oculo-motor nerve emerge. The basis is continuous with the internal capsule of the corpus striatum, and it consists of longitudinal centrifugal fibres, which arise in the cells of the cerebral cortex. These fibres are arranged in two sets, pyramidal and cortico-pontine. The pyramidal fibres form
+
The pyramidal or motor tract derives its fibres from the axons of the pyramidal cells of the cortex of the precentral gyrus, which is situated in front of the central fissure. Having traversed the corona radiata, these fibres pass in succession through (1) the posterior limb of the internal capsule, (2) the middle three-fifths of the basis of the crus cerebri, (3) the ventral portion of the pons, and (4) the pyramid of the medulla oblongata. The motor strand enters the spinal cord in three ways—partly as the direct or ventral cortico-spinal tract, partly as the uncrossed lateral tract, but chiefly as the crossed lateral corticospinal tract. Ultimately the fibres terminate at different levels in arborizations around the motor-cells of the ventral column of grey matter of the opposite side, from which cells the fibres of the motor nerve-roots proceed.
  
  
supr cerebellar peduncle — red nucleus^ fillet-_ posi c long 1 , bundlenucleus of 3 rd nerve
+
The efferent fibres which pass to the motor nuclei of the cranial nerves do not, as a whole, run in the cortico-spinal pathway through the basis pedunculi and basis pontis. They leave this path, usually in the upper part of the mid-brain, and pass down (Fig. 961) in the tegmentum of the mid-brain and pons. They reach the tegmentum also at lower levels, passing usually either medial or lateral to the substantia nigra, but they are not constant in this matter. These cortico-pontine or cortico-bulbar fibres are thus aberrant or extra-pyramidal fibres. The figure, which is modified from Dejerine, shows the nuclei supplied by this group, the remnant of which rejoins the main tract in the medullary pyramid. Each central supply to the nuclei decussates, crossing the middle line nearly at the level of the nucleus to which it is going.
  
supr corpus_
 
  
quadrigeminum
+
The cortico-thalamic tract extends only between the cerebral cortex and the thalamus. Its fibres arise as the axons of the pyramidal cells of various parts of the cerebral cortex, and they terminate in arborizations around the cells of the thalamus.
  
  
 +
The fronto-pontine tract does not extend lower than the pons. It consists of fibres which arise as the axons of the pyramidal cells of the cortex of the prefrontal region—that is to say, the region of the frontal lobe in front of the precentral sulcus. These fibres traverse the anterior limb of the internal capsule, and then descend through the inner or medial fifth of the basis of the crus cerebri into the pons. Within the pons they terminate in arborizations around the cells of the nucleus pontis.
  
corpus mammilare 3 r - d cranial nerve -optic tract -fronto pontine fibres
+
The temporo-pontine tract, like the preceding, does not extend lower than the pons. It consists of fibres which arise as the axons of the pyramidal cells of the cortex of the first and second temporal gyri. These fibres traverse the postlenticular part of the posterior limb of the internal capsule, and then descend through the outer fifth of the basis of the crus cerebri into the ventral part of the pons. Within this part of the pons they terminate in arborizations around the cells of the nucleus pontis.
  
--pyramidal tract _ temporo pontine fibres _ -substantia nigra
+
The corticifugal fibres of the optic radiation consist of fibres which arise as the axons of the pyramidal fibres of the cortex of the occipital lobe. Ihey traverse the post lenticular part of the posterior limb of the internal capsule, and thereafter pass to the lower visual centres— namely, the lateral geniculate body and the upper quadrigeminal body. Within these bodies they terminate in arborizations around their component cells.
  
-exT geniculate body
 
  
-inti- do. do. --fimbria -—dentate fissure -optic radiations
 
  
aqueduct
 
  
 +
Fig. 961. — Plan of Extrapyramidal Fibres running in the Tegmentum to Nuclei of Cranial Nerves (modified from Dejerine).
  
F IG . 958.—The Crura Cerebri and their Relations.
 
  
  
the motor tract from the precentral region of the cerebral cortex, and the cortico-pontine fibres are arranged in two strands—namely, frontopontine and temporo-pontine.
 
  
The pyramidal fibres form the motor tract from the precentral region of the cerebral cortex. T hey traverse the lenticular portion of the posterior limb of the internal capsule, and then occupy the middle three-fifths of the crusta. Thereafter they descend through the ventral portion of the pons and the pyramid of the medulla oblongata. In the lower part of the pyramid they give rise to the crossed and direct pyramidal tracts. The crossed pyramidal tract, having taken part in the decussation of the pyramids, descends in the spinal cord as the lateral corticospinal tract, occupying the posterior part of the lateral column of the opposite side The direct pyramidal tract descends (anterior cortico-spmal tract) in the spinal cord, occupying the medial part of the anterior column of the same side. Its fibres however, cross at intervals to the opposite side.
 
  
 +
Corticipetal Fibres. — The corticipetal or afferent fibres belong to the following tracts:
  
 +
1. Medial lemniscus. 3. Thalamic radiation.
  
 +
2. Superior cerebellar peduncle. 4. Auditory radiation.
  
 +
5. Optic radiation.
  
 +
The medial lemniscus, or principal sensory tract , arises from the nucleus gracilis and nucleus cuneatus of the medulla oblongata, and is the upward prolongation of the posterior column of the spinal cord. Having decussated with its fellow, it ascends through the dorsal part of the pons through the tegmentum of the crus cerebri, and through the subthalamic tegmental region to the thalamus. Within this body its fibres terminate in arborizations around the thalamic cells. As the medial lemniscus ascends towards the thalamus some of its fibres enter the upper quadrigeminal body, in which they end. From the thalamus the fillet-fibres are continued to the cerebral cortex by relays of thalamo-cortical fibres.
  
 +
The superior cerebellar peduncle, having decussated with its fellow, soon comes into contact with the red nucleus. Many of the fibres of the peduncle enter this nucleus and terminate in arborizations around its cells. Numerous fibres encapsule the nucleus, and continue their course upwards, traversing the subthalamic tegmental region, and finally entering the ventral aspect of the thalamus, within which they terminate in arborizations around the thalamic cells. As in the case of the fillet-fibres, they are continued to the cerebral cortex by relays of thalamo-cortical fibres.
  
1560
+
The thalamic radiation is composed of thalamo-cortical fibres which arise as the axons of the cells within the thalamus, that body being regarded as an aggregation of cell-stations in the path of such corticipetal fibres as those of the medial lemniscus and superior cerebellar peduncle. These thalamo-cortical fibres, as stated in the description of the thalamus, issue from that body in four groups or stalks—frontal, parietal, occipital, and inferior or ventral. The fibres of the frontal stalk traverse the anterior limb of the internal capsule, and most of them pass to the cortex of the frontal lobe. The fibres of the parietal stalk pass partly through the internal capsule and partly through the external capsule to the cortex of the parietal lobe and of the central region of the frontal lobe. The fibres of the occipital stalk belong to the optic radiation, to be presently described. The fibres of the inferior or ventral stalk form the ansa lenticularis and ansa peduncularis. The ansa lenticularis enters the nucleus. lentiformis, within which its fibres terminate. the ansa peduncularis passes beneath the nucleus lentiformis and traverses the external capsule, the destination of its fibres being the cortex of the temporal lobe and insula.
  
  
A MANUAL OF ANATOMY
+
The auditory radiation consists of fibres which arise as the axons of the cells of the medial geniculate body. Having issued from that body, they traverse the postlenticular part of the posterior limb of the internal capsule, and pass to the cortex of the middle part of the first temporal gyrus of the temporal lobe.
  
  
As the pyramidal tract descends through the pons and medulla oblongata, some of its fibres pass to the motor nuclei of the cranial nerves in these regions.
+
The corticipetal fibres of the optic radiation are associated with the corticifugal fibres, already described. The corticipetal fibres arise as the axons of the cells of the corpus geniculatum laterale and upper quadrigeminal body. They traverse the postlenticular part of the posterior limb of the internal capsule, and then pass to the cortex of the occipital lobe.
  
The cortico-pontine fibres are arranged in two strands, frontopontine and temporo-pontine. The fibres of the fronto-pontine strand arise from the cells of the cortex of the anterior part of the frontal lobe, and, having traversed the anterior limb of the internal capsule, they are regarded as occupying the medial fifth of the basis pedunculi. The fibres of the temporo-pontine strand arise from the cells of the cortex of the temporal lobe, and having traversed the postlenticular part of the internal capsule, they occupy the lateral fifth of the crusta. In the ventral part of the pons both the fronto-pontine and the temporo-pontine fibres terminate in arborizations around the cells of the nucleus pontis, whereas the pyramidal fibres pass uninterruptedly through the ventral part of the pons.
 
  
Substantia Nigra. — t his is a mass of dark grey matter which is situated between the tegmentum and the basis of the crus cerebri. Like the basis, it is semilunar or crescentic, as seen in transverse section. It contains many multipolar nerve-cells, which are deeply pigmented, and it extends from the upper border of the pons into the subthalamic tegmental region. Laterally it reaches the lateral sulcus on the lateral aspect of the crus, where it is thin, and the oculo-motor sulcus on the medial aspect, where it is thick, and is traversed by the fasciculi of the third cranial or oculo-motor nerve. Its tegmental surface is concave, and the surface directed towards the basis is convex. From the latter surface prolongations extend into the basis.
+
Commissural Fibres. — These fibres are disposed transversely, and serve to connect the grey cortex of one hemisphere with that of the other. They constitute the following commissures: (1) the corpus callosum ; (2) the anterior commissure ; and (3) the lyra, which is known as the hippocampal commissure. The fibres of the corpus callosum, as they enter each hemisphere, are disposed so as to form an extensive callosal radiation, and serve to connect the cortex of one hemisphere with that of the other. The individual portions of cortex so connected may be symmetrical, but to a large extent are not. The fibres arise on one side as (1) the axons of pyramidal or of polymorphous cells, or (2) collaterals of projection or of association fibres; and on the opposite side they terminate in delicate arborizations.
  
1 he substantia nigra does not acquire its pigment before the second or third year after birth.
 
  
Aqueduct of Mid-brain. —The aqueduct is the narrow passage which leads through the mesencephalon from the third to the fourth ventricle (iter a tertio ad quartum ventriculum ). It lies nearer the dorsal than the ventral aspect of the mesencephalon; its direction is from above downwards, and its length is rather more than J inch. Its upper opening is situated on the posterior boundary of the third ventricle immediately underneath the posterior commissure, and its lower opening occupies the superior median angle of the floor of the fourth ventricle. In transverse section the aqueduct is T-shaped in its upper part near the third ventricle, and triangular in its lower part near the fourth ventricle. The passage is lined with ciliated columnar epithelium, external to which there is a thick layer of grey matter, which is spoken of as the central (Sylvian) grey matter. This is continuous superiorly with the grey matter of the floor and lateral walls of the third ventricle, and inferiorly with that which covers the floor of the fourth ventricle. It contains numerous nerve-cells disposed in a scattered manner, but, in addition to these, there are certain definite cell-groups.. These groups constitute the nuclei of origin of the following cranial nerves: the third or oculo-motor, the
+
The anterior commissure, which crosses from side to side in front of the anterior pillars of the fornix, divides on either side into two parts, olfactory and temporal. The olfactory portion enters the olfactory tract. Some of its fibres serve to connect the olfactory bulb of one side with that of the other side; and other fibres connect the olfactory bulb of one side with the temporal lobe of the opposite. The temporal portion enters the white matter of the temporal lobe on either side.
  
 +
The hippocampal commissure lies below the splenial portion of the corpus callosum, and is separated from the roof of the third ventricle by the tela chorioidea. It is a thin layer of arched fibres connecting the posterior pillars and sides of the fornix, and derived mainly from the hippocampus of each side; it is shown in Fig. 933. The commissure, which is not well developed in the human brain, was known as the ‘ lyra ’ in former days.
  
THE NERVOUS SYSTEM
+
The corpus callosum is the great commissure of the neopallium; the hippocampal and anterior commissures, phylogenetically much older, are connections of the rhinencephalon, and hence archipallial.
  
 +
Association Fibres. —These fibres serve to connect different parts of the cortex of the same hemisphere, and they are of two kinds, short and long.
  
1561
+
The short association fibres pass between neighbouring gyri, extending in their course across the bottom of the sulci. Some of them lie beneath the grey cortex, whilst others are contained within its deep part.
  
  
fourth or trochlear, and the mesencephalic root of the fifth nerve. The oculo-motor nucleus is situated in the ventral portion of the grey matter underneath the upper quadrigeminal body, and it extends upwards into the layer of grey matter on the adjacent portion of the lateral wall of the third ventricle. The trochlear nucleus is also situated in the ventral portion of the grey matter, but at a lower level than the oculo-motor nucleus, being placed underneath the upper part of the lower quadrigeminal body. The nucleus of the mesencephalic root of the fifth nerve is extensive, and is situated in the lateral portion of the grey matter.
+
The long association fibres pass between portions of the grey cortex, which are at some distance from each other. They are arranged in bundles, the chief of which are as follows: (1) the superior longitudinal fasciculus; (2) the interior longitudinal fasciculus; (3) the perpendicular fasciculus; (4) the uncinate fasciculus; (5) the cingulum; (6) the occipitofrontal fasciculus; and (7) the fornix.
  
Development. —The aqueduct is the persistent remains of the cavity of the mesencephalon.
+
The superior longitudinal fasciculus consists of fibres which extend from the frontal to the occipital lobe. Posteriorly many of its fibres sweep downwards and forwards into the temporal lobe, and from this circumstance it is sometimes spoken of as the arcuate fasciculus.
  
Posterior Perforated Substance. —This area has been previously described in a general way in connection with the base of the encephalon. It will here be considered more fulfy. It lies at the bottom of a deep depression, called the interpeduncular fossa, which forms the back part of the interpeduncular space. The fossa is bounded posteriorly by the median portion of the upper border of the pons, and laterally by the crura cerebri. Anteriorly it is limited by the corpora mamillaria.
+
The inferior longitudinal fasciculus connects the occipital and temporal lobes, its fibres being disposed upon the lateral walls of the posterior and inferior horns of the lateral ventricle.
  
The locus perforatus is a perforated lamina of grey matter which forms the floor of the interpeduncular fossa, the openings being for the passage of the postero-medial ganglionic branches of the posterior cerebral arteries. This grey lamina extends between the tegmenta of the crura cerebri.
+
The perpendicular fasciculus connects the inferior parietal lobule with the occipito-temporal gyrus.
  
Ganglion Interpedunculare. —This is a collection of nerve-cells situated medially in the lower part of the grey lamina which constitutes the posterior perforated substance. On either side it receives the fibres of the fasciculus retroflexus , which are derived from the ganglion habenulae.
+
The uncinate fasciculus crosses the stem of the lateral fissure, and connects the frontal and temporal lobes.
  
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The cingulum is connected with the rhinencephalon, and lies upon the under surface of the callosal gyrus and the upper surface of the hippocampal gyrus. Its fibres connect the gyri of the lobe with the cerebral cortex.
  
Structure of the Cerebral Hemispheres.
+
The occipito-frontal fasciculus connects the frontal with the occipital and temporal lobes. It lies internal to the corona radiata, in intimate relation to the nucleus caudatus, and as the fibres pass backwards they lie on the outer walls of the inferior and posterior horns of the lateral ventricle.
  
 +
The fornix connects the hippocampus major of one side with the corresponding corpus mamillare, and through the latter with the thalamus by means of the mamillo-thalainic tract (bundle of Vicq d’Azyr), the fibres of which arise in the corpus mamillare.
  
The cerebral hemisphere is composed of grey and white matter. The grey matter is disposed externally, and forms the cerebral cortex, which, with the exception of the rhinencephalon, is known as the neopallium. The white matter occupies the interior, and constitutes the
+
Peculiarities of the Cerebral Cortex-— 1. Calcarine Area.— This area is situated on the medial surface of the occipital lobe in close proximity to the calcarine fissure, and it is known as the visual area. In this region the outer band of Baillarger is very conspicuous, and is known as the white band of Gennari, whilst the inner band of Baillarger is absent.
  
medullary centre.
+
2. Central Area. —In this region, more especially in the cortex of the precentral gyrus, there are groups of very large pyramidal cells, which are known as the giant-cells of Betz, and nerve-fibres are present in large numbers.
  
Cerebral Cortex.— The grey matter forms a continuous covering to the entire hemisphere, dipping into the sulci, so as to cover the opposed surfaces of the gyri, as well as the bottom of the sulci, is thicker over the superficial surfaces of the gyri than at the bottom of the sulci, and attains its greatest thickness over the upper portions of the precentral and postcentral gyri, whilst it is thinnest over the
+
3. Hippocampal Area. —The hippocampus corresponds to the hippocampal or dentate fissure, and is produced by an infolding of the cerebral cortex. It is therefore composed chiefly of grey matter, and is covered superficially by a thin layer of white matter, called the alveus, which is continuous with the fimbria. The hippocampus is composed of the following layers, named in order from the ventricular surface outwards: (1) the alveus, composed of white matter, and covered by the ventricular ependyma; (2) neuroglial layer, consisting of neuroglia fibres and cells; (3) pyramidal layer, composed of large pyramidal cells; (4) stratum radiatum, which is the outer part of the pyramidal layer, and is composed of the dendrites of the apical parts of the pyramidal cells, being thereby rendered striated in appearance; (5) stratum laciniosum, composed of the ramifications of the foregoing apical dendrites intimately intermixed; (6) stratum granulosum, composed of many small cells; and (7) the involuted medullary lamina, consisting of white fibres.
  
  
The cerebral cortex is indistinctly divided into strata by means of layers of a whitish substance. When examined m section it therefore presents a stratified appearance, and is seen to consist of successive
+
Olfactory Tract and Olfactory Bulb. — These are developed as a hollow outgrowth from the anterior cerebral vesicle, more particularly from the part of it which ultimately gives rise to the lateral ventricle, and is known as the telencephalon. In many animals the central cavity persists, and maintains its connection with the lateral ventricle; but in man the cavity disappears, though traces of its ependymal lining remain. External to the vestigial ependyma there is a layer of white matter, and superficial to this there is a layer of grey matter. In the olfactory tract the layer of grey matter is very thin over the ventral or inferior aspect, but over the dorsal or superior aspect it is fairly thick. In the bulb the reverse is the case, the grey matter being thick over the ventral aspect, where it receives the olfactory filaments, but thin over the dorsal aspect.
  
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Structure of the Ventral Grey Matter of the Olfactory Bulb. — The grey matter consists of three layers—namely, (1) the nerve-fibre layer, (2) the glomerular layer, and (3) the granular layer.
  
1562
 
  
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The nerve-fibre layer is the most superficial layer, and is composed of olfactory nerve-fibres. These fibres are non-medullated, and arise as the axons of the olfactory cells of the olfactory mucous membrane of the nasal fossa. Having passed through the foramina of one half of the cribriform plate of the ethmoid bone, they enter the grey matter on the ventral aspect of the bulb, where they break up and form arborizations. These intermingle with the arborizations formed by the dendrites of the mitral cells, to be presently described.
  
A MANUAL OF ANATOMY
+
The glomerular layer is composed of round bodies or glomeruli, which are formed by the interlacements between the arborizations of the olfactory nerve-fibres and those of the dendrites of the mitral cells.
  
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The granular layer lies next to the layer of white matter, and is chiefly characterized by the presence of large mitral cells. These are pyramidal, and one dendrite from each cell passes into the glomerular layer, where it gives rise to a glomerulus in the manner just described in connection with the glomerular layer. Other dendrites intermingle with those of adjacent mitral cells. The axon of each mitral cell enters the white layer of the bulb, and passes along the olfactory tract to the cerebrum.
  
grey and white layers alternating with each other. In most parts of the cerebral cortex there are four superimposed strata; but in certain situations— e.g., over the precentral gyrus—there are as many as six. These strata are as follows, from without inwards:
+
Weight of the Brain. —The average weight of the brain of the adult male is about 48 ounces (1,360 grammes), and that of the adult female about 44 ounces.
  
 +
Arteries of the Encephalon.
  
1. Molecular layer, a superficial
+
Cerebral Part of the Internal Carotid Artery. —The internal carotid artery, having pierced the roof of the cavernous sinus internal to the anterior clinoid process of the sphenoid bone, ascends between the second and third cranial nerves to the inner end of the stem of the lateral fissure. Here it divides into its terminal branches, anterior and middle cerebral.
  
white layer (pale and narrow).
+
Branches are posterior communicating, anterior choroidal, anterior cerebral, and middle cerebral.
  
2. Superficial grey layer, the outer
+
The posterior communicating artery arises from the back part of the internal carotid, and passes backwards to anastomose with the
 +
posterior cerebral artery. It is usually small, but is often larger on one side than the other. Occasionally it is absent.
  
granular.
 
  
3. Outer white band of Baillarger.
 
  
4. Middle grey layer.
+
Fig. 962. — Arteries on the Base of the Brain.
  
  
5. Inner white band of Baillarger.
 
  
6. Inner or deep grey layer, the
+
The anterior choroidal artery arises from the back part of the internal carotid close to its termination. It passes backwards and outwards between the crus cerebri and the "hippocampal gyrus, and enters the lower and anterior extremity of the descending horn of the lateral ventricle by passing through the choroidal fissure. In its course it passes just above the uncus, gives twigs to the hippocampal gyrus and crus cerebri, and terminates in the choroid plexus of the lateral ventricle.
  
polymorphous layer, subjacent to which is the white matter of the medullary centre.
 
  
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Anterior Cerebral Artery. —This is the smaller of the two terminal branches of the internal carotid artery, and it has a more limited distribution than the other terminal branch—namely, the middle cerebral artery. It passes forwards and inwards above the optic nerve, and just internal to the roots of the olfactory tract, to the commencement of the great longitudinal fissure, where it is connected with its fellow of the opposite side by a short transverse vessel, called the anterior communicating artery. After this it enters the great longitudinal fissure, turns round the genu of the corpus callosum, and passes backwards over the upper surface of that body to the splenium, where it anastomoses with the posterior cerebral artery.
  
Layers 3, 4, and 5 are included in the pyramidal layer, in which the cells tend to increase in size as they lie more deeply; the largest lie over the inner band of Baillarger.
 
  
The medullated fibres of the medullary centre pass into the stratified grey cortex in a radiating manner, and within the cortex their course for the most part is perpendicular to the superficial surface, and between the component cells of the cortex.
 
  
Minute Structure of the Cerebral Cortex. —The cortex is composed of nerve-cells and nerve-fibres.
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Fig. 963. — Median Section of Brain with Distribution of Anterior
  
Nerve-cells. —These are arranged in four layers, which are, from without inwards: (1) the molecular layer; (2) the layer of small pyramidal cells; (3) the layer of large pyramidal cells; and (4) the layer of polymorphous cells.
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Cerebral Artery.
  
The molecular or plexiform layer, which is the most superficial, is thin, and consists of cells and fibres. Many of the cells are neurogliacells, the others being nerve-cells. These nerve-cells are for the most part fusiform, and are disposed horizontally. They are known as the horizontal cells of Cajal. Their dendrons and axons are long, the latter forming medullated fibres which are disposed horizontally or parallel to the surface. These furnish minute branches which pass vertically towards the surface. The horizontal cells, according to Cajal, receive impulses from the corticipetal fibres which extend from the thalamus to the cerebral cortex.
 
  
In addition to these fibres there are many others which enter the molecular layer from deeper sources, and form a dense interlacement by their ramifications. The sources from which these extraneous fibres are derived are: (1) the terminal ramifications of the apical dendrons of the pyramidal cells (small and large); (2) the axons of the cells of Martinotti, which lie in the polymorphous layer; and (3) corticipetal fibres derived from the medullary centre of the gyrus.
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Branches .— These are arranged in two groups, antero-medial or ganglionic and cortical.
  
The pyramidal layers represent the second and third layers, and are composed of characteristic pyramidal cells which are peculiar to the cerebral cortex, those of the second layer being small, whilst those of the third layer are large. The layer of small pyramidal cells is narrow, but the layer of large pyramidal cells is of considerable thickness. The giant pyramidal cells of the motor cortex are known as cells of Betz. There is no well-marked line of demarcation between
+
The antero-medial or ganglionic branches, small in size, pass through the lamina cinerea along with twigs from the anterior communicating artery, and supply the front part of the caudate nucleus.
  
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The cortical branches are as follows: (1) medial orbital, to the medial orbital gyrus, olfactory lobe, and gyrus rectus; (2) anterior medial frontal, to the superior frontal gyrus, the anterior two-thirds of the middle frontal gyrus, and the anterior part of the marginal gyrus; (3) middle frontal, to the collosal gyrus, the posterior part of the marginal gyrus, and the upper part of the precentral gyrus; and (4) posterior frontal, to the precuneus and the corpus callosum, the branch of the latter being known as the artery of the corpus callosum.
  
THE NERVOUS SYSTEM 1563
 
  
these two layers, the one passing imperceptibly into the other. They constitute the chief part of the cerebral cortex.
+
Anterior Communicating Artery. — This vessel connects the two anterior cerebral arteries at the entrance to the great longitudinal fissure, and lies over the lamina terminalis in front of the optic commissure. It gives off a few twigs, which accompany the antero-medial branches of each anterior cerebral artery.
  
  
Molecular j Plexiiorm Layer
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Middle Cerebral Artery. — This is the larger of the two terminal branches of the internal carotid artery, and it has a wider distribution than the anterior cerebral artery. It enters the stem of the lateral fissure, in which it passes outwards.
  
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Branches . — These are arranged in two groups, antero-lateral or ganglionic and cortical. The antero-lateral or ganglionic branches
  
Layer of Small £ Pyramidal Cells
 
  
  
Layer of Large Pyramidal ° Cells
+
Pig. 964. Distribution of the Left Middle Cerebral Artery (Charcot).
  
  
Layer of . Polymorphous ** Cells
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F.i. Superior Frontal Gyrus F.2. Middle Frontal Gyrus F.3. Inferior Frontal Gyrus F.A. Ascending Frontal Gyrus P.A. Ascending Parietal Gyrus L.P.S. Superior Parietal Gyrus L.P.I. Inferior Parietal Gyrus P.C. Angular Gyrus L.O. Occipital Lobe
  
  
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L.T. Temporal Lobe
  
Plexus of Exne r (.Superficial Tangential Fibres)
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S. Middle Cerebral Artery entering Lateral Fissure
  
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P. Lenticulo-striate Arteries
  
Band ot Bechterew
+
1. Artery to Inferior Frontal Gyrus
  
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2. Ascending Frontal Artery
  
Outer Band of Bail larger
+
3. Ascending Parietal Artery
  
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4. Parieto-Temporal Artery
  
Radiating
+
5. Arteries to Temporal Lobe
  
Vertical
 
  
Fibres
+
pass through the anterior perforated substance, and foim two sets, medial striate and lateral striate. The medial striate arteries repiesem the ‘ lenticular arteries * of Duret, and they supply the globus pallidus (inner part) of the lentiform nucleus, the internal capsule, and the caudate nucleus. Ihe lateral striate arteries represent the. lenticulostriate ’ and ‘ lenticulo-optic ’ arteries of Duret. The lenticulo-striate arteries supply the putamen (outer part) of the lentiform nucleus and the external capsule. One of the lenticulo-striate aiteries is said to be larger than the others, and is subject to rupture in cases of cerebral haemorrhage. It is often termed the artery of cerebral hcemorrhage (Charcot). Its course is laterally and upwards round the outer aspect of the lentiform nucleus, between it and the external capsule, and then through the internal capsule to the caudate nucleus. The lenticulothalamic arteries supply the posterior part of the lentiform nucleus and the anterior part of the thalamus on its lateral aspect.
  
Inner Band of Baillarger
+
The most important point to notice about all the ganglionic arteries of the brain is that they are ‘ end arteries/ and their branches, once having divided, never anastomose again; hence, if one of them is blocked by an embolus, which is often a piece of fibrin from a diseased heart valve, the area of brain supplied will be cut off from all bloodsupply, and the clinical effects may be very grave.
  
  
Deep Tangential Fibres
 
  
  
•White Medullary Centre
+
Fig. 965. — Coronal Section of the Cerebral Hemispheres made One Centimetre behind the Optic Commissure (Charcot, from Duret).
  
  
Fig , W -Diagram showing the Minute Structure of the Cerebral J Cortex (Poirier).
+
The arteries of this region are shown.
  
The fibres are shown on the right, and the cells on the left.
 
  
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I. Area of Cerebral Artery
  
The apex of each pyramidal cell is directed towards the surface of the evrus and is prolonged into a long tapering dendrite which (Fig. 960) passes into the molecular layer, giving off delicate collatera s
+
II. Area of Middle Cerebral Artery
  
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III. Area of Posterior Cerebral Artery V.V. Sections of Anterior Cornua of Lateral Ventricles
  
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P.P. Anterior Pillars of Fornix C.N. Caudate Nucleus L.S.A. Lenticulo-Striate Arteries CL. Claustrum I.R. Insula E.C. External Capsule
  
  
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L. N. Lentiform Nucleus I.C. Internal Capsule O.T. Optic Tract (cut)
  
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G.M. Grey Matter of Third Ventricle O.C. Optic Chiasma O.N. Optic Nerve A.C. Anterior Cerebral Artery C. Internal Carot'd Artery
  
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M. C. Middle Cerebral Artery L.A. Lenticular Arteries
  
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A.C.H. Artery of Cerebral Hemorrhage
  
  
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The cortical branches arise in the vicinity of the insula, and are: (1) inferior lateral orbital, to the orbital surface of the frontal lobe lateral to the internal orbital sulcus, and to the inferior frontal gyrus; (2) ascending frontal, to the posterior third of the middle frontal gyrus, and to the lower and greater part of the precentral gyrus; (3) ascending parietal, to the postcentral gyrus and superior parietal gyrus; (4) parietotemporal, which traverses the posterior horizontal limb of the lateral fissure, and supplies the inferior parietal lobule and the posterior parts of the superior and middle temporal gyri; and (5) the temporal branches, which emerge from the posterior horizontal limb of the fissure, and supply the anterior and greater parts of the superior and middle temporal gyri.
  
  
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Fourth or Intracranial Part of the Vertebral Artery. —The vertebral artery, on leaving the suboccipital triangle, pierces the dura mater and arachnoid, and enters the cranial cavity through the foramen
  
  
 +
magnum. As it ascends, it lies at first on the side of the medulla oblongata, between the hypoglossal nerve and the anterior root of the suboccipital nerve. It then passes upwards on the ventral surface of the medulla, and on reaching the lower border of the pons it unites with its fellow of the opposite side to form the basilar artery. It will generally be found that one vertebral artery is larger than the other.
  
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Branches. — Posterior meningeal, posterior spinal, posterior inferior cerebellar, anterior spinal, and bulbar.
  
 +
The posterior meningeal artery arises from the vertebral artery just before it pierces the dura mater, and it enters the cerebellar fossa of the occipital bone where it supplies the dura mater.
  
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The posterior spinal artery arises from the vertebral artery immediately after it has pierced the dura mater. It descends upon the side of the spinal cord in front of the posterior roots of the spinal nerves, and it gives off a branch which descends behind these roots. These two arteries, in themselves small, are reinforced by the spinal branches of the second part of the vertebral artery and of the dorsal branches of the intercostal arteries. In this manner lateral anastomotic arterial chains are formed upon each side of the spinal cord in front of and behind the posterior nerve-roots.
  
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The posterior inferior cerebellar artery, of large size, arises a little above the preceding branch. It passes backwards between the vagus and accessory nerves, and then over the restiform body to the vallecula of the cerebellum, where it divides into branches. Some of these supply the inferior vermis, and others ramify on the inferior surface of the cerebellar hemisphere, at the periphery of which they anastomose with branches of the superior cerebellar artery. The artery furnishes branches to the corresponding choroid plexus of the fourth ventricle and to the medulla oblongata.
  
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The anterior spinal branch of the vertebral artery arises from that vessel near its termination. It passes obliquely downwards and inwards over the ventral aspect of the medulla oblongata, and at the median line it unites with its fellow of the opposite side to form the anterior spinal artery of the spinal cord. It furnishes twigs to the medulla oblongata.
  
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The bulbar branches are distributed to the medulla oblongata.
  
 +
Basilar Artery. —This vessel is formed by the union of the two vertebral arteries. It extends from the lower to the upper border of the pons, occupying the median basilar groove on its ventral surface. It lies deep to the arachnoid membrane within the cisterna pontis, and at the upper border of the pons it divides into the two posterior cerebral arteries.
  
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Branches. —These are as follows, on either side: transverse, internal auditory, anterior inferior cerebellar, superior cerebellar, and posterior cerebral.
  
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The transverse or pontine arteries are numerous, and pass outwards on either side to supply the pons, the sensory and motor roots of the fifth cranial nerve, and the middle peduncle of the cerebellum.
  
 +
The internal auditory artery, long and slender, accompanies the auditory nerve through the internal auditory meatus, and is distributed to the internal ear.
  
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The anterior inferior cerebellar artery arises from the centre of the basilar, and passes backwards to be distributed to the anterior part of the inferior surface of the cerebellar hemisphere. It anastomoses with the posterior inferior cerebellar artery, which is a branch of the vertebral.
  
  
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The superior cerebellar artery arises from the basilar close to its termination. It passes outwards parallel to the posterior cerebral artery, from which it is separated by the third cranial nerve. It then winds round the outer side of the crus cerebri below the fourth cranial nerve, and so reaches the superior surface of the cerebellar hemisphere, where it divides into branches. These supply the superior vermis, the upper medullary velum, the tela chorioidea, and the superior surface of the cerebellar hemisphere, at the periphery of which they anastomose with branches of the inferior cerebellar arteries.
  
  
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Posterior Cerebral Artery. — This is one of the terminal branches of the basilar artery at the upper border of the pons. It passes at first outwards beneath the crus cerebri, and parallel to the superior cerebellar artery, from which it is separated by the third cranial nerve. It then winds round the outer side of the crus cerebri, lying between it and the hippocampal gyrus, and above the fourth cranial nerve. In this manner it reaches the tentorial or inferior and medial surfaces of the occipital lobe of the cerebral hemisphere. It receives, not far from its commencement, the posterior communicating artery.
  
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Branches . — These are arranged in three groups—ganglionic, choroidal, and cortical.
  
 +
The ganglionic group includes two sets of branches, postero-medial and postero-lateral.
  
  
 +
Fig. 966. —The Areas of Distribution on the Surface of the Three Main Cerebral Arteries.
  
  
  
  
 +
The postero-medial ganglionic arteries pass medial to the crus cerebri, and pierce the posterior perforated substance. They supply the inner part of the crus cerebri and the posterior part of the thalamus.
  
 +
The postero-lateral ganglionic arteries pass on the lateral side of the crus cerebri, and supply the outer part of the crus, the posterior part of the thalamus, the corpora quadrigemina, and the corpora geniculata.
  
 +
The posterior choroidal arteries are two or three in number, and pass through the choroidal fissure to the tela chorioidea, which they supply, together with the choroid plexus of the lateral ventricle, and the corresponding choroid plexus of the third ventricle.
  
 +
The cortical branches are: (i) anterior temporal, to the anterior parts of the occipito-temporal and hippocampal gyri; (2) posterior temporal, to the posterior parts of the occipito-temporal and hippocampal gyri, and the inferior temporal gyrus; and (3) occipital, to the occipital lobe. One of the occipital branches is called the calcarine artery. It lies in the calcarine fissure, and supplies the lingual or infracalcarine gyrus and the cuneus.
  
 +
Circulus Arteriosus (Circle of Willis).—This circle or (to be more exact) heptagon is formed in front by the anterior communicating artery, which connects the two anterior cerebral arteries; behind by the basilar artery as it divides into the two posterior cerebral arteries; and on either side by (1) the anterior cerebral artery, (2) the trunk of the internal carotid aitery,
  
 +
(3) the posterior communicating artery, and (4) the posterior cerebra artery, in this order from before backwards. The cncle furnishes twigs to the grey cortex of the interpeduncular region. It serves to equalize the blood-pressure in the cerebral arteries, and it also provides for the regular supply of blood to the brain m cases where one of the main arterial trunks may be obstructed.
  
 +
The following parts are contained within the circulus arteriosus, in order from behind forwards: (1) the posterior perforated area; (2) the corpora mamillaria; (3) the tuber cinereum and infundibulum; and
  
 +
(4) the optic chiasma.
  
  
  
 +
13 15 1*
  
  
 +
x. Internal Carotid
  
 +
2. Middle Cerebral
  
 +
3. Anterior Cerebral _
  
 +
4. Anterior Communicating
  
 +
5. Posterior Communicating
  
 +
6. Posterior Cerebral
  
 +
7. Basilar
  
 +
8 Superior Cerebellar 9. Transverse Pontine
  
 +
10. Internal Auditory
  
 +
11. Anterior Inferior Cerebellar
  
 +
12. Posterior Inferior Cerebellar
  
 +
13. Vertebral
  
 +
14. Anterior Spinal
  
 +
15. Posterior Spinal
  
 +
16. Anterior Choroid
  
 +
17. Posterior Choroid
  
 +
18. Central or Ganglionic
  
 +
19. Central or Ganglionic
  
 +
20. Central or Ganglionic
  
 +
(Postero-mesia!)
  
 +
21. Central or Ganglionic
  
 +
(Postero-lateral)
  
  
 +
Fig. 967. —The Arteries at the Base of the Brain, and the Arterial Circle.
  
  
  
 +
===Veins of the Encephalon===
  
 +
The cerebral veins are arranged in two groups—superficial and deep.
  
 +
The superficial cerebral veins are divided into two sets—superior and inferior.
  
 +
The superior cerebral veins return the blood from the upper parts of the outer surfaces of the cerebral hemispheres. They lie in the pia mater, and pierce the arachnoid membrane and inner layer of the dura mater, after which they open into the superior longitudinal sinus, having previously received the veins from the medial surface of either hemisphere. Their direction for the most part is forwards and medially, whilst the direction of the blood-current in the superior longitudinal sinus is backwards.
  
 +
The inferior cerebral veins return the blood from the lower parts of the cerebral hemispheres, and they terminate in the cavernous, superior petrosal, and lateral sinuses. One of these veins is known as the superficial middle cerebral or superficial Sylvian vein. It passes along the lateral fissure, and opens into the front part of the cavernous sinus or else into the spheno-parietal sinus. This vein communicates posteriorly with (1) the superior longitudinal sinus by means of the great or superior anastomotic vein (of Trolard); and (2) the transverse sinus by means of the inferior anastomotic vein (of Labbe).
  
 +
the deep cerebral veins are as follows: (1) the choroidal veins; (2) the veins of the corpora striata; (3) the internal cerebral veins (veins of Galen); (4) the anterior cerebral veins; (5) the deep middle cerebral veins; and (6) the basilar veins.
  
 +
The choroidal vein of each side begins in the choroid plexus of the inferior horn of the lateral ventricle. It ascends at first, and then passes forwards in the lateral margin of the tela chorioidea to the interventricular foramen, where it unites with the vein of the corpus striatum to form the corresponding internal cerebral vein (vein of Galen).
  
 +
The vein of the corpus striatum is formed by branches which issue from the corpus striatum and thalamus. It runs forwards in the groove between these two bodies, lying superficial to the taenia semicircularis, and at the interventricular foramen it joins the choroidal vein to form the corresponding internal cerebral vein.
  
 +
The internal cerebral vein (vein of Galen) of each side is formed close to the interventricular foramen by the union of the choroidal vein, the vein of the corpus striatum, and the vein of the septum lucidum. The two veins, right and left, pass backwards between the two layers of the tela chorioidea, and they unite beneath the splenium of the corpus callosum to form one vessel, called the great cerebral vein, which opens into the front part of the straight sinus. Each vein receives tributaries from the thalamus, the corresponding choroid plexus of the third ventricle, the corpus callosum, and the corpora quadrigemina; and, before joining its fellow, it takes up the basilar vein of its own side. The great cerebral vein receives tributaries from the upper surface of the cerebellum.
  
  
 +
The anterior cerebral vein of each side is situated in the great longitudinal fissure, along with the corresponding artery. Having curved round the genu of the corpus callosum, it passes to the anterior perforated region, where it joins the deep middle cerebral vein to form the basilar vein.
  
 +
The deep middle cerebral vein returns the blood from the insula and lies deeply within the stem of the lateral fissure.
  
 +
The basilar vein of each side begins at the anterior perforated area, where it is formed by the union of the anterior cerebral and deep middle cerebral veins. It passes backwards round the crus cerebri, and opens into the internal cerebral vein of its own side just before that vessel joins its fellow to form the great cerebral vein. The basilar vein receives, close to its commencement, one or more inferior striate veins, which descend from the corpus striatum through the anterior perforated substance. It also receives tributaries from the parts within the interpeduncular space.
  
 +
The cerebellar veins are arranged in two groups—superior and inferior.
  
 +
The superior cerebellar veins terminate in the great cerebral vein, and in the straight, transverse, and superior petrosal sinuses. The inferior cerebellar veins pass to the sigmoid, inferior petrosal, and occipital sinuses.
  
  
 +
Blood-supply of the Different Parts of the Encephalon.
  
 +
The medulla oblongata is supplied by the vertebral, anterior spinal, and
  
 +
posterior inferior cerebellar arteries. .,
  
 +
The pons is supplied by the transverse or pontine branches of the basilar
  
 +
The cerebellum is supplied inferiorly by the posterior inferior cerebellar branches of the vertebral arteries, and the anterior inferior cerebellar branches of the basilar artery. Superiorly it is supplied by the superior cerebellar branches of the basilar artery, and to a limited extent by the posterior inferior cerebellar arteries
  
 +
The crus cerebri is supplied by the postero-medial and postero-lateral branches of the posterior cerebral artery, and by the posterior communicating
  
 +
Th’e posterior perforated substance is pierced by the postero-medial branches
  
 +
of the posterior cerebral arteries. , , . v
  
 +
The corpora quadrigemina are supplied by the postero-lateral ganglionic
  
 +
branches of the posterior cerebral arteries. , +
  
 +
The thalamus is supplied posteriorly by the postero-meclial and posterolateral ganglionic branches of the posterior cerebral artery. Anteriorly its outer part is supplied by the lenticulo-thalamic branches of the middle cerebral artery,
  
 +
and its inner part by the posterior communicating artery. .
  
 +
The anterior perforated substance is pierced by the antero-lateral ganglionic
  
 +
branches of the middle cerebral artery.
  
 +
Frontal Lobe — The superior frontal gyrus, the anterior two-thirds of the middle frontal gyrus, and the upper portion of the precentral gyrus are supplied bv cortical branches of the anterior cerebral artery. The posterior third of the
  
 +
middle frontal gyrus, the inferior frontal gyrus and the .lower “h^ce-ebra* of the precentral gyrus are supplied by cortical branches of the^middle <cerebral „ rfpr * Q n t h e orbital surface the internal orbital gyrus, olfactory lobe, ana gyrus rectus are supplied by the anterior cerebral artery, whilst the remainder is supplied by the middle cerebral artery.
  
  
 +
Parietal Lobe. — The whole of this lobe, practically, is supplied by the middle cerebral artery.
  
 +
Occipital Lobe. — This lobe is supplied by the posterior cerebral artery.
  
 +
Temporal Lobe. — The superior and middle temporal gyri and the pole are supplied by the middle cerebral artery, and the remainder is supplied by the posterior cerebral artery.
  
 +
Medial Surface of the Cerebral Hemisphere. — The anterior cerebral artery has an extensive distribution to this surface, which it supplies as far back as the internal part of the parieto-occipital fissure. The parts behind this fissure— namely, the cuneus and the parts around the calcarine fissure—are supplied by the posterior cerebral artery.
  
 +
The corpus callosum is supplied by the anterior cerebral arteries.
  
 +
Corpus Striatum. —The nucleus caudatus and nucleus lentiformis are supplied for the most part by the antero-lateral or ganglionic branches of the middle cerebral artery, which pass through the anterior perforated substance. According to Duret they form three sets—lenticular, lenticulo-striate, and lenticulo-thalamic. The lenticular (internal striate) arteries supply the globus pallidus (inner part)
  
  
  
 +
Fig. 968. —Brain of an Embryo about Four Weeks Old (from Quain’s ‘ Anatomy ’) (His).
  
 +
1. Telencephalon 4. Metencephalon 7. Spinal Cord
  
 +
2. Thalamencephalon 5. Myelencephalon 8. Pontine Flexure
  
 +
3. Mesencephalon 6. Cervical Flexure 9. Olfactory Lobe
  
 +
1, 2=prosencephalon; 3 = mesencephalon; 4, 5 = rhombencephalon
  
 +
of the lentiform nucleus, the internal capsule, and the caudate nucleus. The lenticulo-striate (external striate) arteries supply the putamen (outer part) of the lenticular nucleus, and the external capsule. One of the lenticulo-striate arteries is larger than the others, and is subject to rupture in cases of cerebral haemorrhage. It is known as the artery of cerebral hcemorrhage (Charcot). Its course is outwards and upwards round the lateral aspect of the lentiform nucleus, between it and the external capsule, and then through the internal capsule to the caudate nucleus. The lenticulo-thalamic arteries supply the posterior part of the lentiform nucleus, and the anterior part of the thalamus on its lateral aspect.
  
 +
1 he front part of the caudate nucleus is supplied by the antero-medial ganglionic branches of the anterior cerebral and anterior communicating arteries.
  
 +
The larger arteries occupy the subarachnoid space, where they divide into branches which enter the pia mater. These in turn give off smaller branches, which enter the cerebral substance, some of them being cortical and others medullary in their distribution.
  
 +
Blood-supply of the Choroid Plexuses. —The choroid plexus of the lateral ventricle derives its blood from (i) the anterior choroidal artery, which is a branch of the internal carotid or the middle cerebral; and (2) the posterior choroidal arteries, which are branches of the posterior cerebral. The choroid plexuses Of the third ventricle derive their blood from the posterior choroidal arteries. The choroid plexuses of the fourth ventricle are supplied from the posterior inferior cerebellar arteries.
  
 +
The tela chorioidea derives its blood from the posterior choroidal arteries and from the superior cerebellar arteries.
  
 +
Lymphatics of the Brain. —There are no lymphatic vessels in the brain. Their place is taken by spaces in the outer coat of the arteries, called perivascular spaces, which are in communication with the subarachnoid space.
  
  
 +
===Development of the Encephalon===
  
 +
A brief outline of the formation of the brain has been given on pp. 55-59* in which can be followed the development of the primary cerebral divisions into fore-, mid- and hind-brain, the formation of the cerebral vesicles, the appearance of the brain flexures, and in general the changes which lead to the existence in their proper positions of the main structures in the brain. The student is advised to read this general account before proceeding to the following descriptions, which deal with the conditions in the developing brain in a more detailed manner.
  
  
 +
Metamorphoses of the Hind-brain.
  
 +
The pontine flexure begins to show about the beginning of the fourth week, is well marked at the beginning of the second month, and has its two limbs very close to one another by the end of this month. The posterior limb of the flexure, down to the nuchal bend, is termed the myelencephalon, the anterior limb the metencephalon, and the narrow junction with the mid-brain is the isthmus.
  
 +
Myelencephalon. — The walls, opened out in their upper parts, with a wide roof-plate, give origin to the medulla oblongata, and the cavity forms the lowei half of the fourth ventricle ; at the extreme lower end the cavity is not enlarged. The development of this lower or hinder portion of the myelencephalon proceeds in general on the lines of development already described for the spinal cord, with certain modifications due to the opening out and change in direction in certain tracts passing to the brain, the crossing of pyramidal fibres, and the presence of certain tracts and nuclei (to be described later) associated with the existence of visceral arches.
  
 +
A little higher up the myelencephalon broadens to make the lower part of the fourth ventricle. The alar and basal laminae are now in the floor of the wide cavity. The broad roof-plate, a single layer of epithelium, is attached at the sides to an everted edge of the alar lamina, known as the rhombic lip, an over hanging the outer parts of the laminae. Such a definite rhombic lip however, is only found in the cranial part of the myelencephalon, where, as will be seen later, the great enlargements appear which are due to growth of the vestibular nuclei.
  
 +
The widened roof-plate is covered by vascular mesenchyme, representing pia mater. At the level of the pontine flexure the ependymal or epithelial roof is invaginated into the fourth ventricle in the form of a transverse fold —plica chorioidea, containing pia mater—which extends between the lateral recesses of the ventricle From this transverse fold two vertical folds —phcce chorioidea, e wise containing pia mater—extend vertically downwards into the ventricle close to the median line. These ependymal folds, containing pia mater, form the two choroid nlexuses of the lateral ventricle.
  
 +
At a comparatively early stage the afferent fibres of the seventh, ninth and tenth nerves pass into the marginal zone of the alar lamina, and form a bund here i ZZausTolitarius; this bundle becomes deeply buried by subsequent
  
 +
thl °Thehvno!gloss afnucleus deveiops, as has been mentioned already on p. 1440 praetor witWnth"ndymal P zone, in the upper part of the column of loosened nuclei seen here in the cervical region formed from the ependymal zone ventro-laterally. The sixth nucleus possibly arises from the extreme cranial end of the same column, but this is not certainly known. In the young embryo the efferent nuclei (except that of the hypoglossal) lie in the basal lamina, where they cause internal depressions by their rapid growth in the thin wall (Fig. 970). These depressions are known as neuromeres, and are a marked though temporary feature of most embryonic brains. In the illustration they are seen from within, and have a curious distribution, in that the sixth neuromere is placed behind the seventh. The fifth has two neuromeres, of which the most cranial is much the deeper; the line of flexure of the hind-brain, which has not yet begun in this specimen, will pass through this deep neuromere of the fifth nerve.
  
  
  
 +
Fig. 969.— Lateral View of Brain, End of Second Month.
  
 +
Cbl, cerebellar rudiment; P, N, pontine and nuchal flexures; M, mid-brain flexure; Pit, hypophysis; Cpb, corpus ponto-bulbare. Roof-plate of hindbrain is only shown in outline.
  
 +
A little later a neuromere will mark, rather indefinitely, the ninth efferent nucleus, but is not seen in the figure.
  
 +
It must be understood that the neuromeres are present only in the basal lamina; this, for practical purposes, is the one seen in the figure, the thin and narrow strip (D) being the only representative of the alar lamina.
  
 +
Behind the region of neuromeres the myelencephalon narrows down to its continuity with the spinal cord, and it is here, extending cranially, that the olive is laid down.
  
 +
The inferior olive, with its medial and dorsal accessory formations, is developed as a modification of the upper part of the ventro-lateral column of neuroblasts in the mantle zone, from which the ventral column is formed at a lower level. The early stages of this development are shown in Fig. 971, while Fig. 972 gives the appearances in subsequent stages. In these it can be seen that the median accessory olive (m.o.) is first defined, the main olivary mass being constructed from the more lateral condensations.
  
  
  
 +
Fig. 970. A, sagittal section of brain of 4-9 mm. embryo; B, section through two adjacent neuromeres.
  
  
  
 +
Fig. 971. — Hind-brain, 13-5 Mm. Embryo.
  
 +
Outline of longitudinal median section on right below; m this cr. is the cranial slope and sp. the spinal cord; TS, tractus solitanus; vl ventro-lateral nuclei. Planes of sections a, b, and c correspond with those showm on the outline.
  
  
  
  
 +
It must be said here that the classical and accepted account of olivary development refers it to the ventral migration of neuroblasts from the ‘ rhombic lip/ The account given above is put forward because it is in accord with observed facts, whereas the older story is very unsatisfactory in several particulars.
  
  
  
 +
Fig. 972. Transverse sections through olivary region in embryos of 15 and 16 mm.; horizontal sections, 18, 21, and 28 mm. in neighbourhood of nuchal flexure.
  
  
 +
The cuneate and gracile nuclei are formed directly from the dorsal neuroblasts.
  
 +
Each pyramid is a ventral bulging of that part of the basal lamina which is on the mesial side of the olivary body, and it is produced by the motor tracts as they descend in the marginal layer from the central area of the cerebral cortex through the pons. This begins in the fourth month.
  
 +
The ponto-bulbar body is represented in the embryo by a collection of small and darkly-staining nuclei lying below the caudal part of the ‘ rhombic lip/ It appears in the latter part of the second month, and its nuclei spread fairly rapidly over the surface of the myelencephalon, especially ventrally and cranially; here they lay down the beginnings of the pontine nuclei. On the myelencephalon further back they appear to be responsible for the various small superficial arcuate nuclei which may be found on the surface of the pyramid, etc. There is also a possibility that the lateral accessory cochlear nucleus may possess a similar origin.
  
 +
As the pyramids, right and left, bulge ventralwards, the floor-plate, which connects the basal laminaj, sinks, and the anterior median fissure is formed, as in the development of the spinal cord. The spongioblastic floor-plate is now invaded by nerve-fibres, most of which cross from one side to the other, these fibres being (1) the anterior superficial arcuate fibres, (2) the deep arcuate fibres, and (3) the cerebello-olivary fibres. In this manner the raphe of the bulb is formed, as in the development of the anterior or white commissure of the spinal cord.
  
 +
In the more cranial portion of the myelencephalon, where the efferent nuclei have been laid down in the neuromeres, the subsequent growth of the alar lamina affords opportunity for the development of the vestibular nuclei, which thus lie just cranial to the great dorsal masses of the gracile and cuneate nuclei. The changes which take place in the floor of the ventricle in this part are verv com plicated and not by any means understood; the plans in Fig. 973 are attempts to show the results of the changes. The first figure gives the positions of the neuromeres on the left, with the sites of the afferent nuclei on the r t§tit. n . e second and third the vestibular masses are associated with considerable widening but there is apparently a marked forward upgrowth of the floor in the basal area which carries the sixth and seventh nuclei forward and brings them against the metencephalic surface. This is no doubt associated with the curious relations between the two nuclei and nerves, but the way in which it comes about, as well as the reason for its occurrence, is not known. The positions of the various nuclei are given approximately in the plans, and a general idea of their origins and changes can be obtained by a study of the figures.
  
  
  
564
+
Fig. 973._ Plans to show Areas in Floor of Fourth Ventricle, with Position of Developing Structures.
  
A MANUAL OF ANATOMY
 
  
  
Terminal Ramifications
+
Metencephalon. — From this are developed the pons, cerebellum, its upper and middle peduncles, and the superior and inferior medullary vela Its cavity forms the upper part of the fourth ventricle .
  
  
in its course. Near the surface of the molecular layer it divides into terminal filaments, which are disposed horizontally and mingle with the tangential fibres. The base of the pyramidal cell is directed towards the medullary centre of the gyrus, and from its centre an axon is given off, which enters the medullary centre, giving off collaterals in its course. From each side of the body of the cell, as well as from each lateral angle of its base, dendrites are given off.
+
The pons develops as a ventral thickening on the lower end of the region, immediately cranial to the pontine flexure. Its nuclei appear to owe their origin to the neuroblasts which have spread over the surface from the ponto-bulbar body; presumably they increase subsequently in situ, but no definite indications of mitotic activity have been
  
The polymorphous layer is the deepest layer, and is composed of cells which have different shapes. Each cell gives off several dendrites, which pass towards, but do not enter, the molecular layer. The axon of each cell enters the medullary centre as a nerve-fibre.
 
  
In addition to the foregoing cells of the cerebral cortex, two other kinds of cells are met with amongst the pyramidal and polymorphous cells—namely, the cells of Golgi and the cells of Martinotti. The cells of Golgi are characterized by the fact that the axon of each almost immediately divides into several branches, which pass towards the surface, but soon terminate without entering the molecular layer. The cells of Martinotti are chiefly met with in the polymorphous layer. The axon of each cell passes towards the surface, and enters the molecular
 
  
 +
Fig. 974. — Semi-diagrammatic Figures showing Cerebellar Rudiments.
  
  
Human Cerebral Cortex (Ramon y Cajal).
+
found among them. The down-growing cerebro-spinal fibres find their way into and among these nuclei in the third month and subsequently.
  
 +
The cerebellum is developed from the alar laminae of the metencephalon, the thickening involving the roof-plate in its growth. The
  
layer, where it divides into terminal branches, which form part of the tangential fibres of this layer.
+
two lateral cerebellar plates formed by the laminae are at first inclined to each other at an angle (Fig. 976), but as the angle of the pontine flexure becomes more closed and the metencephalon widens, the paired cerebellar rudiments come nearly into line with one another (Figs. 974, 977). The lateral plates, being thickenings in the floor of the cavity, project at first into the cavity, covered by the roof-plate, which is attached to the margins outside them ; subsequently the attachment of the roof-plate is turned in (Fig. 977) below the bulging lateral formations, so that it becomes attached, descriptively, to the anterior and lower aspect of the transversely disposed cerebellar rudiment. It is in the taenial fold to which the roof-plate is attached here that the floccular enlargement occurs a little later.
  
 +
In some lower vertebrates the cerebellum develops altogether within the cavity of the hind-brain, as in the early human stage; the later change in the human conditions enables the structure to expand freely outside the ventricle.
  
Nerve-fibres of the Cortex. —1 hese are arranged in two groups— vertical and tangential.
+
The upper part of the roof-plate of the rhombencephalon, at the angle of junction of the lateral plates, is invaded by them, and forms
  
The vertical (radial) fibres are disposed in radiating bundles, which issue from the medullary centre, and traverse the polymorphous and large pyramidaf layers, after which they become indistinguishable. The polymorphous and large pyramidal cells lie m the spaces between these bundles, and assume a columnar arrangement. The fibres of the radiating bundles gradually become less numerous, some of them becoming the axons of the polymorphous cells, but most of them becoming the axons of the large pyramidal cells. The radiating bundles contain centripetal cortical fibres, which pass into the molecular layer and end in terminal ramifications, forming part of its tangential fibres.
 
  
  
 +
Fig. 975.— Brain of Third Month Embryo seen
  
 +
FROM BEHIND, TO SHOW *
  
 +
the Transverse Cerebellar Rudiment.
  
THE NERVOUS SYSTEM
+
This stage is between those shown in the previous figure.
  
  
1565
 
  
The tangential fibres are disposed horizontally at different levels, and form the following strata: (1) the superficial tangential fibres (plexus of Exner), which occupy the superficial part of the molecular layer; (2) the band of Bechterew, which is situated in the superficial part of the small pyramidal layer; (3) the outer band of Baillarger (band of Vicq d’Azyr), which intersects the large pyramidal layer; (4) the inner band of Baillarger, which is situated between the large pyramidal and polymorphous layers; and (3) the deep tangential fibres (intracortical association fibres), which are situated in the deep part of the polymorphous layer.
+
the basis in which the vermis develops; growth here is slow, and the lateral lobes in their enlargement come to overlap it and cover it in.
  
The tangential fibres are formed by (1) the collaterals of the polymorphous and pyramidal cells and of the cells of Martinotti; (2) the ramifications of the axons of the cells of Golgi; and (3) association fibres.
+
The lateral plates come into evidence during the second month; the transversely disposed plate (Fig. 975) is well marked in the middle of the third month, when the flocculus may be recognized, and after this the development proceeds slowly. After the third month the vermis shows transverse fissures, and in the fifth month these are found also on the lateral lobes.
  
Medullary Centre of Cerebral Hemisphere. —The white matter of the medullary centre consists of medullated nerve-fibres, which pursue different courses, and are arranged in three groups—namely, projection fibres, commissural fibres, and association fibres.
 
  
The projection fibres connect the cerebral cortex with parts at a lower level, and they are of two kinds—corticipetal or afferent, and corticifugal or efferent. The commissural fibres pass from one hemisphere to the other, and connect portions of the cerebral cortex of opposite hemispheres. The association fibres are confined to one side of the median plane, and they bring different parts of the cerebral cortex of the same hemisphere into association with one another.
 
  
Projection Fibres. —These fibres, as stated, are both corticipetal and corticifugal, and the majority of them constitute the internalcapsule of the corpus striatum, and the diverging arrangement of its fibres known as the corona radiata, which passes to all parts of the cerebral cortex. Some projection fibres, however, do not traverse the internal capsule and corona radiata—£.g., the fibres of the ansa peduncularis.
+
Fig 076_Ventricular Aspect of Cranial Slope of Hind-brain in Em bryos OF 12, 16, AND 28 Mm., TO SHOW BEGINNINGS OF CEREBELLAR FOLD.
  
Corticifugal Fibres.— The corticifugal or efferent fibres constitute the following tracts:
 
  
1. Pyramidal or motor. 3 - Fronto-pontine.
+
The fissure cutting off the flocculus extends across the region of the vermis, marking off the nodule here. At the end of the third month a fissura prima forms across the vermis, making the lowei edge of the future culmen, and a little later a fissura secunda forms below the future pyramid . Other secondary fissures follow, and some ol the fissures of the vermis extend into the lateral lobes, but most of the
  
2. Cortico-thalamic. 4- Temporo-pontine.
+
fissures here are separate local formations. , A
  
5. Optic radiation (portion).
+
Cerebellar thickening, spreading into the roof-plate above and below the main development, forms the upper and lower medullary vela, the first extending (valve of Vieussens) to the closed isthmus, the second being continuous below with the undeveloped ependymal or epithelial roof-plate of the myelencephalon.
  
The pyramidal or motor tract derives its fibres from the axons of the pyramidal cells of the cortex of the precentral gyrus, which is situated in front of the central fissure. Having traversed the corona radiata, these fibres pass in succession through (1) the posterior limb of the internal capsule, (2) the middle three-fifths of the basis of the crus cerebri, (3) the ventral portion of the pons, and (4) the pyramid of the medulla oblongata. The motor strand enters the spinal cord in three ways—partly as the direct or ventral cortico-spinal tract, partly as the uncrossed lateral tract, but chiefly as the crossed lateral
+
On either side of the valve of Vieussens the roof-plate is thickened by the superior cerebellar peduncles.
  
  
1566
 
  
 +
Fig. 977.— Views from behind of the Hind-brain in Embryos of 35 and 48 Mm.
  
A MANUAL OF ANATOMY
+
EV, IV, extra- and intra-ventricular surfaces.
  
 +
The superior peduncles of the cerebellum, right and left, arise from the cells
  
corticospinal tract. Ultimately the fibres terminate at different levels in arborizations around the motor-cells of the ventral column of grey matter of the opposite side, from which cells the fibres of the motor nerve-roots proceed.
+
of the nucleus dentatus of the corresponding lateral cerebellar hemisphere. Emerging from the anterior parts of the dentate nuclei, the peduncles give rise to two thickenings of the roof-plate of the metencephalon, one on either side of the valve of Vieussens. Thereafter they enter the mesencephalon or mid-brain, and, after decussating, each passes to the corresponding red nucleus of the tegmentum of either crus cerebri, which constitutes its lower cell-station.
  
The efferent fibres which pass to the motor nuclei of the cranial nerves do not, as a whole, run in the cortico-spinal pathway through the basis pedunculi and basis pontis. They leave this path, usually in the upper part of the mid-brain, and pass down (Fig. 961) in the tegmentum of the mid-brain and pons. They reach the tegmentum also at lower levels, passing usually either medial or lateral to the substantia nigra, but they are not constant in this matter. These cortico-pontine or
+
The middle peduncles of the cerebellum ( brachia pontis) are developed from the cells of the pontine nuclei of the pons right and left. The fibres of each peduncle issue from the lateral portion of the corresponding pontine nucleus, and enter the adjacent cerebellar hemisphere.
  
cortico-bulbar fibres are thus aberrant or extra-pyramidal fibres. The figure, which is modified from Dejerine, shows the nuclei supplied by this group, the remnant of which rejoins the main tract in the medullary pyramid. Each central supply to the nuclei decussates, crossing the middle line nearly at the level of the nucleus to which it is going.
+
Mesencephalon. — This portion makes a sharp curve (Fig. 969) as it develops. It has a large cavity, which is slowdy lessened in size by the growing thickness of the walls, finally remaining as the small aqueduct. The thickened walls around this are composed of a floor lamina and roof lamina (tectum); in the former are formed the crura cerebri, while the tectum gives rise to the corpora quadrigemina.
  
The cortico-thalamic tract extends only between the cerebral cortex and the thalamus. Its fibres arise as the axons of the pyramidal cells of various parts of the cerebral cortex, and they terminate in arborizations around the cells of the thalamus.
+
The isthmus is a part of some interest; it is essentially a derivative of the hind-brain, so that its name isthmus rhombencephali is correct. The isthmus is produced at the spot where the two regions of growth, metencephalic and mesencephalic, meet each other, but there is in addition a definite forward extension of the basal lamina from the hind-brain into the opening, of which it makes the immediate wall. This extension carries with it the trochlear nucleus, which develops in the basal lamina of the hind-brain just above the opening; the decussation of the nerves, originally in the floor-plate of the hind-brain, is carried down also and lies therefore on the dorsum of the isthmus.
  
The fronto-pontine tract does not extend lower than the pons. It consists of fibres which arise as the axons of the pyramidal cells of the cortex of the prefrontal region—that is to say, the region of the frontal lobe in front of the precentral sulcus. These fibres traverse the anterior limb of the internal capsule, and then descend through the inner or medial fifth of the basis of the crus cerebri into the pons. Within the pons they terminate in arborizations around the cells of the nucleus pontis.
 
  
The temporo-pontine tract, like the preceding, does not extend lower than the pons. It consists of fibres which arise as the axons of the pyramidal cells of the cortex of the first and second temporal gyri. These fibres traverse the postlenticular part of the posterior limb of the internal capsule, and then descend through the outer fifth of the basis of the crus cerebri into the ventral part of the pons. Within this part of the pons they terminate in arborizations around the cells of the nucleus pontis.
 
  
The corticifugal fibres of the optic radiation consist of fibres which arise as the axons of the pyramidal fibres of the cortex of the occipital
+
Fig. 978._Schemes to show the Parts of the Third Ventricle formed FROM THE THALAMENCEPHALON (Bi.ACK LlNE) AND TELENCEPHALON (INTERRUPTED Line).
  
 +
A little later there is an extension forwards from the alar lamina of the hindbrain, passing on the outer side of the basal extension, and carrying with it (sensory) nuclei from the upper part of the trigeminal nucleus; this appears to be the beginning of the mesencephalic root of the nerve, further short connections developing subsequently. .
  
  
N. III.
+
The mid-brain presents dorsally a median longitudinal groove, which separates two rounded eminences, known as the corpora bigemina . At a latei period a transverse groove appears, which divides each of the corpoia bigemina into two, thus giving rise to the corpora quadrigemina.
  
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The third nucleus forms in the ventral mantle zone, and the fourth nucleus o-ains its position here secondarily. The red nucleus is probably, formed in situ, from the mantle zone of the floor lamina. I he corpora quadrigemina, formed from the alar laminae (and probably from the roof-plate secondarily involved), are hollow at first. -They become solid in the fourth and fifth months. The bases peduneulornm begin to appear in the fourth month in the central parts of the marginal zone.
  
' ~ ' N. IV.
+
Diencephalon or Thalamencephalon.— This is the anterior primary vesicle (Fig 978) Its cavity forms the greater part of the third ventricle (the anterior portion being derived from the secondary outgrowth, the telencephalon). Its walls develop into the thalamus, corpora mamillaria, tuber cmereum, mfundibulum, and posterior lobe of pituitary, and from the roof-plate grow the pineal body and the ependymal roof of the ventricle. Moreover, when it is first formed, the thalamencephalon gives origin to the optic outgrowth on each side.
  
  
N. V.
+
The pineal body, or epiphysis cerebri, is developed from the dorsal part of the ependymal roof of the third ventricle. It appears as a diverticulum of the ependymal roof close to the mesencephalon, and it is directed backwards, so that it comes to lie over the corpora quadrigemina. The distal end is blind, and in connection with it a number of closed follicles are formed which contain calcareous particles forming the acervulus cerebri, or brain-sand. The proximal part of the diverticulum forms the stalk of the pineal body, which contains the pineal recess and opens into the third ventricle.
  
N. VI.
+
The basal laminae, smaller than the alar, give origin to the tuber cinereum and the outgrowth which makes the infundibular process of the pituitary formation (see p. 1171). These structures are (Fig. 978) on the lower part of the posterior aspect of the fore-brain immediately behind the site of the optic outgrowth. Corpora mamillaria are formed from basal laminae just behind and above the tuber. The position of these structures is due to the length and curve of the mid-brain; when this gets relatively shorter, and the nasal fossae grow upwards from below, the fore-brain is rotated upwards to some extent, and the structures thus come to lie more below the third ventricle.
  
N. VII.
 
  
N. Amb. N. XII.
 
  
  
Fig. 961. — Plan of Extrapyramidal Fibres running in the Tegmentum to Nuclei of Cranial Nerves (modified from Dejerine).
+
Fig. 979.— Diagram showing Parts of the Fore-brain and Structures Derived from These.
  
 +
D, diencephalon; Tel, telencephalon; T, E, M, are thalamus, epithalamus, and metathalamus; H is hypothalamus; CV, cerebral vesicle; CS, corpus striatum.
  
  
 +
The optic outgrowths, although they are actually low down with reference to the fore-brain, are derivatives from its alar region, and the interlaminar sulcus (big. 978) reaches its lowest or most cephalic point between the optic pouch and the infundibulum. This sulcus is here the hypothalamic sulcus, but the continuation of the ‘ sulcus of Monro ’ towards the foramen is a secondary effec t produced by the growth of the thalamus.
  
 +
For development of the eye, see next chapter.
  
 +
The thalamus begins early in the second month as an enlargement in the anterior part of the dorsal lamina due to growth of the mantle cells. It increases rapidly and narrows the ventricle, so that in the fourth or fifth month the two bodies come into contact and fuse to a small extent, leaving as an attachment between them when they draw apart again the connexus thalami or massa intermedia.
  
  
  
  
 +
Fig. 980. — Schemes to show how the Corpus Striatum alters by its Growth the Shape of the Cavity.
  
 +
occ, occipital growth, with formation of posterior horn; i.c., line in which pyramidal motor fibres issue through corpus striatum.
  
  
 +
Later, in the fourth to fifth month, growth of the alar laminae farther back than the thalamic formation makes the pulvinar and geniculate bodies, and becomes secondarily continuous with the thalamus.
  
THE NERVOUS SYSTEM
+
Telencephalon. —This (Fig. 979) is an outgrowth or forward bulging of the terminal part of the thalamencephalon; it gives off on each side the two cerebral vesicles. These hollow vesicles have cavities widely open into that of the telencephalon, and thus into the third ventricle, of which the telencephalon makes the most anterior part ..
  
 +
The cerebral vesicles, growing fairly rapidly, stand up above the general level of the fore-brain, being separated from each other by the rudiment of the great longitudinal fissure. The direction of their main growth is upwards and backwards from the interventricular foramen, which is the opening from the telencephalon into a vesicle. They also grow forward for a little distance in front of the foramen, and of course their increasing size leads to an increasing prominence laterally. Thus they cover successively the diencephalon, the mid-bram, and finally the hind-brain and its formations, so that these are not to be seen from above. The vesicles begin to cover the mid-brain (Fig. 981) during the third month, and grow over the cerebellum in the fifth month. .
  
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The interventricular foramen does not increase in size pari passti with the growing brain, hence becomes relatively very small.
  
lobe. Ihey traverse the post lenticular part of the posterior limb of the internal capsule, and thereafter pass to the lower visual centres— namely, the lateral geniculate body and the upper quadrigeminal body. Within these bodies they terminate in arborizations around their component cells.
+
The anterior wall of the central unpaired or original telencephalic growth is the lamina terminalis. This is the direct path from one cerebral
  
Corticipetal Fibres. —The corticipetal or afferent fibres belong to the following tracts:
+
fn the other whence it becomes the path .
  
1. Medial lemniscus. 3. Thalamic radiation.
+
Ilong which commissural fibres between the hemispheres will proceed in their
  
2. Superior cerebellar peduncle. 4. Auditory radiation.
+
P " Thl V w fl S n a s S of'the cerebral vesicles are very thin at first, and thicken slowly. But the thkkening of the corpus striatum is visible at an early stage m each vesicle bednrdng to stand up into the cavity as a growthi ofthe floor and lower partof the outer wall. It grows rapidly, so that in the third month (Fig. 981)
  
5. Optic radiation.
 
  
The medial lemniscus, or principal sensory tract , arises from the nucleus gracilis and nucleus cuneatus of the medulla oblongata, and is the upward prolongation of the posterior column of the spinal cord. Having decussated with its fellow, it ascends through the dorsal part of the pons through the tegmentum of the crus cerebri, and through the subthalamic tegmental region to the thalamus. Within this body its fibres terminate in arborizations around the thalamic cells. As the medial lemniscus ascends towards the thalamus some of its fibres enter the upper quadrigeminal body, in which they end. From the thalamus the fillet-fibres are continued to the cerebral cortex by relays of thalamo-cortical fibres.
 
  
The superior cerebellar peduncle, having decussated with its fellow, soon comes into contact with the red nucleus. Many of the fibres of the peduncle enter this nucleus and terminate in arborizations around its cells. Numerous fibres encapsule the nucleus, and continue their course upwards, traversing the subthalamic tegmental region, and finally entering the ventral aspect of the thalamus, within which they terminate in arborizations around the thalamic cells. As in the case of the fillet-fibres, they are continued to the cerebral cortex by relays of thalamo-cortical fibres.
+
Fig. 981. —Brain of Embryo in Middle of Third Month seen from
  
The thalamic radiation is composed of thalamo-cortical fibres which arise as the axons of the cells within the thalamus, that body being regarded as an aggregation of cell-stations in the path of such corticipetal fibres as those of the medial lemniscus and superior cerebellar peduncle. These thalamo-cortical fibres, as stated in the description of the thalamus, issue from that body in four groups or stalks—frontal, parietal, occipital, and inferior or ventral. The fibres of the frontal stalk traverse the anterior limb of the internal capsule, and most of them pass to the cortex of the frontal lobe. The fibres of the parietal stalk pass partly through the internal capsule and partly through the external capsule to the cortex of the parietal lobe and of the central region of the frontal lobe. The fibres of the occipital stalk belong to the optic radiation, to be presently described. The fibres of the inferior or ventral stalk form the ansa lenticularis and ansa peduncularis. The ansa lenticularis enters the nucleus. lentiformis, within which its fibres terminate. the ansa peduncularis passes beneath the nucleus lentiformis and traverses the external capsule, the
+
ABOVE, THE UPPER PART of the Right Cerebral Vesicle being removed to expose the Corpus Striatum (CS), Thalamus (OT), Midbrain (MB).
  
  
1568
 
  
  
A MANUAL OF ANATOMY
 
  
  
destination of its fibres being the cortex of the temporal lobe and insula.
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it makes a marked prominence, while the greater part of the remaining wall of the vesicle is still thin. The corpus striatum, thus forming a floor for the lateral ventricle, is the cause, by its further growth, of alteration in the shape of that cavity. This is shown in Fig. 980. The mass, at first low, grows in an upward and backward direction, projecting into the cavity, and thus leading to the formation of an inferior horn ; the forward-turned end of this horn is the result of further growth of the body. The deep cleft seen on the inner side of the mass in Fig. 981 is obliterated by fusion following on growth of the corpus on one side and of the thalamus on the other, and the floor of the ‘ body ’ of the cavity is raised accordingly.
  
The auditory radiation consists of fibres which arise as the axons of the cells of the medial geniculate body. Having issued from that body, they traverse the postlenticular part of the posterior limb of the internal capsule, and pass to the cortex of the middle part of the first temporal gyrus of the temporal lobe.
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The corpus striatum is one of the primitive formations in the brain connected with its primitive functions. A section across the fore-brain and vesicles in the second month is represented diagrammatically in Fig. 982, where the thick mass of the corpus striatum is in contrast with the thin wall of the rest of the vesicle. This thin wall is the rudimentary neopallium, that part of the cerebrum which in man is associated with the higher functions of the brain, and will, when it begins to grow, completely overshadow in size the original portion. The result of neopallial growth is shown in the second diagram; the mass of the corpus is not affected, while the rapid increase of the area of the neopallium leads to its overlapping the inert striate mass. Overlapping cannot take place on the inner and basal aspects, but growth outwards and backwards and downwards
  
The corticipetal fibres of the optic radiation are associated with the corticifugal fibres, already described. The corticipetal fibres arise as the axons of the cells of the corpus geniculatum laterale and upper quadrigeminal body. They traverse the postlenticular part of the posterior limb of the internal capsule, and then pass to the cortex of the occipital lobe.
 
  
Commissural Fibres. —These fibres are disposed transversely, and serve to connect the grey cortex of one hemisphere with that of the other. They constitute the following commissures: (1) the corpus callosum ; (2) the anterior commissure ; and (3) the lyra, which is known as the hippocampal commissure. The fibres of the corpus callosum, as they enter each hemisphere, are disposed so as to form an extensive callosal radiation, and serve to connect the cortex of one hemisphere with that of the other. The individual portions of cortex so connected may be symmetrical, but to a large extent are not. The fibres arise on one side as (1) the axons of pyramidal or of polymorphous cells, or (2) collaterals of projection or of association fibres; and on the opposite side they terminate in delicate arborizations.
 
  
The anterior commissure, which crosses from side to side in front of the anterior pillars of the fornix, divides on either side into two parts, olfactory and temporal. The olfactory portion enters the olfactory tract. Some of its fibres serve to connect the olfactory bulb of one side with that of the other side; and other fibres connect the olfactory bulb of one side with the temporal lobe of the opposite. The temporal portion enters the white matter of the temporal lobe on either side.
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Fig. 982. —Schematic Sections to show how the Corpus Striatum (CS) is
  
The hippocampal commissure lies below the splenial portion of the corpus callosum, and is separated from the roof of the third ventricle by the tela chorioidea. It is a thin layer of arched fibres connecting the posterior pillars and sides of the fornix, and derived mainly from the hippocampus of each side; it is shown in Fig. 933. The commissure, which is not well developed in the human brain, was known as the ‘ lyra ’ in former days.
+
OVERLAPPED BY THE GROWTH OF THE NEOPALLIUM, AND IS DIVIDED INTO CAUDATE and Lenticular Parts by the Pyramidal Fibres. OT, thalamus.
  
The corpus callosum is the great commissure of the neopallium; the hippocampal and anterior commissures, phylogenetically much older, are connections of the rhinencephalon, and hence archipallial.
+
is unrestricted, so we find the surface area corresponding with the corpus striatum is overlapped by opercula from behind and above and in front. The surface area corresponding with the striate body is the insula, and the opercula covering it in make by their presence the lateral fissure.
  
Association Fibres. —These fibres serve to connect different parts of the cortex of the same hemisphere, and they are of two kinds, short and long.
+
Growth of the neopallial area implies formation of processes from its nervecells, and during the third month the pyramidal motor fibres begin to extend down toward the lower regions. They pass, as indicated in Fig. 982, through the corpus striatum to reach the marginal region beside the thalamus, and m doing so divide the corpus into caudate and lentiform masses, the caudate mass lying between them (1 internal capsule) and the ventricular cavity, the lentiform ganglion between them and the surface; hence the lentiform and caudate nuclei are always separated from one another by fibres of the internal capsule, and these fibres are always separated from the lateral ventricle by the caudate mass, and from the surface by the lentiform body. The mass of fibres, however, passing out of the striate body below and behind, come to separate the lenticular part here from the thalamus, with which they are coming into relation.
  
The short association fibres pass between neighbouring gyri, extending in their course across the bottom of the sulci. Some of them
+
The capsular fibres, passing through the corpus striatum, are affected by its upward growth, and thus make their passage and exit in a line (Fig. 980) curved like the surface of the striate body; this being so, it is easily understood that a section downwards, as along the arrow, would cut, in order from above, neopallium, cavity, caudate nucleus, internal capsule, lentiform nucleus, capsule, caudate nucleus, cavity, and finally neopallium again,
  
  
/
 
  
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In Fig. 982 the medial wall of the cavity of the vesicle is seen to remain thin. This wall is invaginated into the cavity as the ependymal covering of the choroid plexus of the lateral ventricle. The line of this thin wall lies just above the floor thickening of the corpus striatum, and is affected by the growth of this body, so that it assumes a curve corresponding with the curved shape of the ventricle, being invaginated into this throughout its length. This thin inner wall is continuous with the roof-plate of the third ventricle at the interventricular foramen, and the invaginations which cover the plexuses of the ventricles thus become continuous at this point.
  
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The anterior part of the hemisphere vesicle corresponds to the frontal lobe; the lower part, as far forwards as the stem of the lateral fissure, becomes the temporal lobe ; and the upper and posterior part represents the parietal lobe.
  
THE NERVOUS SYSTEM 1569
 
  
lie beneath the grey cortex, whilst others are contained within its deep part.
 
  
The long association fibres pass between portions of the grey cortex, which are at some distance from each other. They are arranged in bundles, the chief of which are as follows: (1) the superior longitudinal fasciculus; (2) the interior longitudinal fasciculus; (3) the perpendicular fasciculus; (4) the uncinate fasciculus; (5) the cingulum; (6) the occipitofrontal fasciculus; and (7) the fornix.
+
Fig. 983. — Side Views of Left Hemisphere at Beginning of Fifth and End of Sixth and Seventh Months (modified from Koli.mann).
  
The superior longitudinal fasciculus consists of fibres which extend from the frontal to the occipital lobe. Posteriorly many of its fibres sweep downwards and forwards into the temporal lobe, and from this circumstance it is sometimes spoken of as the arcuate fasciculus.
+
The occipital lobe is formed at a later period as the hemisphere grows backwards. The limbic lobe is developed in connection with the medial surface of the hemisphere. The olfactory lobe is developed as a hollow protrusion from the ventral aspect of the frontal portion of the hemisphere vesicle.
  
The inferior longitudinal fasciculus connects the occipital and temporal lobes, its fibres being disposed upon the lateral walls of the posterior and inferior horns of the lateral ventricle.
+
The surface of each hemisphere becomes very much broken up into gyri or convolutions, this being effected bv means of fissures. I he fissures are of two kinds—complete and incomplete/ The incomplete fissures are merely sulci produced by the growth of the gyri, and they do not involve the entire thickness of the walls of the cerebral hemispheres. The complete or primary fissures are infoldings of the walls of the cerebral hemispheres, and involve their entire thickness. They consequently give rise internally to certain prominences upon the wall of each lateral ventricle—namely, the lateral choroid plexus, hippocampus, calcar avis, and eminentia collaterals. The primary fissures are as follows:
  
The perpendicular fasciculus connects the inferior parietal lobule with the occipito-temporal gyrus.
+
Lateral. Calcarine (anterior part).
  
The uncinate fasciculus crosses the stem of the lateral fissure, and connects the frontal and temporal lobes.
+
Choroidal. Parieto-occipital.
  
The cingulum is connected with the rhinencephalon, and lies upon the under surface of the callosal gyrus and the upper surface of the hippocampal gyrus. Its fibres connect the gyri of the lobe with the cerebral cortex.
+
Hippocampal. Collateral (central part).
  
The occipito-frontal fasciculus connects the frontal with the occipital and temporal lobes. It lies internal to the corona radiata, in intimate relation to the nucleus caudatus, and as the fibres pass backwards they lie on the outer walls of the inferior and posterior horns of the lateral ventricle.
+
With the exception of the lateral fissure, already described, the other complete fissures appear on the medial surface of the vesicle of the cerebral hemisphere.
  
The fornix connects the hippocampus major of one side with the corresponding corpus mamillare, and through the latter with the thalamus by means of the mamillo-thalainic tract (bundle of Vicq d’Azyr), the fibres of which arise in the corpus mamillare.
+
The choroidal fissure is not really a fissure, but merely a groove or sulcus produced by an infolding of the vesicular wall, which is here composed entirely of ependyma. It commences above and behind the interventricular foramen of the corresponding side, and it terminates close to the tip of the temporal pole, where the inferior or descending horn of the lateral ventricle ends. Between these two points it passes backwards, downwards, and then forwards into the future temporal lobe in a crescentic manner, so as to embrace the stalk of the cerebral hemisphere. After the ependymal infolding has taken place, vascular mesenchyme dips in between its two folds, and so a plica chorioidea is formed. From this choroidal fold the lateral choroid plexus of the corresponding side is formed. This plexus projects into the lateral ventricle, but is excluded from the ventricular cavity by the ependyma of the wall, previously infolded. When the lateral choroid plexus is withdrawn, the thin ependymal covering of the plexus comes away with it, or is broken down. Under "these circumstances the choroidal fissure is really a fissure, inasmuch as the lateral ventricle now opens upon the surface through it. The internal prominence produced by the choroidal fissure is the lateral choroid plexus covered by ependyma, this prominence being verv conspicuous.
  
Peculiarities of the Cerebral Cortex-— 1. Calcarine Area.— This area is situated on the medial surface of the occipital lobe in close proximity to the calcarine fissure, and it is known as the visual area. In this region the outer band of Baillarger is very conspicuous, and is known as the white band of Gennari, whilst the inner band of Baillarger is absent.
+
The other complete fissures will be found described in connection with the cerebral hemispheres. All the primary fissures are formed before the fourth month.
  
2. Central Area. —In this region, more especially in the cortex of the precentral gyrus, there are groups of very large pyramidal cells, which are known as the giant-cells of Betz, and nerve-fibres are present in large numbers.
+
The incomplete fissures are very numerous. The first two to make their appearance are the calloso-marginal fissure, or sulcus cinguli, on the medial surface of the cerebral hemisphere, and the central sulcus, or fissure of Rolando, on the external surface. These, along with the other incomplete fissures, will be found described in connection with the cerebral hemispheres. - These are developed in the later foetal months, and inconstant tertiary fissures appear for years after birth.
  
3. Hippocampal Area. —The hippocampus corresponds to the hippocampal or dentate fissure, and is produced by an infolding of the cerebral cortex. It is therefore composed chiefly of grey matter, and is covered superficially by a thin layer of white matter, called the alveus, which is continuous with the fimbria. The hippocampus is composed of the following layers, named in order from the ventricular surface outwards: (1) the alveus, composed of white matter, and covered by the ventricular ependyma; (2) neuroglial layer, consisting of neuroglia fibres and cells; (3) pyramidal layer, composed of large pyramidal cells; (4) stratum radiatum, which is the outer part of the pyramidal layer, and is composed of the dendrites of the apical parts of the pyramidal cells, being thereby rendered striated in appearance; (5) stratum laciniosum, composed of the ramifications
+
The cavity of the vesicle of the cerebral hemisphere forms on either side the lateral ventricle, which is very much curtailed by the thickening undergone by the vesicular walls, and the internal prominences produced by the complete fissures. As the frontal lobe undergoes development the body of the ventricle extends forwards into it, and so the anterior horn of the ventricle is formed. As the occipital lobe becomes developed at a later period the body of the ventricle extends backwards into it, and so the posterior horn is formed. Meanwhile the interventricular foramen on either side, originally large, is being gradually much diminished in size.
  
99
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Basal Ganglia. —The basal ganglia of each cerebral hemisphere are as follows:
  
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Corpus striatum.
  
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Claustrum.
  
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Amygdaloid nucleus.
  
A MANUAL OF ANATOMY
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ihey are all developed from the deep part of the much thickened cortical substance which forms the floor of the lateral fossa. The claustrum and amygdaloid nucleus remain of small size, but the nucleus caudatus of the corpus striatum forms a conspicuous prominence in the lateral ventricle as it bulges into that cavity.
  
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Commissures. —The commissures are as follows:
  
of the foregoing apical dendrites intimately intermixed; (6) stratum granulosum, composed of many small cells; and (7) the involuted medullary lamina, consisting of white fibres.
+
1. Corpus callosum. 4. Anterior.
  
Olfactory Tract and Olfactory Bulb. —These are developed as a hollow outgrowth from the anterior cerebral vesicle, more particularly from the part of it which ultimately gives rise to the lateral ventricle, and is known as the telencephalon. In many animals the central cavity persists, and maintains its connection with the lateral ventricle; but in man the cavity disappears, though traces of its ependymal lining remain. External to the vestigial ependyma there is a layer of white matter, and superficial to this there is a layer of grey matter. In the olfactory tract the layer of grey matter is very thin over the ventral or inferior aspect, but over the dorsal or superior aspect it is fairly thick. In the bulb the reverse is the case, the grey matter being thick over the ventral aspect, where it receives the olfactory filaments, but thin over the dorsal aspect.
+
2. Fornix. 5. Posterior.
  
Structure of the Ventral Grey Matter of the Olfactory Bulb. —The
+
3. Hippocampal. 6. Middle.
  
grey matter consists of three layers—namely, (1) the nerve-fibre layer, (2) the glomerular layer, and (3) the granular layer.
 
  
The nerve-fibre layer is the most superficial layer, and is composed of olfactory nerve-fibres. These fibres are non-medullated, and arise as the axons of the olfactory cells of the olfactory mucous membrane of the nasal fossa. Having passed through the foramina of one half of the cribriform plate of the ethmoid bone, they enter the grey matter on the ventral aspect of the bulb, where they break up and form arborizations. These intermingle with the arborizations formed by the dendrites of the mitral cells, to be presently described.
+
7. Habenular.
  
The glomerular layer is composed of round bodies or glomeruli, which are formed by the interlacements between the arborizations of the olfactory nerve-fibres and those of the dendrites of the mitral cells.
 
  
The granular layer lies next to the layer of white matter, and is chiefly characterized by the presence of large mitral cells. These are pyramidal, and one dendrite from each cell passes into the glomerular layer, where it gives rise to a glomerulus in the manner just described in connection with the glomerular layer. Other dendrites intermingle with those of adjacent mitral cells. The axon of each mitral cell enters the white layer of the bulb, and passes along the olfactory tract to the cerebrum.
 
  
Weight of the Brain. —The average weight of the brain of the adult male is about 48 ounces (1,360 grammes), and that of the adult female about 44 ounces.
+
At an early period in the development of the cerebral hemispheres the interhemispherical (great longitudinal) fissure leads directly down to the roof of the diencephalon. At a later period the roof of the diencephalon is separated from the great longitudinal fissure by two commissures placed one above the other. The upper commissure is the corpus callosum, and the lower one is the fornix. These may be looked on as drawn back from the lamina terminalis by the growing vesicles, although this is not quite an accurate statement of their formation.
  
Arteries of the Encephalon.
 
  
Cerebral Part of the Internal Carotid Artery. —The internal carotid artery, having pierced the roof of the cavernous sinus internal to the anterior clinoid process of the sphenoid bone, ascends between the second and third cranial nerves to the inner end of the stem of the
 
  
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Fig. 984.— Schematic Figure to show the General Formation of the Main Commissures.
  
THE NERVOUS SYSTEM 1571
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The thalamus (OT) is supposed to be cut away in part, exposing the region of the corpus striatum; this has grown up "(producing the curved form of ventricle already described), and the choroidal fissure (, ch.f .) is therefore a curved line. Fibres of the internal capsule ( i.c.) also come through the mass of the corpus striatum in a curved line, and lie between the thalamus and lenticular part of corpus striatum. Hippocampal formations lie on the other side of the choroidal fissure, and association fibres here make the fimbria ( fimb .) and fornix (/). Commissural fibres cross the middle line and are cut at h (hippocampal commissure) and ac (anterior commissure) ; these are in the lamina terminalis. As the neopallium grows, its commissure, the corpus callosum, begins to form; it is at first in the lamina terminalis (cc.), but with increasing growth it extends forwards (A) and backwards (P). The rostrum, shown by the dotted line, is subsequently formed by fusion of the walls of the two'vesicles, the septum lucidum being that part of the wall between this and A. The backward extension carries with it the hippocampal commissure; these backward movements, involving also the hippocampus, are associated with great growth of the front part of the brain.
  
lateral fissure. Here it divides into its terminal branches, anterior and middle cerebral.
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Fornix—Anterior Commissure— These appear in the third month. In Fig 984 is shown the inner aspect of a cerebral vesicle in which the lamina terminalis is visible. Thickenings begin in this from fibres crossing between the olfactory and insular regions', these make the anterior commissure. . A little later fibres extend from each hippocampus to the sides of the lamina, where they turn into the side walls of the telencephalon, and reach much later the basal laminae of the thalamencephalon, constituting the anterior pillars of the fornix. The rest of the fornix is carried back with the growing vesicle.
  
Branches are posterior communicating, anterior choroidal, anterior cerebral, and middle cerebral.
 
  
The posterior communicating artery arises from the back part of the internal carotid, and passes backwards to anastomose with the
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Corpus Callosum. — Some time later, as the neopallium grows, its commissural fibres begin to become evident as the corpus callosum. These at first make use of the path already utilized by the earlier commissural fibres, and are found crossing at and above the upper part of the lamina terminalis as far back as the level of the interventricular foramen, as seen in the figure. As the neopallium grows, however, its commissural fibres become too numerous to be confined to this area, and their line of crossing extends backwards and forwards. The hinder extension necessarily lies with the fornix above the choroidal fissure, while the forward extension is between the two anterior expansions of the vesicles or hemispheres. These anterior fibres of the corpus callosum come from the frontal lobe above the anterior horn of the ventricle, hence that part of the wall of the hemisphere below these fibres is the medial wall of this horn. The hemispheres become approximated and fused below this small area of medial wall as the result of growth of neopallium round it, and commissural fibres now find their way through the line of fusion below the medial wall of the anterior horn. This makes the rostrum, and its continuity in front and above with the original forward extension of the corpus callosum cuts off the two areas of medial wall from the rest of this wall in the longitudinal fissure; they now form the two layers of the septum lucidum, and the cavity between them is only the corresponding part of the space of the fissure, similarly cut off. The fusion between the medial walls is not confined to the region just considered, but is found behind this in the neighbourhood of the callosal fibres and the fornix; this explains the posterior extension of the region of the septum lucidum.
  
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The hippocampal commissure appears on the back part of the ventral aspect of the plate formed by the fused areas of the cerebral hemispheres, to which position it has been carried by the backgrowth of the corpus callosum. Its fibres pass across from one hippocampus to the other, and they correspond to the region known as the lyra or psalterium.
  
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The posterior commissure is formed in connection with the back part of the roof of the diencephalon behind the pineal diverticulum.
  
Fig. 962.—Arteries on the Base of the Brain.
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The so-called middle commissure is not a commissure properly so called, but is formed by the fusion over a limited area of the grey matter of the medial surfaces of the thalami, and properly termed connexus thalami.
  
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The habenular commissure is produced by the decussating fibres of the thalamic stria?, these fibres, as they decussate, forming the dorsal part of the pedicle of the pineal body.
  
posterior cerebral artery. It is usually small, but is often larger on one side than the other. Occasionally it is absent.
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Meninges of the Encephalon. —The walls of all the cerebral vesicles are invested by mesoderm, and this tissue becomes differentiated into the three meninges—namely, the dura mater, arachnoid, and pia mater.
  
The anterior choroidal artery arises from the back part of the internal carotid close to its termination. It passes backwards and outwards between the crus cerebri and the "hippocampal gyrus, and enters the lower and anterior extremity of the descending horn of the
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Choroid Plexuses. —The choroid plexuses of the two lateral, third, and fourth ventricles are developed as infoldings of the ependymal walls of the ventricles. Vascular mesenchyme (mesoderm) dips in between the two layers of each infolding, and in this manner plicce c-horioidece are formed. These choroidal folds give rise to the choroid plexuses, which as they project into the ventricles carry the ependymal walls, already infolded, before them.
  
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Tela Chorioidea. —The vascular mesoderm ( pia mater) investing the neural tube is converted into a double layer interposed between fore-brain and cerebral vesicles as a result of the backgrowth of the latter. It is clear that this velum interpositum extends to the interventricular foramen, where its two layers are continuous, and where vessels of the cerebral layer can join those of the earlier one. The dotted lines in Fig. 982 show the position of these two layers on section; it can be seen that the ‘ cerebral ’ layer reaches out to the thin medial wall of the vesicle, and its marginal vessels can invaginate this to form the choroid plexus of the lateral ventricle, while the deeper layer rests on the roof of the third ventricle, and makes its choroid plexus. The continuity of the two layers at the foramen explains why the lateral vein runs there to join the internal cerebral vein, which is in the lower layer.
  
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==Development of the Peripheral Nervous System==
  
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The peripheral nerves are arranged in two groups—namely, spinal, which are derived from the spinal cord; and cranial, which arise from the brain.
  
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The spinal nerves are composed of two kinds of fibres—efferent, centrifugal, or motor; and afferent, centripetal, or sensory.
  
A MANUAL OF ANATOMY
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A motor spinal nerve-fibre arises as the axon of a neuroblast or nerve-cell in the mantle layer of the neural tube (see Development of the Spinal Cord).
  
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A sensory spinal nerve-fibre is developed from a cell of a spinal ganglion, and these ganglia are developed from the corresponding neural crest.
  
U572
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Neural Crests. — The neural or ganglionic crests, right and left, are ridges of ectodermic cells which lie on either side of the neural tube. They are derived from a single crest of ectoderm, which is formed by the fusion of the ectoderm over each neural fold, this single crest being situated medially on the dorsal aspect of the neural tube along che line of fusion of the neural folds to close the tube. Subsequently the medial crest divides into right and left halves, which cover the dorsolateral aspects of the neural tube.
  
lateral ventricle by passing through the choroidal fissure. In its course it passes just above the uncus, gives twigs to the hippocampal gyrus and crus cerebri, and terminates in the choroid plexus of the lateral ventricle.
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Each {{neural crest}} becomes broken up into a number of segments, or ganglia, there being four pairs for the head region, and thirty-one pairs for the region of the trunk.
  
Anterior Cerebral Artery. —This is the smaller of the two terminal branches of the internal carotid artery, and it has a more limited distribution than the other terminal branch—namely, the middle cerebral artery. It passes forwards and inwards above the optic nerve, and just internal to the roots of the olfactory tract, to the commencement of the great longitudinal fissure, where it is connected with its fellow of the opposite side by a short transverse vessel, called the anterior communicating artery. After this it enters the great longitudinal fissure, turns round the genu of the corpus callosum, and passes back
 
  
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Spinal Ganglia. — The spinal ganglia are arranged in thirtyone pairs, right and left.
  
Fig. 963.—Median Section of Brain with Distribution of Anterior
 
  
Cerebral Artery.
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Each cell of a ganglion acquires two poles — afferent or centripetal, and efferent or centrifugal—and at this stage it is consequently a bipolar cell.
  
wards over the upper surface of that body to the splenium, where it anastomoses with the posterior cerebral artery.
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The centripetal or proximal pole, which is the axon of a ganglionic cell, grows into the dorsal part of the wall of the neural tube and forms part of the dorsal or posterior root of a spinal nerve. Within the marginal layer of the neural tube the centripetal pole or nerve-fibre divides into tw r o branches, ascending and descending, which give off collaterals and terminate in arborizations. The centrifugal or distal pole joins the ventral or anterior nerve-root of the same segment of the spinal cord on the distal side of the ganglion to form a spinal nerve.
  
Branches .—These are arranged in two groups, antero-medial or ganglionic and cortical.
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Though the cells of a spinal ganglion are originally bipolar, they become m the course of growth unipolar, the single pole having a T-shape. d his is brought about by an excessive growth of one wall of the bipolar cell, which biings the two original poles into contact, when they fuse, and are now connected with the cell by one stalk or pole, which divides into a centripetal and a centrifugal process.
  
The antero-medial or ganglionic branches, small in size, pass through the lamina cinerea along with twigs from the anterior communicating artery, and supply the front part of the caudate nucleus.
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Whilst the fibres of the dorsal or posterior roots of the spinal nerves grow into the mantle layer of the neural tube from the cells of the spinal ganglia, the fibres of the ventral or anterior roots arise within the mantle lacei as the axons of its neuroblasts or nerve-cells. The fibres of the anterior roots therefore grow out from the neural tube.
  
The cortical branches are as follows: (1) medial orbital, to the medial orbital gyrus, olfactory lobe, and gyrus rectus; (2) anterior medial frontal, to the superior frontal gyrus, the anterior two-thirds of the middle frontal gyrus, and the anterior part of the marginal gyrus; (3) middle frontal, to the collosal gyrus, the posterior part of the marginal gyrus, and the upper part of the precentral gyrus; and (4) posterior
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Cranial Nerves. _The development of the cranial nerves, with the exception
  
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of the olfactory and optic nerves, corresponds for the most part with the development of the spinal nerves. The motor cranial nerve-fibres arise as the axons of nerve-cells of the brain, and groiv into the brain, whereas the sensory cranial nerve-fibres grow into the brain from the cells of the cephalic ganglia.
  
  
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Neural Crest (Ectoderm)
  
THE NERVOUS SYSTEM
 
  
  
1573
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Fig. 985. — Development of the Neural or Ganglion Crest (Keibel and Mall) (after Von Lenhossek and Kollmann).
  
  
frontal, to the precuneus and the corpus callosum, the branch of the latter being known as the artery of the corpus callosum.
 
  
Anterior Communicating Artery. —This vessel connects the two anterior cerebral arteries at the entrance to the great longitudinal fissure, and lies over the lamina terminalis in front of the optic commissure. It gives off a few twigs, which accompany the antero-medial branches of each anterior cerebral artery.
 
  
Middle Cerebral Artery. —This is the larger of the two terminal branches of the internal carotid artery, and it has a wider distribution than the anterior cerebral artery. It enters the stem of the lateral fissure, in which it passes outwards.
 
  
Branches .—These are arranged in two groups, antero-lateral or ganglionic and cortical. The antero-lateral or ganglionic branches
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Cephalic Ganglia. — The cephalic ganglia, like the spinal ganglia, are developed from the neural crests, and they constitute four pairs of ganglionic groups— namely, trigeminal, acoustico-facial, glosso-pharyngeal, and vagal. They are all comparable to the spinal ganglia.
  
  
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The trigeminal ganglion is connected with the sensory root of the fifth cranial nerve. The centripetal poles of its nerve-cells pass inwards into the brain, forming the large sensory root of the nerve, and the centrifugal poles of its cells pass peripherally, forming the ophthalmic, superior maxillary, and sensory part of the inferior maxillary nerves. The trigeminal ganglion is thus clearly comparable to a spinal ganglion. The small motor root of the fifth nerve is homologous to the motor or anterior root of a spinal nerve, inasmuch as its fibres arise as the axons of nerve-cells within the brain.
  
Pig. 964._Distribution of the Left Middle Cerebral Artery (Charcot).
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The acoustico-facial ganglion resolves itself into facial and acoustic parts.
  
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The facial ganglion, known as the geniculate ganglion, is connected with the genu of the facial nerve in the aqueduct of Fallopius. The centripetal poles of the nerve-cells of this ganglion form the sensory root of the facial nerve—the pars intermedia of Wrisberg —which passes inwards to the fasciculus solitarius and upper part of the glosso-pharyngeal nucleus. Many of the centrifugal poles of the cells issue from the ganglion as the chorda tympani nerve, which passes to the anterior two-thirds of the tongue as a nerve of special sense.
  
F.i. Superior Frontal Gyrus F.2. Middle Frontal Gyrus F.3. Inferior Frontal Gyrus F.A. Ascending Frontal Gyrus P.A. Ascending Parietal Gyrus L.P.S. Superior Parietal Gyrus L.P.I. Inferior Parietal Gyrus P.C. Angular Gyrus L.O. Occipital Lobe
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The acoustic ganglion is represented by the vestibular and cochlear ganglia in connection with the auditory nerve.
  
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The vestibular ganglion is connected with the vestibular division of the auditory nerve in the internal auditory meatus. As in the other ganglia, the centripetal poles of the cells of this ganglion form the centripetal fibres of the vestibular nerve, which pass inwards to the brain. The centrifugal poles of the cells leave the ganglion, and form the peripheral part of the nerve as regards its distribution.
  
L.T. Temporal Lobe
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The cochlear ganglion, known as the ganglion spirale, is connected with the cochlear division of the auditory nerve, and is situated in the spiral canal of the modiolus. Its nerve-cells are related to nerve-fibres, as in the case of the vestibular ganglion.
  
S. Middle Cerebral Artery entering Lateral Fissure
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The glosso-pharyngeal ganglion, which is broken up into a superior (jugular) ganglion and an inferior (petrous) ganglion, is comparable to a spinal ganglion. The centripetal poles of the nerve-cells, which issue from the ganglion, represent the centripetal sensory fibres of the glosso-pharyngeal nerve, passing into the brain. The centrifugal poles of the nerve-cells, issuing from the ganglion, represent the peripheral sensory fibres of the nerve.
  
P. Lenticulo-striate Arteries
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The vagal ganglion, which is broken up into the upper ganglion (of the root) and the lower ganglion (of the trunk), is disposed towards the sensory fibres of the vagus nerve, as in the case of the other ganglia.
  
1. Artery to Inferior Frontal Gyrus
 
  
2. Ascending Frontal Artery
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==Meninges of the Encephalon==
  
3. Ascending Parietal Artery
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===Dura Mater===
  
4. Parieto-Temporal Artery
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The dura mater is a strong fibrous membrane which surrounds the encephalon, and is composed of two layers—outer and inner. The outer or endosteal layer serves as the internal periosteum or endocranium of the cranial bones, and the inner or sustentacular layer supports the encephalon. It is more firmly adherent to the bones forming the base of the skull than to those over the cranial vault, and it is also firmly attached along the course of the sutures. At the various openings it is prolonged outwards, blending with the sheaths of the transmitted nerves, and also becoming continuous with the external periosteum or pericranium. At the sphenoidal fissure it passes into the orbit to form the orbital periosteum. At the lower margin of the foramen magnum the two layers of which the dura mater is composed separate. The external layer blends with the periosteum of the occipital bone around the margin of the foramen magnum. The internal layer is prolonged into the spinal canal, and forms the theca of the spinal cord. The outer surface of the dura mater is rough and flocculent, owing to fibrous processes which connect it to the inner surfaces of the cranial bones. Its inner surface is smooth and covered by endothelium. Superiorly, on either side of the superior longitudinal sinus there are several small granular nodules, called arachnoid granulations, which are best marked in old age. They indent the parietal bone, and protrude into the superior longitudinal sinus, carrying with them prolongations from the endothelial lining of the sinus, which separate them from the blood.
  
5. Arteries to Temporal Lobe
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The bodies are enlargements of the villi of the arachnoid membrane (see p. 1609).
  
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Structure. — The dura mater consists of fibrous and elastic tissues arranged as parallel bundles.
  
pass through the anterior perforated substance, and foim two sets, medial striate and lateral striate. The medial striate arteries repiesem the ‘ lenticular arteries * of Duret, and they supply the globus pallidus (inner part) of the lentiform nucleus, the internal capsule, and the caudate nucleus. Ihe lateral striate arteries represent the. lenticulostriate ’ and ‘ lenticulo-optic ’ arteries of Duret. The lenticulo-striate arteries supply the putamen (outer part) of the lentiform nucleus and the external capsule. One of the lenticulo-striate aiteries is said to be larger than the others, and is subject to rupture in cases of cerebral haemorrhage. It is often termed the artery of cerebral hcemorrhage (Charcot). Its course is laterally and upwards round the outer aspect of the lentiform nucleus, between it and the external capsule, and then through the internal capsule to the caudate nucleus. The lenticulo
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The intracranial dura mater differs from the dura mater of the spinal cord in the following respects: (1) it consists of two layers— outer or periosteal, and inner or sustentacular—whereas the spinal dura mater has only one layer, representing the sustentacular layer; (2) it furnishes certain processes or septa, which project into the cranial cavity, and separate parts of the encephalon from each other, whereas the spinal dura mater sends no septa into the spinal cord; and (3) it contains venous sinuses, which are absent in the spinal dura mater, or are represented by the extradural venous plexuses (see p. 1434) •
  
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Subdural Space. — This is the interval between the dura mater and the arachnoid membrane. There is really no space, but simply sufficient interval to contain a minute quantity of serous fluid for lubricating purposes. The dura mater and the arachnoid are therefore practically in contact with each other.
  
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Blood-supply. — The cranial dura mater is supplied by the meningeal arteries, which are extradural and supply the inner table of the cranial bones. These vessels are very numerous, and the chief are as follows on either side, from before backwards: (1) anterior meningeal, two in number, from the anterior and posterior ethmoidal arteries; (2) meningeal, from the cavernous part of the internal carotid artery; (3) small meningeal, from the middle meningeal, or from the first part of the maxillary artery; (4) middle meningeal, from the first part of the internal maxillary; (5) meningeal branches of the ascending pharyngeal artery, (6) posterior meningeal branch of the occipital artery; and (7) posterior meningeal, from the vertebral artery.
  
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The anterior meningeal branch of the anterior ethmoidal artery arises from that vessel as it accompanies the nasal nerve on the cribriform plate of the ethmoid bone, and it takes part in the supply of the dura mater of the anterior fossa
  
1574
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The anterior meningeal branch of the posterior ethmoidal artery arises from that vessel after it has entered the cranial cavity through a minute foramen between the cribriform plate of the ethmoid and the sphenoid. It has a limited distribution to the dura mater in this region.
  
  
A MANUAL OF ANATOMY
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The meningeal branch of the internal carotid artery arises from the cavernous part of that vessel, and enters the middle fossa to supply the dura mater.
  
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The small meningeal artery is usually a branch of the middle meningeal, but it may arise from the first part of the maxillary artery. It enters the cranial cavity through the foramen ovale, and supplies the adjacent dura mater and the trigeminal ganglion.
  
thalamic arteries supply the posterior part of the lentiform nucleus and the anterior part of the thalamus on its lateral aspect.
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The middle meningeal artery, as stated, is a branch of the first part of the maxillary artery. Its diameter is that of the foramen spinosum, through which it enters the cranial cavity. After passing into the cranium it divides into two branches, anterior and posterior. The anterior branch passes forwards, outwards, and upwards in a groove on the upper surface of the great wing of the sphenoid bone to the inner aspect of the antero-inferior angle of the parietal bone, where there is a groove, or sometimes a short canal. The position of the middle meningeal artery at this level is ascertained by taking a point on the exterior of the skull ii inches behind the zygomatic process of the frontal bone and inches above the zygomatic arch. From this point the artery ascends in a
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branching meningeal groove near the anterior border of the parietal bone as high as the superior longitudinal sinus. In this part of its course it furnishes numerous branches forwards and backwards.
  
The most important point to notice about all the ganglionic arteries of the brain is that they are ‘ end arteries/ and their branches, once having divided, never anastomose again; hence, if one of them is blocked by an embolus, which is often a piece of fibrin from a diseased heart valve, the area of brain supplied will be cut off from all bloodsupply, and the clinical effects may be very grave.
 
  
The cortical branches arise in the vicinity of the insula, and are: (i) inferior lateral orbital, to the orbital surface of the frontal lobe lateral to the internal orbital sulcus, and to the inferior frontal gyrus;
 
  
  
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Fig. 986. — The Veins of the Diploe.
  
Fig. 965.- —Coronal Section of the Cerebral Hemispheres made One Centimetre behind the Optic Commissure (Charcot, from Duret).
 
  
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The outer tables of the cranial bones have been removed.
  
The arteries of this region are shown.
 
  
  
I. Area of Cerebral Artery
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The posterior branch passes backwards on the squamous part of the temporal bone, and then on to the inner aspect of the inferior border of the parietal bone, where there is a meningeal groove about the centre. From this point it ascends in a branching groove as high as the superior longitudinal sinus, giving off branches forwards and backwards.
  
II. Area of Middle Cerebral Artery
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The distribution of the middle meningeal artery extends as high as the superior longitudinal sinus forwards on to the frontal bone, and backwards on to the occipital bone. Besides supplying the dura mater and the inner table and diploe of the bones, the vessel furnishes the following branches: (1) Ganglionic to the trigeminal ganglion; (2) a petrosal brcinch, which passes through the hiatus (hallopii) to supply the facial nerve in its canal, and anastomose with the stylo-mastoid branch of the posterior auricular artery; and (3) an orbital branch, which enters the orbit through the sphenoidal fissure, and anastomoses with the ophthalmic artery.
  
III. Area of Posterior Cerebral Artery V.V. Sections of Anterior Cornua of Lateral Ventricles
 
  
P.P. Anterior Pillars of Fornix C.N. Caudate Nucleus L.S.A. Lenticulo-Striate Arteries CL. Claustrum I.R. Insula E.C. External Capsule
 
  
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The meningeal branches of the ascending pharyngeal artery, which is a branch of the external carotid, are three in number: One passes through the foramen lacerum into the middle fossa; another passes through the jugular foramen into' the posterior fossa; and the third passes through the anterior condylar foramen, likewise into the posterior fossa.
  
L. N. Lentiform Nucleus I.C. Internal Capsule O.T. Optic Tract (cut)
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The posterior meningeal branch of the occipital artery passes through the jugular or mastoid foramen into the posterior fossa.
  
G.M. Grey Matter of Third Ventricle O.C. Optic Chiasma O.N. Optic Nerve A.C. Anterior Cerebral Artery C. Internal Carot'd Artery
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The posterior meningeal branch of the vertebral artery passes through the foramen magnum into the cerebellar fossa of the occipital bone.
  
M. C. Middle Cerebral Artery L.A. Lenticular Arteries
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As a rule, only one of these various posterior meningeal arteries is large enough to carry injection. .#•
  
A.C.H. Artery of Cerebral Hemorrhage
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Meningeal Veins. —The sinuses or veins with the middle meningeal artery are two in number. They pass through the foramen ovale, and terminate in the pterygoid plexus of veins. The other meningeal veins are disposed in one of two ways. Some of them accompany the corresponding arteries and terminate in extracranial veins; whilst others end in the various intracranial venous sinuses, in part directly and in part by means of venous lacunae.
  
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Veins of the Diploe. —These vessels are situated in the cancellous tissue between the outer and inner plates of the cranial bones, and are exposed after removal of the outer plate. They are destitute of valves, and are arranged in the form of a network, from which the blood is returned by four diploic veins on either side—namely, frontal, anterior temporal, posterior temporal, and occipital. These terminate partly in extracranial veins, and partly in the intracranial venous sinuses and meningeal veins.
  
(2) ascending frontal, to the posterior third of the middle frontal gyrus, and to the lower and greater part of the precentral gyrus; (3) ascending parietal, to the postcentral gyrus and superior parietal gyrus; (4) parietotemporal, which traverses the posterior horizontal limb of the lateral fissure, and supplies the inferior parietal lobule and the posterior parts of the superior and middle temporal gyri; and (5) the temporal branches, which emerge from the posterior horizontal limb of the fissure, and supply the anterior and greater parts of the superior and middle temporal gyri.
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The frontal diploic vein passes downwards and escapes through an opening in the outer plate of the frontal bone at the supra-orbital notch, where it joins the communicating vein which passes between the supra-orbital and ophthalmic veins. It receives radicles from the frontal air-sinus of the same side.
  
Fourth or Intracranial Part of the Vertebral Artery. —The vertebral artery, on leaving the suboccipital triangle, pierces the dura mater and arachnoid, and enters the cranial cavity through the foramen
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The anterior temporal diploic vein is confined to the back part of the frontal and anterior part of the parietal regions, and descends to terminate in two ways. It partly joins one of the extracranial deep temporal veins by passing through an opening in the outer plate of the great wing of the sphenoid, and in part it ends in a meningeal vein, or in the spheno-parietal, or it may be the cavernous sinus.
  
  
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The posterior temporal diploic vein is confined to the parietal region, and descends to the postero-inferior angle of the parietal bone, where it pierces the inner plate of that bone, and terminates in the transverse sinus..
  
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The occipital diploic vein occupies the occipital region, and pierces the inner table of the occipital bone, to terminate in the transverse or sigmoid sinus.
  
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Nerves of the Dura Mater— The dura mater receives nerves from (i) the sympathetic plexuses which accompany the arteries; (2) the three divisions o the fifth cranial nerve; (3) the ganglion of the root of the va gus nerve , an (4) the hypoglossal nerve. Headache is said to be due to irritation of these nerves, especially to one of the branches of the fifth, known as the nervus tentorn.
  
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Processes of the Dura Mater.— It has been already explained that the dura mater is composed of two layers outer or periosteal, and inner or sustentacular. The processes are formed by the inner or sustentacular layer, and are four in number the faix cerebri, tentorium cerebelli, falx cerebelli, and diaphragma sellse.
  
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The falx cerebri is an extensive sickle-shaped process, which occupies the great longitudinal fissure, where it lies between the two cerebral hemispheres. Anteriorly it is almost pointed, and is attached to the crista salli of the ethmoid bone. Posteriorly it is broad, and is attached to the upper surface of the tentorium cerebelli along the median line the straight sinus being situated at the place of junction The supenor border is convex, and is attached m the median line to the frontal, parietal, and occipital bones, extending upon the latter bone only as low as the internal occipital protuberance. The superior sagittal sinus is situated along this border. The inferior border is concave and free. It overhangs the upper surface of the corpus callosum, from which it is separated by a slight interval. The inferior sagittal sinus is situated within this border. The lateral surfaces face the medial surface of the cerebral hemispheres.
  
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The tentorium cerebelli is an extensive crescentic sheet which covers the cerebellum. Superiorly it supports the posterior parts of the cerebral hemispheres, and is elevated along the median line, whence it slopes towards the attached borders. Anteriorly it presents a free,
  
  
  
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Fig. 987.—The Falx Cerebri, Tentorium Cerebelli, and Venous Sinuses
  
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of the Dura Mater (Left View).
  
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i- Superior Sagittal Sinus 4. Transverse Sinus
  
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2. Inferior Sagittal Sinus 5. Superior Petrosal Sinus
  
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3. Straight Sinus 6. Internal Jugular Vein
  
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sharp, concave border, which forms, with the dorsum sellae of the sphenoid bone anteriorly, an oval opening, called the foramen ovale tentorii , within which the mesencephalon is placed. Posteriorly and laterally the tentorium cerebelli is convex, and is attached as follows, from behind forwards: (i) to the horizontal ridge on the inner surface of the tabular part of the occipital bone, where the process contains the transverse sinus; (2) to the inner aspect of the postero-inferior angle of the parietal bone, where the process also contains this sinus; and (3) 1° the superior border of the petrous part of the temporal bone, where the process contains the superior petrosal sinus. Close to the apex of th e pars petrosa the outer and anterior borders of the tentorium cerebelli cross, the outer border passing inwards to be attached to the posterior clinoid process of the sphenoid bone, and the anterior border passing forwards to be attached to the anterior clinoid process of that bone.
  
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In all the carnivora the tentorium is ossified.
  
  
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The falx cerebelli extends forwards into the posterior cerebellar notch, where it lies between the two cerebellar hemispheres. It is falciform in outline. Superiorly it is attached to the posterior part of the inferior surface of the tentorium cerebelli in the median line. Posteriorly it is attached to the internal occipital crest, and this portion bifurcates interiorly, the two divisions being connected to the lateral margins of the vermiform fossa. Along this posterior attachment it contains the occipital sinus and its two divisions. Anteriorly it ends in a free border.
  
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In the ornithorhynchus and many cetacea the falx is ossified.
  
  
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The diaphragma sellse is a small circular fold horizontally placed, which forms a roof for the sella turcica or hypophysial fossa of the sphenoid bone, and almost entirely covers the hypophysis. At its centre there is a small opening for the passage of the infundibulum.
  
THE NERVOUS SYSTEM
+
Venous Sinuses of the Dura Mater.—These are blood-channels or spaces situated between the two layers of the dura mater, and lined with endothelium. They are as follows:
  
  
1575
+
Superior sagittal sinus. Inferior sagittal sinus. Straight sinus.
  
 +
Transverse sinuses (two). Occipital sinus. Cavernous sinuses (two).
  
magnum. As it ascends, it lies at first on the side of the medulla oblongata, between the hypoglossal nerve and the anterior root of the suboccipital nerve. It then passes upwards on the ventral surface of the medulla, and on reaching the lower border of the pons it unites with its fellow of the opposite side to form the basilar artery. It will generally be found that one vertebral artery is larger than the other.
 
  
Branches. —Posterior meningeal, posterior spinal, posterior inferior cerebellar, anterior spinal, and bulbar.
+
Spheno-parietal sinuses (two). Circular sinus.
  
The posterior meningeal artery arises from the vertebral artery just before it pierces the dura mater, and it enters the cerebellar fossa of the occipital bone where it supplies the dura mater.
+
Superior petrosal sinuses (two). Inferior petrosal sinuses (two). Basilar sinus.
  
The posterior spinal artery arises from the vertebral artery immediately after it has pierced the dura mater. It descends upon the side of the spinal cord in front of the posterior roots of the spinal nerves, and it gives off a branch which descends behind these roots. These two arteries, in themselves small, are reinforced by the spinal branches of the second part of the vertebral artery and of the dorsal branches of the intercostal arteries. In this manner lateral anastomotic arterial chains are formed upon each side of the spinal cord in front of and behind the posterior nerve-roots.
+
Petro-squamous sinuses (two).
  
The posterior inferior cerebellar artery, of large size, arises a little above the preceding branch. It passes backwards between the vagus and accessory nerves, and then over the restiform body to the vallecula of the cerebellum, where it divides into branches. Some of these supply the inferior vermis, and others ramify on the inferior surface of the cerebellar hemisphere, at the periphery of which they anastomose with branches of the superior cerebellar artery. The artery furnishes branches to the corresponding choroid plexus of the fourth ventricle and to the medulla oblongata.
 
  
The anterior spinal branch of the vertebral artery arises from that vessel near its termination. It passes obliquely downwards and inwards over the ventral aspect of the medulla oblongata, and at the median line it unites with its fellow of the opposite side to form the anterior spinal artery of the spinal cord. It furnishes twigs to the medulla oblongata.
+
The superior sagittal sinus is situated in the median line within the superior convex border of the falx cerebri. It extends from the crista galli of the ethmoid bone to the internal occipital protuberance, where as a rule it turns sharply to the right, and opens into the right transverse sinus. It sometimes, however, turns to the left, and opens into the left transverse sinus. Its posterior extremity is dilated, and forms the confluens sinuum, which usually occupies a depression on the right side of the internal occipital protuberance, and is connected with the dilatation at the posterior extremity of the straight sinus by a transverse vessel. It increases in size as it passes backwards, and its shape is triangular in section, the base being directed towards the cranial vault. The apex is directed downwards, and in this region the sinus is crossed by a number of delicate fibrous bands. Opening into the sinus on either side there are venous spaces, called lacunae laterales, which are situated within the dura mater, and projecting into these lacunae from below, or into the sinus itself, there are seveial arachnoid granulations, covered by the endothelial lining. The sinus receives the superior cerebral veins and some of the meningeal veins of the falx cerebri. The former open into it from behind forwards, so far at least as the more posterior vessels are concerned, so that the blood-flow in these veins is opposed to the current of blood in the sinus, which is from before backwards.
  
The bulbar branches are distributed to the medulla oblongata.
+
The superior sagittal sinus sometimes communicates anteriorly with the veins of the roof of the nose through the foramen ccecum, and
  
Basilar Artery. —This vessel is formed by the union of the two vertebral arteries. It extends from the lower to the upper border of the pons, occupying the median basilar groove on its ventral surface. It lies deep to the arachnoid membrane within the cisterna pontis, and at the upper border of the pons it divides into the two posterior cerebral arteries.
 
  
Branches. —These are as follows, on either side: transverse, internal auditory, anterior inferior cerebellar, superior cerebellar, and posterior cerebral.
+
Frontal Air-Sinus
  
The transverse or pontine arteries are numerous, and pass outwards on either side to supply the pons, the sensory and motor roots of the fifth cranial nerve, and the middle peduncle of the cerebellum.
 
  
  
 +
Anterior Fossa
  
 +
Cerebellar Fossa
  
1576 A MANUAL OF ANATOMY
 
  
 +
Accessory Nerv Hypoglossal Nerve
  
The internal auditory artery, long and slender, accompanies the auditory nerve through the internal auditory meatus, and is distributed to the internal ear.
 
  
The anterior inferior cerebellar artery arises from the centre of the basilar, and passes backwards to be distributed to the anterior part of the inferior surface of the cerebellar hemisphere. It anastomoses
+
Spinal Cord ' Occipital Sinus
  
with the posterior inferior
 
  
 +
Optic Nerve ^ Ophthalmic Artery
  
cerebellar artery, which is a branch of the vertebral.
 
  
The superior cerebellar artery arises from the basilar close to its termination. It passes outwards parallel to the posterior cerebral artery, from which it is separated by the third cranial nerve. It then winds round the outer side of the crus cerebri below the fourth cranial nerve, and so reaches the superior surface of the cerebellar hemisphere, where it divides into branches. These supply the superior vermis, the upper medullary velum, the tela chorioidea, and the superior surface of the cerebellar hemisphere, at the periphery of which they anastomose with branches of the inferior cerebellar arteries.
+
Third Nerve
  
Posterior Cerebral Artery. —This is one of the terminal branches of the basilar artery at the upper border of the pons. It passes at first outwards beneath the crus cerebri, and parallel to the superior cerebellar artery, from which it is separated by the third cranial nerve. It then winds round the outer side of the crus cerebri, lying between it and the hippocampal gyrus, and above the fourth cranial nerve. In this manner it reaches the tentorial or inferior and medial surfaces of the occipital lobe of the cerebral hemisphere. It receives, not far from its commencement, the posterior communicating artery.
+
Fourth Nerve
  
Branches .—These are arranged in three groups—ganglionic, choroidal, and cortical.
+
Region of Meckel’s Cave
  
The ganglionic group includes two sets of branches, postero-medial and postero-lateral.
+
Fifth Nerve
  
 +
Facial Nerve’
  
Fig. 966. —The Areas of Distribution on the Surface of the Three Main Cerebral Arteries.
+
Auditory Nerve
  
 +
Glosso-pharyngeal Nerve
  
 +
Vagus Nerve
  
  
 +
Transverse Sinus
  
  
 +
Cerebral Fossa
  
 +
^ Confluens Sinuum
  
  
 +
Border of Small Wing ✓ ' of Sphenoid
  
  
 +
Ant. Pt. of Circ. Sinus ^ (Ant. Intercav. Sinus)
  
THE NERVOUS SYSTEM 1577
+
Hypophysis
  
The postero-medial ganglionic arteries pass medial to the crus cerebri, and pierce the posterior perforated substance. They supply the inner part of the crus cerebri and the posterior part of the thalamus.
 
  
The postero-lateral ganglionic arteries pass on the lateral side of the crus cerebri, and supply the outer part of the crus, the posterior part of the thalamus, the corpora quadrigemina, and the corpora geniculata.
+
Cavernous Sinus Sixth Nerve
  
The posterior choroidal arteries are two or three in number, and pass through the choroidal fissure to the tela chorioidea, which they supply, together with the choroid plexus of the lateral ventricle, and the corresponding choroid plexus of the third ventricle.
 
  
The cortical branches are: (i) anterior temporal, to the anterior parts of the occipito-temporal and hippocampal gyri; (2) posterior temporal, to the posterior parts of the occipito-temporal and hippocampal gyri, and the inferior temporal gyrus; and (3) occipital, to the occipital lobe. One of the occipital branches is called the calcarine artery. It lies in the calcarine fissure, and supplies the lingual or infracalcarine gyrus and the cuneus.
+
Middle Fossa
  
Circulus Arteriosus (Circle of Willis).—This circle or (to be more exact) heptagon is formed in front by the anterior communicating artery,
 
  
which connects the two .
+
Basilar Sinus Sup. Petrosal Sinus - Inf. Petrosal Sinus
  
anterior cerebral arteries; behind by the basilar artery as it divides into the two posterior cerebral arteries; and on either side by (1) the anterior cerebral artery, (2) the trunk of the internal carotid aitery,
+
Sigmoid Sinus
  
(3) the posterior communicating artery, and (4) the posterior cerebra artery, in this order from before backwards. The cncle furnishes twigs to the grey cortex of the interpeduncular region. It serves to equalize the blood-pressure in the cerebral arteries, and it also provides for the regular supply of blood to the brain m cases where one of the main arterial trunks may be obstructed.
 
  
The following parts are contained within the circulus arteriosus, in order from behind forwards: (1) the posterior perforated area; (2) the corpora mamillaria; (3) the tuber cinereum and infundibulum; and
+
Fig. 988. — Ihe Internal Uase or- the Skull, showing the Cranial Nerves and Venous Sinuses.
  
(4) the optic chiasma.
 
  
 +
it communicates with the veins of the scalp by means of an emissary vein, which passes through the parietal foramen of each side, when that is present.
  
 +
The inferior sagittal sinus (vein) is of small size, and is situated in the lower free border of the falx cerebri over its posterior two-thirds. Its direction is backwards, and it opens into the front part of the straight sinus at the anterior margin of the tentorium cerebelli. It is circular, and increases in size as it passes backwards. Its tributaries are derived from the lower part of the falx cerebri, and from the adjacent parts of the medial surfaces of the cerebral hemispheres.
  
13 15 1*
 
  
 +
The straight sinus is situated at the junction of the falx cerebri with the tentorium cerebelli, and is formed by the union between the inferior longitudinal sinus and the great (internal) cerebral vein at the anterior margin of the tentorium. It is triangular in section, and increases in size from before backwards. Its course is downwards and backwards in the median line to the left side, as a rule, of the internal occipital protuberance, where it becomes slightly dilated, and is continued into the left transverse sinus. Its terminal dilatation is connected with the confluens sinuum by a transverse vessel. In those cases where the superior longitudinal sinus passes into the left transverse sinus the straight sinus passes into the right transverse sinus. In addition to the inferior longitudinal sinus and the great cerebral vein, the straight sinus receives some of the superior cerebellar veins and tributaries from the falx cerebri and tentorium cerebelli.
  
x. Internal Carotid
+
The transverse sinuses extend on each side from the internal occipital protuberance to the postero-lateral compartment of the jugular foramen, through which it passes, to terminate in the bulb of the internal jugular vein. The sinus pursues a curved course. It passes outwards in the transverse groove on the inner surface of the tabular part of the occipital bone as far as the inner surface of the postero-inferior angle of the parietal bone. Here it ascends for a little in a groove, and, having described a sharp curve, it passes downwards and medially in the sigmoid groove on the inner surface of the mastoid portion of the temporal bone. Finally, it turns forwards in the groove on the upper surface of the jugular process of the occipital bone to the jugular foramen. As far as the postero-inferior angle of the parietal bone the transverse sinus is contained within the attached border of the tentorium cerebelli, and just before leaving this it receives the superior petrosal sinus. The right transverse sinus is, as a rule, formed by the superior longitudinal sinus, and in these circumstances is larger than the left, which is usually formed by the straight sinus. The reverse, however, may be the case. Before leaving the torcular the two sinuses communicate freely.
  
2. Middle Cerebral
+
The transverse sinus of each side, in addition to the superior petrosal sinus, receives tributaries from the posterior part of the cerebrum and the superior and inferior surfaces of the cerebellum. It also receives the posterior temporal and occipital diploic veins, and the petrosquamous sinus, when that sinus is present. Near its termination it communicates with the marginal sinus of the same side. The sinus communicates with extracranial veins by means of two large emissary veins, posterior condylar and mastoid, which pass through the corresponding foramina. These foramina, however, being inconstant, the emissary veins are sometimes wanting.
  
3. Anterior Cerebral _
+
The name ‘ transverse ’ is strictly applicable only to that part ot the sinus which is contained within the attached border of the tentorium cerebelli , and extends from the internal occipital protuberance to the postero-inferior angle of the parietal bone. Beyond this latter point the sinus is known as the sigmoid sinus.
  
4. Anterior Communicating
+
The occipital sinus is situated within the falx cerebelli along its attachment to the internal occipital crest. It is of small size, and is formed inferiorly by the union of the two marginal sinuses (inferior occipital) which lie on either side of the vermiform fossa and foramen magnum, where they communicate with the posterior intraspinal veins and the terminal part of each lateral sinus. Superiorly it opens into the confluens sinuum. In certain cases the marginal or inferior occipital sinuses remain separate, and then each opens into the corresponding transverse sinus. The occipital sinus receives tributaries from the falx cerebelli and the inferior surface of the cerebellum, and it establishes a communication between the beginning and end of the transverse sinuses.
  
5. Posterior Communicating
+
The cavernous sinuses are so named because the interior of each is broken up by fibrous filaments, which impart to it a reticular appearance. Each sinus is situated on the side of the body of the sphenoid bone, and extends from the inner extremity of the sphenoidal fissure to the apex of the petrous part of the temporal bone. Anteriorly it receives the ophthalmic vein or veins from the orbit, and posteriorly it terminates by dividing into the superior and inferior petrosal sinuses. In addition to the ophthalmic vein or veins, the cavernous sinus receives the spheno-parietal sinus and some of the inferior cerebral veins. It communicates with the angular vein of the face through the superior ophthalmic vein; with its fellow of the opposite side through the circular sinus; with the transverse sinus by the superior petrosal sinus; with the internal jugular vein by the inferior petrosal sinus; with the pterygoid plexus through the inferior ophthalmic vein, and by an emissary vein which passes through the foramen ovale, or through the foramen Vesalii ’; and with the pharyngeal plexus by an emissary vein which passes through the foramen lacerum medium. The internal carotid artery, with the cavernous sympathetic plexus, lies within the sinus, and the sixth cranial nerve (Fig. 989) is close to the outer side of the artery. In connection with the roof and outer wall of the sinus there are the third and fourth, as well as the ophthalmic and superior maxillary divisions of the fifth cranial nerves.
  
6. Posterior Cerebral
+
The spheno-parietal sinus is of small size, and is situated on the inferior surface of the lesser wing of the sphenoid bone. It generally begins in connection with the anterior temporal diploic vein, and ends in the anterior part of the cavernous sinus.
  
7. Basilar
+
The circular sinus fills any small part of the hypophysial fossa which
  
8 Superior Cerebellar 9. Transverse Pontine
+
is not occupied by the gland; on each side it opens freely into the cavernous sinus.
  
10. Internal Auditory
+
The superior petrosal sinus is situated along the superior border of the petrous part of the temporal bone, and lies within the attached margin of the tentorium cerebelli. It begins at the back part of the cavernous sinus, and, having passed laterally and backwards, it opens into the transverse sinus as that is about to enter the sigmoid groove of the pars mastoidea. It receives tributaries from the cerebellum and tympanum.
  
11. Anterior Inferior Cerebellar
+
The inferior petrosal sinus occupies the groove at the junction of the basilar process of the occipital bone with the petrous part of the temporal bone. It begins at the back part of the cavernous sinus, and, passing backwards and slightly laterally, it leaves the cranial cavity through the antero-medial compartment of the jugular foramen, to terminate in the bulb of the internal jugular vein. It receives tributaries from the inferior surface of the cerebellum, and from the internal ear.
  
12. Posterior Inferior Cerebellar
 
  
13. Vertebral
 
  
14. Anterior Spinal
+
Fig. 989. Right Internal Carotid put in Position on Base of Skull and Crossed by Sixth Nerve.
  
15. Posterior Spinal
+
Greater superficial petrosal nerve is also seen entering foramen lacerum.
  
16. Anterior Choroid
+
The basilar sinus is really a plexus of veins situated within the dura mater over the basilar process of the occipital bone, extending as low as the anterior margin of the foramen magnum, where it communicates with the anterior intraspinal veins. It connects the anterior ends of the inferior petrosal sinuses with each other.
  
17. Posterior Choroid
+
The petrosquamous sinus is situated along the junction of the petrous and squamous parts of the temporal bone. Its direction is backwards, and it opens into the transverse sinus as that is entering the
  
18. Central or Ganglionic
+
siermoid fossa of the pars mastoidea.
  
19. Central or Ganglionic
+
Each petro-squamous sinus represents the continuation of the transverse sinus in early life, to terminate in the primitive jugular vein. Often there are no traces of the petro-squamous sinus.
  
20. Central or Ganglionic
 
  
(Postero-mesia!)
+
Emissary Veins. — These are vessels which pass through foramina in the cranial wall, and establish communications between the intracranial venous sinuses and the extracranial veins. The principal emissary veins are mastoid, nasal, posterior condylar, parietal, and occipital. In addition to these there are emissary veins, which pass through (i) the foramen ovale, (2) the foramen Vesalii (when present), and (3) the foramen lacerum and the carotid canal.
  
21. Central or Ganglionic
+
The transverse sinus in many cases has two emissary veins, mastoid and posterior condylar.
  
(Postero-lateral)
+
The mastoid emissary vein is of large size. It passes through the mastoid foramen, and usuallv opens into the posterior auricular vein.
  
 +
The posterior condylar emissary vein passes through the posterior condylar foramen, and opens into the suboccipital plexus, from which the blood is carried away by the vertebral and deep cervical veins.
  
Fig. 967. —The Arteries at the Base of the Brain, and the Arterial Circle.
+
The mastoid and posterior condylar veins are not constant.
  
 +
The superior sagittal sinus may have three emissary veins—parietal and occipital and nasal.
  
 +
The parietal emissary vein passes through the parietal foramen, and opens into the occipital plexus, or into radicles of the superficial temporal vein.
  
 +
The occipital emissary vein passes from the confluens sinuum to the occipital plexus, being transmitted through a minute foramen which pierces the external occipital protuberance.
  
 +
The nasal emissary vein passes through the foramen caecum; like all other emissary veins it is often absent. It has been seen already that the cavernous sinus communicates with (1) the angular vein of the face, (2) the pterygoid plexus, and (3) the pharyngeal plexus. The marginal or inferior occipital sinus communicates with the posterior intraspinal veins, and the basilar sinus communicates with the anterior intraspinal veins.
  
  
A MANUAL OF ANATOMY
+
===Arachnoid Membrane===
  
 +
This is a very delicate membrane which loosely surrounds the encephalon, and is situated between the dura mater and pia mater. It does not dip into the fissures, except in the case of the great longitudinal fissure, its general course being over the gyri and other eminences and depressions of the encephalon. It is conspicuous at the base of the encephalon in the region of the interpeduncular space, pons, and medulla oblongata. Its outer surface is practically in close contact with the inner surface of the dura mater, the extremely slight interval containing a very little lubricating serous fluid, and being known as the subdural space. Between the arachnoid and the pia mater there is the interval known as the subarachnoid space. The membrane furnishes sheaths to the various cranial nerves.
  
1578
+
Subarachnoid Space. — This space lies between the arachnoid and the pia mater. It is crossed by delicate trabeculae of connective tissue, which pass between the two membranes, in a reticular manner. The meshes of this reticulum contain the cerebro-spinal fluid.
  
Veins of the Encephalon.
+
The subarachnoid space communicates with the ventricles of the brain by one main opening, the median aperture of the ventricle or foramen of Magendie, which is situated in the median line of the roof of the ventricle a little above the lower angle.* In some situations— as, for example, over the gyri—the arachnoid and pia mater are in close contact, but in other localities the two membranes are more or less widely separated by intervals, called cisternas. The most important of these are the cisterna magna, cisterna pontis, cisterna basalis, and cisterna venae magnae.
  
The cerebral veins are arranged in two groups—superficial and deep.
 
  
The superficial cerebral veins are divided into two sets—superior and inferior.
+
The modern view is that there are also foramina at the lateral angles of the ventricle, lateral apertures.
  
The superior cerebral veins return the blood from the upper parts of the outer surfaces of the cerebral hemispheres. They lie in the pia mater, and pierce the arachnoid membrane and inner layer of the dura mater, after which they open into the superior longitudinal sinus, having previously received the veins from the medial surface of either hemisphere. Their direction for the most part is forwards and medially, whilst the direction of the blood-current in the superior longitudinal sinus is backwards.
 
  
The inferior cerebral veins return the blood from the lower parts of the cerebral hemispheres, and they terminate in the cavernous, superior petrosal, and lateral sinuses. One of these veins is known as the superficial middle cerebral or superficial Sylvian vein. It passes along the lateral fissure, and opens into the front part of the cavernous sinus or else into the spheno-parietal sinus. This vein communicates posteriorly with (1) the superior longitudinal sinus by means of the great or superior anastomotic vein (of Trolard); and (2) the transverse sinus by means of the inferior anastomotic vein (of Labbe).
 
  
the deep cerebral veins are as follows: (1) the choroidal veins; (2) the veins of the corpora striata; (3) the internal cerebral veins (veins of Galen); (4) the anterior cerebral veins; (5) the deep middle cerebral veins; and (6) the basilar veins.
 
  
The choroidal vein of each side begins in the choroid plexus of the inferior horn of the lateral ventricle. It ascends at first, and then passes forwards in the lateral margin of the tela chorioidea to the interventricular foramen, where it unites with the vein of the corpus striatum to form the corresponding internal cerebral vein (vein of Galen).
+
The cisterna cerebello-medullaris (or magna) lies between the posterior part of the inferior surface of the cerebellum and the medulla oblongata. It is of large size, the arachnoid, which here passes from cerebellum to medulla, being widely separated from the pia mater. It is continuous through the foramen magnum with the posterior part of the subarachnoid space of the spinal cord.
  
The vein of the corpus striatum is formed by branches which issue from the corpus striatum and thalamus. It runs forwards in the groove between these two bodies, lying superficial to the taenia semicircularis, and at the interventricular foramen it joins the choroidal vein to form the corresponding internal cerebral vein.
+
The cisterna pontis is situated on the ventral aspect of the pons. Interiorly it is continuous with the anterior part of the subarachnoid space of the spinal cord, and in the region of the medulla oblongata it is continuous otherwise with the previous cisterna and interpeduncularis. It contains the basilar artery.
  
The internal cerebral vein (vein of Galen) of each side is formed close to the interventricular foramen by the union of the choroidal vein, the vein of the corpus striatum, and the vein of the septum lucidum. The two veins, right and left, pass backwards between the two layers of the tela chorioidea, and they unite beneath the splenium of the corpus callosum to form one vessel, called the great cerebral vein, which opens into the front part of the straight sinus. Each vein receives tributaries from the thalamus, the corresponding choroid plexus of the third ventricle, the corpus callosum, and the corpora quadrigemina; and, before joining its fellow, it takes up the basilar vein of its own side. The great cerebral vein receives tributaries from the upper surface of the cerebellum.
+
The cisterna basalis or interpeduncularis is situated in front of the pons, in which situation the arachnoid extends over the interpeduncular space from one temporal lobe to the other. It contains the arteries which form the circulus arteriosus. This cisterna is prolonged outwards on each side into the stem of the lateral fissure, each of these prolongations containing the middle cerebral artery. Anteriorly it extends in front of the optic commissure into the great longitudinal fissure over the upper surface of the corpus callosum, this prolongation containing the anterior cerebral arteries.
  
 +
The cisterna venae magnae lies just behind the entrance of the great transverse fissure, between the splenium and the corpora quadrigemina, where the great cerebral vein comes out.
  
THE NERVOUS SYSTEM
+
The subarachnoid fluid can be drained away in two directions. It can enter the lymph-spaces of the cranial nerves upon which the arachnoid is prolonged outwards in the form of sheaths; and it can enter the lacunae laterales, and through the intervention of the Pacchionian bodies make its way into the superior longitudinal sinus.
  
 +
Structure of the Arachnoid Membrane. —The arachnoid consists of fine fibrous tissue arranged in interlacing bundles, the intervals between these bundles being occupied by delicate cellular membranes. Several such layers, intimately blended together, form the membrane.
  
1579
+
Beneath the arachnoid, and constituting a part of it, there is a reticulum of subarachnoid trabeculce. These trabeculae consist, as in the case of the arachnoid proper, of fine fibrous tissue, but the intertrabecular spaces, instead of being occupied by cellular membranes, contain cerebro-spinal (subarachnoid) fluid. The trabecular reticulum connects the arachnoid with the subjacent pia mater. The superficial surface of the arachnoid is covered with a delicate layer of endothelium.
  
 +
Arachnoid Granulations— These are small granular bodies which are situated along the course of the superior longitudinal sinus, into which some of them project. They are seldom met with in adults in other sinuses— e.g., the lateral and straight sinuses. Each body is a villous projection of the arachnoid membrane, with which it is connected by a narrow pedicle. Some bodies project into the superior longitudinal sinus; others project from below into the lacunae laterales. In all cases the bodies pierce the dura mater and carry before them the lining of the sinus. Superficially the bodies give rise to the depressions on the internal surface of the parietal bone near the superior border. Each body contains a prolongation of the subarachnoid space and reticulum. This is surrounded by a prolongation of the arachnoid membrane, and external to this is the endothelial lining of the sinus or of the lacuna. The granulations probably are channels through which the subarachnoid fluid can be drained away from the subarachnoid space into the lacunae laterales, and thence into the superior sagittal sinus, as well as into the other sinuses— e.g., the transverse and straight sinuses. They are rarely met with in children under twelve, and then most commonly in the transverse sinus.
  
The anterior cerebral vein of each side is situated in the great longitudinal fissure, along with the corresponding artery. Having curved round the genu of the corpus callosum, it passes to the anterior perforated region, where it joins the deep middle cerebral vein to form the basilar vein.
 
  
The deep middle cerebral vein returns the blood from the insula and lies deeply within the stem of the lateral fissure.
+
===Pia Mater===
  
The basilar vein of each side begins at the anterior perforated area, where it is formed by the union of the anterior cerebral and deep middle cerebral veins. It passes backwards round the crus cerebri, and opens into the internal cerebral vein of its own side just before that vessel joins its fellow to form the great cerebral vein. The basilar vein receives, close to its commencement, one or more inferior striate veins, which descend from the corpus striatum through the anterior perforated substance. It also receives tributaries from the parts within the interpeduncular space.
+
The pia mater is the most internal covering of the encephalon. It is a very vascular membrane, which invests and is closely adherent to the entire surface. From its internal surface delicate processes pass into the cerebral substance, which represent the minute bloodvessels, surrounded by pia-matral sheaths. The pia mater not only invests the external surface, but also dips into the sulci, and covers the opposed surfaces of the gyri. It also furnishes sheaths to the various cranial nerves, which blend with their perineurium. It gives rise to two weblike expansions—namely, the tela chorioidea inferior and tela chorioidea superior.
  
The cerebellar veins are arranged in two groups—superior and
+
The tela chorioidea inferior is situated in the lower part of the roof of the fourth ventricle, and from it are derived the choroid plexuses of that ventricle. The tela chorioidea superior (or velum interpositum) is an invagination of the pia mater through the transverse fissure beneath the splenium of the corpus callosum. It lies underneath the body of the fornix, and its lower surface is covered by the ependymal lining of the third ventricle, the latter forming the roof of that cavity. The tela chorioidea superior furnishes the choroid plexuses of the two lateral and third ventricles.
  
inferior.
+
The pia mater of the encephalon differs from the pia mater of the spinal cord in being thinner and less adherent to the nervous substance. The greater thinness is due to the fact that it is destitute of the outer layer which characterizes the spinal pia mater.
  
The superior cerebellar veins terminate in the great cerebral vein, and in the straight, transverse, and superior petrosal sinuses. The inferior cerebellar veins pass to the sigmoid, inferior petrosal, and occipital sinuses.
+
Structure. — The pia mater of the encephalon consists of a single layer of areolar tissue, which contains a great many small bloodvessels, these being derived from the larger vessels lying in the subarachnoid space.
  
  
Blood-supply of the Different Parts of the Encephalon.
+
==The Cranial Nerves==
  
The medulla oblongata is supplied by the vertebral, anterior spinal, and
+
The cranial nerves are arranged in twelve pairs. They have received numerical names according to the order in which they leave the cranial cavity from before backwards, and they also have descriptive names. The different pairs of nerves are as follows, in order from before backwards:
  
posterior inferior cerebellar arteries. .,
 
  
The pons is supplied by the transverse or pontine branches of the basilar
+
First, or olfactory.
  
The cerebellum is supplied inferiorly by the posterior inferior cerebellar branches of the vertebral arteries, and the anterior inferior cerebellar branches of the basilar artery. Superiorly it is supplied by the superior cerebellar branches of the basilar artery, and to a limited extent by the posterior inferior cerebellar
+
Second, or optic.
  
arteries
+
Third, or oculo-motor.
  
The crus cerebri is supplied by the postero-medial and postero-lateral branches of the posterior cerebral artery, and by the posterior communicating
+
Fourth, or trochlear (pathetic). Fifth, or trigeminal (trifacial). Sixth, or abducent.
  
Th’e posterior perforated substance is pierced by the postero-medial branches
 
  
of the posterior cerebral arteries. , , . v
+
Seventh, or facial.
  
The corpora quadrigemina are supplied by the postero-lateral ganglionic
+
Eighth, or auditory.
  
branches of the posterior cerebral arteries. , +
+
Ninth, or glosso-pharyngeal. Tenth, or vagus.
  
The thalamus is supplied posteriorly by the postero-meclial and posterolateral ganglionic branches of the posterior cerebral artery. Anteriorly its outer part is supplied by the lenticulo-thalamic branches of the middle cerebral artery,
+
Eleventh, or accessory. Twelfth, or hypoglossal.
  
and its inner part by the posterior communicating artery. .
 
  
The anterior perforated substance is pierced by the antero-lateral ganglionic
+
The cranial nerves are connected to certain parts of the encephalon, and these connections constitute their superficial or apparent origins. The fibres, however, can be traced to certain collections of grey matter, which are called nuclei. From the deep positions occupied by these nuclei they constitute the deep origins of the nerves.
  
branches of the middle cerebral artery. , ., . . c ,,
+
First or olfactory nerve consists of the olfactory filaments or nerves, which are about twenty in number.
  
Frontal Lobe— The superior frontal gyrus, the anterior two-thirds of the middle frontal gyrus, and the upper portion of the precentral gyrus are supplied bv cortical branches of the anterior cerebral artery. The posterior third of the
+
The olfactory nerves are non-medullated. They arise as the axons of the olfactory cells of the olfactory mucous membrane of the nasal fossa; and enter the cranial cavity through the foramina of one half of the cribriform plate of the ethmoid bone. Thereafter they enter the grey matter on the ventral or inferior aspect of the olfactory bulb, and terminate in arborizations which intermingle with the arborizations formed by the dendrites of the mitral cells situated in the granular layer of the bulb (see p. 1570).
  
middle frontal gyrus, the inferior frontal gyrus and the .lower “h^ce-ebra* of the precentral gyrus are supplied by cortical branches of the^middle <cerebral „ rfpr * Q n t h e orbital surface the internal orbital gyrus, olfactory lobe, ana gyrus rectus are supplied by the anterior cerebral artery, whilst the remainder is supplied by the middle cerebral artery.
+
Second or Optic Nerve.—This nerve arises from the brain by means of the optic tract, the deep connections of which have been already described (p. 1545). Each optic tract passes forwards and inwards to the optic commissure or chiasma, which is situated in front of the interpeduncular space. 1 he optic nerve of each side arises from the anterior part of the optic chiasma. It courses forwards and outwards to the optic foramen, through which it passes into the orbit, piercing the dura mater, and receiving a sheath from it, as well as from the arachnoid membrane. Having reached the back part of the eyeball, it pierces the sclerotic and choroid coats ^ inch to the nasal or inner side of the axis of the eyeball, and terminates in an expansion which forms the most internal layer of the retina, called the nerve-fibre or optic layer.
  
 +
Neither this nor the preceding is, strictly speaking, a nerve at all.
  
1580 A MANUAL OF ANATOMY
+
Third or Oculo-motor Nerve.—The fibres of this nerve arise from the oculo-motor nucleus, which is situated in the grey matter of the ventral aspect (floor) of the aqueduct on a level with the upper quadrigeminal body, and extends superiorly for a short distance on to the lateral wall of the third ventricle. The nucleus is intimately related to the medial longitudinal bundle, by means of which it is connected with the trochlear and abducent nuclei. All three nuclei receive collaterals from the bundle; and in this manner a functional association between these nuclei is maintained, and harmonious action is insure on the part of the muscles which are supplied by the nerves arising from them. It consists of several groups of cells. As many as seven groups are ascribed to each oculo-motor nucleus by Perlia, which correspond to the seven muscles supplied by the oculo-motor nerve, and are disposed symmetrically. In addition to these, there is a medially-placed group, the cells of which furnish fibres to both oculo-motor nerves. The fibres which arise from the individual groups of each nucleus are regarded as supplying particular orbital muscles. Certain of the oculo-motor fibres of one side arise from the nucleus of the opposite side, the fibres from either side decussating at the median line. Moreover, each oculo-motor nerve is said to receive fibres from the abducent nucleus of the opposite side, which ascend in the posterior longitudinal bundle and cross to the other side.
  
Parietal Lobe. —The whole of this lobe, practically, is supplied by the middle cerebral artery.
 
  
Occipital Lobe. —This lobe is supplied by the posterior cerebral artery.
 
  
Temporal Lobe. —The superior and middle temporal gyri and the pole are supplied by the middle cerebral artery, and the remainder is supplied by the posterior cerebral artery.
 
  
Medial Surface of the Cerebral Hemisphere. —The anterior cerebral artery has an extensive distribution to this surface, which it supplies as far back as the internal part of the parieto-occipital fissure. The parts behind this fissure— namely, the cuneus and the parts around the calcarine fissure—are supplied by the posterior cerebral artery.
+
Fig. 990. — The Base of the Encephalon, and the - Cranial Nerves. 1, frontal lobe (orbital surface); 2, temporal lobe; 3, cerebellum.
  
The corpus callosum is supplied by the anterior cerebral arteries.
 
  
Corpus Striatum. —The nucleus caudatus and nucleus lentiformis are supplied for the most part by the antero-lateral or ganglionic branches of the middle cerebral artery, which pass through the anterior perforated substance. According to Duret they form three sets—lenticular, lenticulo-striate, and lenticulo-thalamic. The lenticular (internal striate) arteries supply the globus pallidus (inner part)
 
  
 +
Two views are entertained in regard to the nerve-supply of the medial rectus muscle. According to one view, the muscle of one side is supplied by those fibres which have crossed from the oculo-motor nucleus of the opposite side. The other view is that the muscle of one side is supplied by those fibres which have crossed from the abducent nucleus of the opposite side. According to this latter view, the nerve-fibres which supply the lateral rectus muscle of one side and those which supply the medial rectus muscle of the opposite side arise from the same nucleus—namely, the abducent nucleus—and vice versa.
  
 +
Probably the whole of the oculo-motor nucleus is not in series with the medial somatic group to which the fourth, sixth, and twelfth nuclei belong, but that some of it corresponds to the more lateral group containing the seventh, ninth, and tenth nuclei. Fibres from this part probably go to the ciliary muscle and iris.
  
Fig. 968. —Brain of an Embryo about Four Weeks Old (from
 
  
Quain’s ‘ Anatomy ’) (His).
 
  
1. Telencephalon 4. Metencephalon 7. Spinal Cord
+
Fig qqi._Scheme showing the Different Cell-groups which constitute,
  
2. Thalamencephalon 5. Myelencephalon 8. Pontine Flexure
+
'according to Perlia, the Nucleus of Origin of the Third Cranial
  
3. Mesencephalon 6. Cervical Flexure 9. Olfactory Lobe
+
OR OCULO-MOTOR NERVE (FROM TESTUT, AFTER PERLIA).
  
1, 2=prosencephalon; 3 = mesencephalon; 4, 5 = rhombencephalon
 
  
of the lentiform nucleus, the internal capsule, and the caudate nucleus. The lenticulo-striate (external striate) arteries supply the putamen (outer part) of the lenticular nucleus, and the external capsule. One of the lenticulo-striate arteries is larger than the others, and is subject to rupture in cases of cerebral haemorrhage. It is known as the artery of cerebral hcemorrhage (Charcot). Its course is outwards and upwards round the lateral aspect of the lentiform nucleus, between it and the external capsule, and then through the internal capsule to the caudate nucleus. The lenticulo-thalamic arteries supply the posterior part of the lentiform nucleus, and the anterior part of the thalamus on its lateral aspect.
+
1. Posterior Dorsal Nucleus 1'. Posterior Ventral Nucleus
  
1 he front part of the caudate nucleus is supplied by the antero-medial ganglionic branches of the anterior cerebral and anterior communicating arteries.
+
2. Anterior Dorsal Nucleus 2'. Anterior Ventral Nucleus
  
The larger arteries occupy the subarachnoid space, where they divide into branches which enter the pia mater. These in turn give off smaller branches, which enter the cerebral substance, some of them being cortical and others medullary in their distribution.
 
  
Blood-supply of the Choroid Plexuses. —The choroid plexus of the lateral ventricle derives its blood from (i) the anterior choroidal artery, which is a
+
3
  
 +
4
  
 +
5
  
THE NERVOUS SYSTEM
 
  
 +
Central Nucleus ,
  
1581
+
Jucleus of Edinger and Westphal Lntero-medial Nucleus
  
  
branch of the internal carotid or the middle cerebral; and (2) the posterior choroidal arteries, which are branches of the posterior cerebral. The choroid plexuses Of the third ventricle derive their blood from the posterior choroidal arteries. The choroid plexuses of the fourth ventricle are supplied from the posterior inferior cerebellar arteries.
+
6. Antero-lateral Nucleus
  
The tela chorioidea derives its blood from the posterior choroidal arteries and from the superior cerebellar arteries.
+
7. Trunk of Oculo-Motor Nerve
  
Lymphatics of the Brain. —There are no lymphatic vessels in the brain. Their place is taken by spaces in the outer coat of the arteries, called perivascular spaces, which are in communication with the subarachnoid space.
+
8. Crossed Fibres
  
 +
9. Nucleus of Origin of Fourth Nerve
  
Development of the Encephalon.
+
g'. Intercrossing of Fourth Cranial Nerves
  
A brief outline of the formation of the brain has been given on pp. 55-59* in which can be followed the development of the primary cerebral divisions into fore-, mid- and hind-brain, the formation of the cerebral vesicles, the appearance of the brain flexures, and in general the changes which lead to the existence in their proper positions of the main structures in the brain. The student is advised to read this general account before proceeding to the following descriptions, which deal with the conditions in the developing brain in a more detailed manner.
+
10. Third Ventricle M. Middle Line.
  
  
Metamorphoses of the Hind-brain.
+
The old view, that the orbicularis oculi is supplied from this nucleus, by the medial longitudinal bundle and the facial trunk, is no longer held by anatomists.
  
The pontine flexure begins to show about the beginning of the fourth week, is well marked at the beginning of the second month, and has its two limbs very close to one another by the end of this month. The posterior limb of the flexure, down to the nuchal bend, is termed the myelencephalon, the anterior limb the metencephalon, and the narrow junction with the mid-brain is the
+
Course of the Fibres o£ the Third Nerve.The fibres pass forwards from their origin through the tegmentum, the red nucleus, and the medial portion of the substantia nigra, and afterwards make their superficial appearance at the oculo-motor sulcus on the medial aspect of the crus cerebri.
  
isthmus.
+
The third or oculo-motor nerve supplies the following seven muscles.
  
Myelencephalon. —The walls, opened out in their upper parts, with a wide roof-plate, give origin to the medulla oblongata, and the cavity forms the lowei half of the fourth ventricle ; at the extreme lower end the cavity is not enlarged. The development of this lower or hinder portion of the myelencephalon proceeds in general on the lines of development already described for the spinal cord, with certain modifications due to the opening out and change in direction in certain tracts passing to the brain, the crossing of pyramidal fibres, and the presence of certain tracts and nuclei (to be described later) associated with the
+
the levator palpebne superioris; the superior, inferior, and internal recti; the inferior oblique; the sphincter pupillae; and the ciliary muscle.
  
existence of visceral arches. , ,
+
Fourth (Trochlear, or Pathetic) Nerve. —The fibres of this nerve arise from the trochlear nucleus, which is situated in the grey matter of the ventral aspect (floor) of the aqueduct on a level with the upper part of the lower quadrigeminal body. The nucleus is intimately related to the medial longitudinal bundle, by means of which it is connected with the oculo-motor nucleus.
  
A little higher up the myelencephalon broadens to make the lower part of
+
The fibres are at first directed laterally and backwards, and then medially to the upper part of the superior medullary velum, which they enter. Here the nerve crosses to the opposite side, decussating with its fellow, after which it emerges from the upper end of the superior medullary velum close below the lower quadrigeminal body, and by the side of the frenulum veli. After this the nerve turns over the superior peduncle of the cerebellum, and is then directed forwards, round the outer aspect of the crus cerebri, between which and the temporal lobe it makes its superficial appearance.
  
the fourth ventricle. The alar and basal laminae are now in the floor of the wide cavity. The broad roof-plate, a single layer of epithelium, is attached at the sides to an everted edge of the alar lamina, known as the rhombic lip, an over hanging the outer parts of the laminae. Such a definite rhombic lip however, is only found in the cranial part of the myelencephalon, where, as will be seen later, the great enlargements appear which are due to growth of the vestibular nuclei.
 
  
The widened roof-plate is covered by vascular mesenchyme, representing pia mater. At the level of the pontine flexure the ependymal or epithelial roof is invaginated into the fourth ventricle in the form of a transverse fold —plica chorioidea, containing pia mater—which extends between the lateral recesses of the ventricle From this transverse fold two vertical folds —phcce chorioidea, e wise containing pia mater—extend vertically downwards into the ventricle close to the median line. These ependymal folds, containing pia mater, form the two
 
  
choroid nlexuses of the lateral ventricle. . , .
 
  
At a comparatively early stage the afferent fibres of the seventh, ninth and
+
Fig. 992. — Deep Origins of Third, Fourth, and Fifth Cranial Nerves.
  
tenth nerves pass into the marginal zone of the alar lamina, and form a bund here i ZZausTolitarius; this bundle becomes deeply buried by subsequent
 
  
thl °Thehvno!gloss afnucleus deveiops, as has been mentioned already on p. 1440 praetor witWnth"ndymal P zone, in the upper part of the column of
+
The fourth nerve supplies the superior oblique muscle of the eyeball.
  
 +
Fifth Cranial, Trigeminal (or Trifacial) Nerve. —The fifth cranial nerve resembles a spinal nerve in having two roots—sensory and motor
  
1582
+
the former being large, and having a ganglion, called the trigeminal ganglion.
  
  
A MANUAL OF ANATOMY
+
Sensory Root. — The fibres of this root are derived from the central poles of the bipolar cells of the trigeminal ganglion. After entering the pons each fibre divides into two branches, ascending and descending, as in the case of the fibres of the dorsal or sensory root of a spinal nerve. The terminal nuclei of these ascending and descending sensory fibres are two in number—upper and lower.
  
 +
♦The upper sensory nucleus is situated in the outer portion of the dorsal part of the pons, where it lies close to the lateral side of the pontine or principal motor nucleus of the nerve. The ascending sensory fibres, after a short course, enter this nucleus and terminate in arborizations around its cells.
  
loosened nuclei seen here in the cervical region formed from the ependymal zone ventro-laterally. The sixth nucleus possibly arises from the extreme cranial end of the same column, but this is not certainly known. In the young embryo the efferent nuclei (except that of the hypoglossal) lie in the basal lamina, where they cause internal depressions by their rapid growth in the thin wall (Fig. 970). These depressions are known as neuromeres, and are a marked though temporary feature of most embryonic brains. In the illustration they are seen from within, and have a curious distribution, in that the sixth neuromere is placed behind the seventh. The fifth has two neuromeres, of which the most cranial is much the deeper; the line of flexure of the hind-brain, which has not yet begun in this specimen, will pass through this deep neuromere of the fifth nerve.
+
The lower sensory nucleus, continuing the line of the upper sensory nucleus, is an upward prolongation of the substantia gelatinosa from the tubercle and funiculus gelatinosus in the medulla oblongata. The nucleus is traceable as low as the dorsal grey horn of the spinal cord on a level with the second cervical spinal nerve, where it is close to the substantia gelatinosa. The descending sensory fibres, which are numerous, and constitute the spinal root of the fifth nerve, pass downwards through the pons and medulla oblongata into the spinal cord as low as the level of the second cervical spinal nerve. They are accompanied by the lower sensory nucleus, and at different levels they enter this nucleus and terminate in arborizations around its cells.
  
 +
The disposition of the fibres and cells within the spinal root is of a reversed order—that is, the ophthalmic nerve is associated with the lower part of the spinal root, above this the maxillary, with the mandibular at the upper end.
  
 +
The ascending or mesencephalic nucleus of the fifth extends along the grey matter on the side of the aqueduct as far as the level of the lower part of the upper corpus quadrigeminum. Its lower limit is lateral to the substantia coerulea in the upper part of the fourth ventricle. The mesencephalic root has only been recognized as sensory within the last few years, and there is reason to suppose that it receives proprioceptive impulses from certain muscles.
  
Fig. 969.—Lateral View of Brain, End of Second Month.
+
Motor Nucleus. —This nucleus is placed in the lateral part of the tegmental region of the pons, deep to the floor of the upper or pontine portion of the fourth ventricle, and immediately medial to the upper sensory nucleus of the nerve. Its fibres run ventro-laterally to emerge as the small motor root of the nerve.
  
Cbl, cerebellar rudiment; P, N, pontine and nuchal flexures; M, mid-brain flexure; Pit, hypophysis; Cpb, corpus ponto-bulbare. Roof-plate of hindbrain is only shown in outline.
+
Most of the axons of the cells of the terminal sensory nuclei pass inwards to the raphe and cross to the opposite side. They then become longitudinal and ascend in company with the medial lemniscus or chief sensory tract, their destination being the thalamus of the side to which they have crossed. They thus constitute a trigemino-thalamic ascending tract. From these fibres collaterals are furnished to (1) the facial nucleus, and (2) the ventral vago-glosso-pharyngeal nucleus, or nucleus am biguus, from the cells of which latter the efferent or motor fibres of the pneumogastric or vagus nerve arise.
  
A little later a neuromere will mark, rather indefinitely, the ninth efferent nucleus, but is not seen in the figure.
+
A few of the axons, however, enter the pontine or chief motor nucleus, and also the mesencephalic sensory nucleus, of the nerve, and terminate in arborizations around its cells.
  
It must be understood that the neuromeres are present only in the basal lamina; this, for practical purposes, is the one seen in the figure, the thin and narrow strip (D) being the only representative of the alar lamina.
+
The large sensory and small motor roots appear close together, on the lateral aspect of the ventral surface of the pons, the motor root lying above and slightly internal to the sensory root. The sensory root enters and the motor root leaves the pons.
  
Behind the region of neuromeres the myelencephalon narrows down to its continuity with the spinal cord, and it is here, extending cranially, that the olive is laid down.
+
Distribution. —The fifth cranial nerve has an extensive distribution by means of its three divisions—ophthalmic, superior maxillary, and inferior maxillary.
  
The inferior olive, with its medial and dorsal accessory formations, is developed as a modification of the upper part of the ventro-lateral column of neuroblasts in the mantle zone, from which the ventral column is formed at a lower level. The early stages of this development are shown in Fig. 971, while Fig. 972
+
Ophthalmic Nerve (Sensory). —(i) The front part of the cranium; (2) the integument of (a) the upper eyelid, and ( b ) the root and tip of the nose; (3) the anterior part of the nasal mucous membrane, and the conjunctiva; (4) the eyeball; and (5) the lacrimal gland.
  
 +
Maxillary Nerve (Sensory). —(1) The integument of the zygomatic and anterior part of the temporal regions; (2) the integument of (, a) part of the lower eyelid, ( b) the side of the nose, (c) the upper lip, and (d) that part of the face between the lower eyelid and the upper lip; (3) the upper teeth, and the mucous membrane of the upper gum;
  
THE NERVOUS SYSTEM
+
(4) a large part of the nasal mucous membrane; (5) the mucous membrane of the maxillary air-sinus (or antrum of Highmore); (6) the mucous membrane of (a) the naso-pharynx, and ( b ) the soft and hard palate and the tonsil.
  
 +
Mandibular Nerve (Sensory and Motor). — The sensory distribution of this nerve is as follows: (1) the integument of (a) the temporal region, ( b ) the outer surface of the pinna, and (c) the external auditory meatus;
  
1583
+
(2) the integument of the lower lip, and that which covers the mandible; a recurrent branch runs along the petro-squamous suture, supplying the mucous membrane of the tympanum and of the mastoid antrum;
  
 +
(3) the temporo-mandibular joint; (4) the parotid salivary gland;
  
 +
(5) the mucous membrane lining the buccinator muscle, and the integument covering that muscle (by means of the long or sensory buccal nerve ); (6) the mucous membrane (fungiform and conical papillae) of the anterior two-thirds of the tongue (common sensation); (7) the submandibular and sublingual salivary glands; and (8) the pulps of the lower teeth, and the mucous membrane of the lower gum.
  
 +
The motor distribution of the mandibular nerve is as follows: (1) The muscles of mastication—namely, (a) the masseter, ( b ) the temporal, and (c) the pterygoid muscles; (2) the mylo-hyoid muscle and anterior belly of the digastric; (3) the tensor tympani muscle by means of a branch from the otic ganglion; and (4) the tensor palati muscle through the otic ganglion.
  
Fig. 970.
+
Sixth or Abducent Nerve. —The fibres of this nerve arise from the abducent nucleus, which is situated in the dorsal part of the pons close to the median line. It lies above the striae acusticae on the floor of the fourth ventricle subjacent to the eminentia teres. The fibres emerge from the inner part of the nucleus, and pass through the lower part of the pons in a forward and slightly downward and lateral direction to the lower border of the pons just lateral to the pyramid of the medulla oblongata, where the nerve makes its superficial appearance.
  
A, sagittal section of brain of 4-9 mm. embryo; B, section through two adjacent
+
The abducent nucleus receives collaterals from the medial or posterior longitudinal bundle, and a functional connection is thereby established between that nucleus and the oculo-motor nucleus. The medial rectus muscle of one side and the lateral rectus of the other side are thus associated muscles.
  
neuromeres.
+
The sixth nerve supplies the lateral rectus muscle of the eyeball.
  
  
  
Fig. 971. —Hind-brain, 13-5 Mm. Embryo.
+
Fig. 993. — Deep Origins of Sixth, Seventh, and Eighth Cranial Nerves.
  
Outline of longitudinal median section on right below; m this cr. is the cranial slope and sp. the spinal cord; TS, tractus solitanus; vl ventro-lateral nuclei. Planes of sections a, b, and c correspond with those showm on the outline.
+
Seventh or Facial Nerve. — The facial nerve is composed of two parts. One of these consists of efferent or motor fibres, and is known as the facial nerve proper. The other part, of small size, consists of afferent or sensory fibres. The facial nerve proper arises from the facial nucleus , which is situated deeply in the dorsal part of the lower portion of the pons. The fibres of the nerve pursue an intricate course before appearing superficially. They at first pass backwards and inwards to the floor of the fourth ventricle. Here they turn upwards, lying close to the median line in the form of a single bundle. The nerve then makes a sharp bend laterally, and passes forwards through the pons in a downward and outward direction to its place of emergence.
  
  
  
  
 +
In its course within the pons the nerve is intimately related to the dorsal aspect of the abducent nucleus.
  
 +
1 he intrapontine part of the facial nerve proper is intimately related to the following structures:
  
A MANUAL OF ANATOMY
+
1. The abducent nucleus. 4. The spinal root of the fifth
  
 +
2. The superior olive. nerve.
  
1584
+
3. The corpus trapezoides. 5. The medial or posterior longitudinal bundle.
  
gives the appearances in subsequent stages. In these it can be seen that the median accessory olive (m.o.) is first defined, the main olivary mass being constructed from the more lateral condensations.
+
The motor facial nucleus receives fibres from the following sources: (1) The corpus trapezoides, being thereby brought into connection with the cochlear division of the auditory nerve; (2) the spinal root of the fifth cranial or trigeminal nerve, which is the sensory nerve of the face; and (3) the pyramidal tract of the opposite side, being thereby brought into connection with the precentral motor area of the cerebral cortex.
  
It must be said here that the classical and accepted account of olivary development refers it to the ventral migration of neuroblasts from the ‘ rhombic lip/ The account given above is put forward because it is in accord with observed facts, whereas the older story is very unsatisfactory in several particulars.
 
  
  
  
Fig. 972.
 
  
Transverse sections through olivary region in embryos of 15 and 16 mm.; horizontal sections, 18, 21, and 28 mm. in neighbourhood of nuchal flexure.
+
Fig. 994. — Diagrammatic Section through the Pons, to show Deep Origins of Sixth (Red) and Seventh (Black) Cranial Nerves.
  
The cuneate and gracile nuclei are formed directly from the dorsal neuroblasts.
 
  
Each pyramid is a ventral bulging of that part of the basal lamina which is on the mesial side of the olivary body, and it is produced by the motor tracts as they descend in the marginal layer from the central area of the cerebral cortex through the pons. This begins in the fourth month.
 
  
The ponto-bulbar body is represented in the embryo by a collection of small and darkly-staining nuclei lying below the caudal part of the ‘ rhombic lip/ It appears in the latter part of the second month, and its nuclei spread fairly rapidly over the surface of the myelencephalon, especially ventrally and cranially; here they lay down the beginnings of the pontine nuclei. On the myelencephalon further back they appear to be responsible for the various small superficial
+
All the foregoing fibres terminate within the nucleus in arborizations around its component cells.
  
 +
The sensory portion of the facial nerve arises from the central poles of the bipolar cells of the geniculate ganglion on the facial nerve in the facial canal. This ganglion resembles the ganglion of the fifth nerve and the spinal ganglia, and most of the peripheral poles of its bipolar cells give rise to the chorda tympani nerve. The pars intermedia passes from the facial canal into the internal auditory meatus, after leaving which it runs to the lower border of the pons, where it lies between the facial nerve proper and the auditory nerve. The nerve then enters the medulla oblongata, and passes downwards to the upper part of the nucleus of the fasciculus solitarius (see Glosso-pharyngeal Nerve), and its fibres terminate in arborizations around the cells of the upper part of that nucleus. In this situation it is closely associated with the terminal afferent or sensory fibres of the glosso-pharyngeal nerve.
  
 +
The facial nerve proper emerges from the brain at the lower border of the pons in front of, and internal to, the auditory nerve; and the sensory part enters between the facial nerve proper and the auditory nerve.
  
  
  
 +
Fig. 995. — Terminal Nuclei of the Vestibular Nerve, with their Superior Connections (Schematic) (L. Testut’s * Anatomie Humaine ’).
  
THE NERVOUS SYSTEM
 
  
 +
1. Cochlear Root, with its Two Nuclei
  
1585
+
2. Accessory Nucleus
  
 +
3. Lateral Nucleus (or Tuberculum Acusticum)
  
arcuate nuclei which may be found on the surface of the pyramid, etc. There is also a possibility that the lateral accessory cochlear nucleus may possess a similar origin.
+
4. Vestibular Root
  
As the pyramids, right and left, bulge ventralwards, the floor-plate, which connects the basal laminaj, sinks, and the anterior median fissure is formed, as in the development of the spinal cord. The spongioblastic floor-plate is now invaded by nerve-fibres, most of which cross from one side to the other, these fibres being (1) the anterior superficial arcuate fibres, (2) the deep arcuate fibres, and (3) the cerebello-olivary fibres. In this manner the raphe of the bulb is formed, as in the development of the anterior or white commissure of the spinal cord.
+
5. Medial Nucleus
  
In the more cranial portion of the myelencephalon, where the efferent nuclei have been laid down in the neuromeres, the subsequent growth of the alar lamina affords opportunity for the development of the vestibular nuclei, which thus lie just cranial to the great dorsal masses of the gracile and cuneate nuclei. The
+
6. Lateral Vestibular Nucleus (or Nucleus of Deiters)
  
 +
7. Superior Nucleus (of Bechterew)
  
 +
8. Inferior Root or Nucleus of Auditory Nerve
  
Fig. 973._ Plans to show Areas in Floor of Fourth Ventricle, with
+
9. Ascending Cerebellar Fibres
  
Position of Developing Structures.
+
10. Fibres passing to Raphe
  
changes which take place in the floor of the ventricle in this part are verv com plicated and not by any means understood; the plans in Fig. 973 are attempts to show the results of the changes. The first figure gives the positions of the neuromeres on the left, with the sites of the afferent nuclei on the r t§tit. n . e second and third the vestibular masses are associated with considerable widening but there is apparently a marked forward upgrowth of the floor in the basal area which carries the sixth and seventh nuclei forward and brings them against the metencephalic surface. This is no doubt associated with the curious relations between the two nuclei and nerves, but the way in which it comes about, as well as the reason for its occurrence, is not known. The positions of the various nuclei are given approximately in the plans, and a general idea of their origins and changes can be obtained by a study of the figures.
+
11. Oblique Fibres
  
Metencephalon.— -From this are developed the pons, cerebellum, its upper and middle peduncles, and the superior and inferior medullary vela Its cavity forms the upper part of the fourth ventricle .
+
12. Lemniscus
  
 +
13. Inferior Sensory Root of Fifth Cranial Nerve
  
100
+
14. Pyramidal Fibres
  
 +
15. Raph6
  
 +
16. Fourth Ventricle
  
 +
17. Inferior Peduncle of Cerebellum (Restiform Body)
  
 +
18. Origin of Auditory Striae
  
  
 +
Distribution—Motor Part (Facial Nerve Proper).— (1) The muscles of the face, including the buccinator ; (2) the occipito-frontalis; (3) the muscles of the auricle; (4) the posterior belly of the digastric and the stylo-hyoid; (5) the platysma myoides; and (6) the stapedius muscle within the tympanic cavity.
  
 +
Sensory Part (Sensory Root and Chorda Tympani). —The anterior two-thirds of the tongue (sense of taste).
  
 +
The chorda tympani nerve conveys secretory and vaso-dilator fibres from the facial nerve proper to the submandibular and sublingual salivary glands. .
  
 +
The large superficial petrosal nerve from the geniculate ganglion
  
 +
of the facial nerve is concerned in the supply of the mucous membrane of the palate, the path being as follows: (i) Large superficial petrosal nerve (facial fibres); (2) the nerve of pterygoid canal; (3) spheno-palatine ganglion; and (4) the descending palatine nerves.
  
 +
Eighth, Auditory, or Acoustic Nerve. —The auditory nerve is the nerve of hearing and of equilibrium. It is an afferent or centripetal nerve which conducts impressions from the membranous labyrinth (cochlea and vestibule) to the medulla oblongata and pons, and thence to the cerebrum and cerebellum. It consists of two divisions—namely, the cochlear nerve or root, and the vestibular nerve or root.
  
1586
 
 
 
A MANUAL OF ANATOMY
 
 
 
1 he pons develops as a ventral thickening on the lower end of the region, immediately cranial to the pontine flexure. Its nuclei appear to owe their origin to the neuroblasts which have spread over the surface from the ponto-bulbar body; presumably they increase subsequently in situ, but no definite indications of mitotic activity have been
 
 
 
 
Fig. 974.— Semi-diagrammatic Figures showing Cerebellar Rudiments.
 
 
 
found among them. The down-growing cerebro-spinal fibres find their way into and among these nuclei in the third month and subsequently.
 
 
The cerebellum is developed from the alar laminae of the metencephalon, the thickening involving the roof-plate in its growth. The
 
 
two lateral cerebellar plates formed by the laminae are at first inclined to each other at an angle (Fig. 976), but as the angle of the pontine flexure becomes more closed and the metencephalon widens, the paired cerebellar rudiments come nearly into line with one another (Figs. 974, 977). The lateral plates, being thickenings in the floor of the cavity, project at first into the cavity, covered by the roof-plate, which is attached to the margins outside them ; subsequently the attachment of the roof-plate is turned in (Fig. 977) below the bulging lateral formations, so that it becomes attached, descriptively, to the anterior and lower aspect of the transversely disposed cerebellar rudiment. It is in the taenial fold to which the roof-plate is attached here that the floccular enlargement occurs a little later.
 
 
In some lower vertebrates the cerebellum develops altogether within the cavity of the hind-brain, as in the early human stage; the later change in the human conditions enables the structure to expand freely outside the ventricle.
 
 
The upper part of the roof-plate of the rhombencephalon, at the angle of junction of the lateral plates, is invaded by them, and forms
 
 
 
 
Fig. 975.— Brain of Third Month Embryo seen
 
 
FROM BEHIND, TO SHOW *
 
 
the Transverse Cerebellar Rudiment.
 
 
This stage is between those shown in the previous figure.
 
 
 
/
 
 
 
 
 
 
 
 
THE NERVOUS SYSTEM
 
 
 
1587
 
 
 
the basis in which the vermis develops; growth here is slow, and the lateral lobes in their enlargement come to overlap it and cover it in.
 
 
The lateral plates come into evidence during the second month; the transversely disposed plate (Fig. 975) is well marked in the middle of the third month, when the flocculus may be recognized, and after this the development proceeds slowly. After the third month the vermis shows transverse fissures, and in the fifth month these are found also on the lateral lobes.
 
 
 
 
Fig 076_Ventricular Aspect of Cranial Slope of Hind-brain in Em bryos OF 12, 16, AND 28 Mm., TO SHOW BEGINNINGS OF CEREBELLAR FOLD.
 
 
 
The fissure cutting off the flocculus extends across the region of the vermis, marking off the nodule here. At the end of the third month a fissura prima forms across the vermis, making the lowei edge of the future culmen, and a little later a fissura secunda forms below the future pyramid . Other secondary fissures follow, and some ol the fissures of the vermis extend into the lateral lobes, but most of the
 
 
fissures here are separate local formations. , A
 
 
Cerebellar thickening, spreading into the roof-plate above and below the main development, forms the upper and lower medullary vela, the first extending (valve of Vieussens) to the closed isthmus, the
 
 
 
 
 
 
 
1588
 
 
 
A MANUAL OF ANATOMY
 
 
 
second being continuous below with the undeveloped ependymal or epithelial roof-plate of the myelencephalon.
 
 
On either side of the valve of Vieussens the roof-plate is thickened by the superior cerebellar peduncles.
 
 
 
 
Fig. 977.—Views from behind of the Hind-brain in Embryos of 35
 
 
and 48 Mm.
 
 
EV, IV, extra- and intra-ventricular surfaces.
 
 
The superior peduncles of the cerebellum, right and left, arise from the cells
 
 
of the nucleus dentatus of the corresponding lateral cerebellar hemisphere. Emerging from the anterior parts of the dentate nuclei, the peduncles give rise to two thickenings of the roof-plate of the metencephalon, one on either side of the valve of Vieussens. Thereafter they enter the mesencephalon or mid-brain,
 
 
 
 
THE NERVOUS SYSTEM
 
 
 
1589
 
 
 
and, after decussating, each passes to the corresponding red nucleus of the tegmentum of either crus cerebri, which constitutes its lower cell-station.
 
 
The middle peduncles of the cerebellum ( brachia pontis) are developed from the cells of the pontine nuclei of the pons right and left. The fibres of each peduncle issue from the lateral portion of the corresponding pontine nucleus, and enter the adjacent cerebellar hemisphere.
 
 
Mesencephalon. —This portion makes a sharp curve (Fig. 969) as it develops. It has a large cavity, which is slowdy lessened in size by the growing thickness of the walls, finally remaining as the small aqueduct. The thickened walls around this are composed of a floor lamina and roof lamina (tectum); in the former are formed the crura cerebri, while the tectum gives rise to the corpora quadrigemina.
 
 
The isthmus is a part of some interest; it is essentially a derivative of the hind-brain, so that its name isthmus rhombencephali is correct. The isthmus is produced at the spot where the two regions of growth, metencephalic and mesencephalic, meet each other, but there is in addition a definite forward extension of the basal lamina from the hind-brain into the opening, of which it makes the immediate wall. This extension carries with it the trochlear nucleus, which develops in the basal lamina of the hind-brain just above the opening; the decussation of the nerves, originally in the floor-plate of the hind-brain, is carried down also and lies therefore on the dorsum of the isthmus.
 
 
 
 
Fig. 978._Schemes to show the Parts of the Third Ventricle formed
 
 
FROM THE THALAMENCEPHALON (Bi.ACK LlNE) AND TELENCEPHALON (INTERRUPTED Line).
 
 
A little later there is an extension forwards from the alar lamina of the hindbrain, passing on the outer side of the basal extension, and carrying with it (sensory) nuclei from the upper part of the trigeminal nucleus; this appears to be the beginning of the mesencephalic root of the nerve, further short connections
 
 
developing subsequently. .
 
 
The mid-brain presents dorsally a median longitudinal groove, which separates two rounded eminences, known as the corpora bigemina . At a latei period a transverse groove appears, which divides each of the corpoia bigemina into two, thus giving rise to the corpora quadrigemina.
 
 
The third nucleus forms in the ventral mantle zone, and the fourth nucleus o-ains its position here secondarily. The red nucleus is probably, formed in situ, from the mantle zone of the floor lamina. I he corpora quadrigemina, formed from the alar laminae (and probably from the roof-plate secondarily involved), are hollow at first. -They become solid in the fourth and fifth months. The bases peduneulornm begin to appear in the fourth month in the central
 
 
parts of the marginal zone. . . . . . ,
 
 
Diencephalon or Thalamencephalon.— This is the anterior primary vesicle (Fig 978) Its cavity forms the greater part of the third ventricle (the anterior portion being derived from the secondary outgrowth, the telencephalon). Its walls develop into the thalamus, corpora mamillaria, tuber cmereum, mfundibulum, and posterior lobe of pituitary, and from the roof-plate grow the pineal
 
 
 
 
1590
 
 
 
A MANUAL OF ANATOMY
 
 
 
body and the ependymal roof of the ventricle. Moreover, when it is first formed, the thalamencephalon gives origin to the optic outgrowth on each side.
 
 
The pineal body, or epiphysis cerebri, is developed from the dorsal part of the ependymal roof of the third ventricle. It appears as a diverticulum of the ependymal roof close to the mesencephalon, and it is directed backwards, so that it comes to lie over the corpora quadrigemina. The distal end is blind, and in connection with it a number of closed follicles are formed which contain calcareous particles forming the acervulus cerebri, or brain-sand. The proximal part of the diverticulum forms the stalk of the pineal body, which contains the pineal recess and opens into the third ventricle.
 
 
The basal laminae, smaller than the alar, give origin to the tuber cinereum and the outgrowth which makes the infundibular process of the pituitary formation (see p. 1171). These structures are (Fig. 978) on the lower part of the
 
 
 
 
Fig. 979.— Diagram showing Parts of the Fore-brain and Structures
 
 
Derived from These.
 
 
D, diencephalon; Tel, telencephalon; T, E, M, are thalamus, epithalamus, and metathalamus; H is hypothalamus; CV, cerebral vesicle; CS, corpus striatum.
 
 
posterior aspect of the fore-brain immediately behind the site of the optic outgrowth. Corpora mamillaria are formed from basal laminae just behind and above the tuber. The position of these structures is due to the length and curve of the mid-brain; when this gets relatively shorter, and the nasal fossae grow upwards from below, the fore-brain is rotated upwards to some extent, and the structures thus come to lie more below the third ventricle.
 
 
The optic outgrowths, although they are actually low down with reference to the fore-brain, are derivatives from its alar region, and the interlaminar sulcus (big. 978) reaches its lowest or most cephalic point between the optic pouch and the infundibulum. This sulcus is here the hypothalamic sulcus, but the continuation of the ‘ sulcus of Monro ’ towards the foramen is a secondary effec t produced by the growth of the thalamus.
 
 
For development of the eye, see next chapter.
 
 
The thalamus begins early in the second month as an enlargement in
 
 
 
 
 
 
 
 
THE NERVOUS SYSTEM
 
 
 
i 59 i
 
 
 
the anterior part of the dorsal lamina due to growth of the mantle cells. It increases rapidly and narrows the ventricle, so that in the fourth or fifth month the two bodies come into contact and fuse to a small extent, leaving as an attachment between them when they draw apart again the connexus thalami or massa intermedia.
 
 
 
 
 
Fig. 980. —Schemes to show how the Corpus Striatum alters by its Growth the Shape of the Cavity.
 
 
occ, occipital growth, with formation of posterior horn; i.c., line in which pyramidal motor fibres issue through corpus striatum.
 
 
 
Later, in the fourth to fifth month, growth of the alar laminae farther back than the thalamic formation makes the pulvinar and geniculate bodies, and becomes secondarily continuous with the thalamus.
 
 
Telencephalon. —This (Fig. 979) is an outgrowth or forward bulging of the terminal part of the thalamencephalon; it gives off on each side the two cerebral vesicles. These hollow vesicles have cavities widely open into that of the telencephalon, and thus into the third ventricle, of which the telencephalon makes the most anterior part ..
 
 
The cerebral vesicles, growing fairly rapidly, stand up above the general level of the fore-brain, being separated from each other by the rudiment of the great longitudinal fissure. The direction of their main growth is upwards and backwards from the interventricular foramen, which is the opening from the telencephalon into a vesicle. They also grow forward for a little distance in front of the foramen, and of course their increasing size leads to an increasing prominence laterally. Thus they cover successively the diencephalon, the mid-bram, and finally the hind-brain and its formations, so that these are not to be seen from above. The vesicles begin to cover the mid-brain (Fig. 981) during the third month, and grow over the cerebellum in the fifth month. .
 
 
The interventricular foramen does not increase in size pari passti with the growing brain, hence
 
 
becomes relatively very small.
 
 
The anterior wall of the central unpaired or original telencephalic growth is the lamina terminalis. This is the direct path from one cerebral
 
 
fn the other whence it becomes the path .
 
 
Ilong which commissural fibres between the hemispheres will proceed in their
 
 
P " Thl V w fl S n a s S of'the cerebral vesicles are very thin at first, and thicken slowly. But the thkkening of the corpus striatum is visible at an early stage m each vesicle bednrdng to stand up into the cavity as a growthi ofthe floor and lower partof the outer wall. It grows rapidly, so that in the third month (Fig. 981)
 
 
 
 
Fig. 981. —Brain of Embryo in Middle of Third Month seen from
 
 
ABOVE, THE UPPER PART of the Right Cerebral Vesicle being removed to expose the Corpus Striatum (CS), Thalamus (OT), Midbrain (MB).
 
 
 
 
 
 
 
 
1592
 
 
 
A MANUAL OF ANATOMY
 
 
 
it makes a marked prominence, while the greater part of the remaining wall of the vesicle is still thin. The corpus striatum, thus forming a floor for the lateral ventricle, is the cause, by its further growth, of alteration in the shape of that cavity. This is shown in Fig. 980. The mass, at first low, grows in an upward and backward direction, projecting into the cavity, and thus leading to the formation of an inferior horn ; the forward-turned end of this horn is the result of further growth of the body. The deep cleft seen on the inner side of the mass in Fig. 981 is obliterated by fusion following on growth of the corpus on one side and of the thalamus on the other, and the floor of the ‘ body ’ of the cavity is raised accordingly.
 
 
The corpus striatum is one of the primitive formations in the brain connected with its primitive functions. A section across the fore-brain and vesicles in the second month is represented diagrammatically in Fig. 982, where the thick mass of the corpus striatum is in contrast with the thin wall of the rest of the vesicle. This thin wall is the rudimentary neopallium, that part of the cerebrum which in man is associated with the higher functions of the brain, and will, when it begins to grow, completely overshadow in size the original portion. The result of neopallial growth is shown in the second diagram; the mass of the corpus is not affected, while the rapid increase of the area of the neopallium leads to its overlapping the inert striate mass. Overlapping cannot take place on the inner and basal aspects, but growth outwards and backwards and downwards
 
 
 
 
Fig. 982. —Schematic Sections to show how the Corpus Striatum (CS) is
 
 
OVERLAPPED BY THE GROWTH OF THE NEOPALLIUM, AND IS DIVIDED INTO CAUDATE and Lenticular Parts by the Pyramidal Fibres. OT, thalamus.
 
 
is unrestricted, so we find the surface area corresponding with the corpus striatum is overlapped by opercula from behind and above and in front. The surface area corresponding with the striate body is the insula, and the opercula covering it in make by their presence the lateral fissure.
 
 
Growth of the neopallial area implies formation of processes from its nervecells, and during the third month the pyramidal motor fibres begin to extend down toward the lower regions. They pass, as indicated in Fig. 982, through the corpus striatum to reach the marginal region beside the thalamus, and m doing so divide the corpus into caudate and lentiform masses, the caudate mass lying between them (1 internal capsule) and the ventricular cavity, the lentiform ganglion between them and the surface; hence the lentiform and caudate nuclei are always separated from one another by fibres of the internal capsule, and these fibres are always separated from the lateral ventricle by the caudate mass, and from the surface by the lentiform body. The mass of fibres, however, passing out of the striate body below and behind, come to separate the lenticular part here from the thalamus, with which they are coming into relation.
 
 
The capsular fibres, passing through the corpus striatum, are affected by its upward growth, and thus make their passage and exit in a line (Fig. 980) curved like the surface of the striate body; this being so, it is easily understood that a section downwards, as along the arrow, would cut, in order from above, neopallium, cavity, caudate nucleus, internal capsule, lentiform nucleus, capsule, caudate nucleus, cavity, and finally neopallium again,
 
 
 
 
 
 
 
 
 
THE NERVOUS SYSTEM
 
 
 
1593
 
 
 
In tig. 982 the medial wall of the cavity of the vesicle is seen to remain thin. This wall is invaginated into the cavity as the ependymal covering of the choroid plexus of the lateral ventricle. The line of this thin wall lies just above the floor thickening of the corpus striatum, and is affected by the growth of this body, so that it assumes a curve corresponding with the curved shape of the ventricle, being invaginated into this throughout its length. This thin inner wall is continuous with the roof-plate of the third ventricle at the interventricular foramen, and the invaginations which cover the plexuses of the ventricles thus become continuous at this point.
 
 
The anterior part of the hemisphere vesicle corresponds to the frontal lobe; the lower part, as far forwards as the stem of the lateral fissure, becomes the temporal lobe ; and the upper and posterior part represents the parietal lobe.
 
 
 
 
Fig. 983.—Side Views of Left Hemisphere at Beginning of Fifth and End of Sixth and Seventh Months (modified from Koli.mann).
 
 
The occipital lobe is formed at a later period as the hemisphere grows backwards. The limbic lobe is developed in connection with the medial surface of the hemisphere. The olfactory lobe is developed as a hollow protrusion from the ventral aspect of the frontal portion of the hemisphere vesicle.
 
 
The surface of each hemisphere becomes very much broken up into gyri or convolutions, this being effected bv means of fissures. I he fissures are of two kinds—complete and incomplete/ The incomplete fissures are merely sulci produced by the growth of the gyri, and they do not involve the entire thickness of the walls of the cerebral hemispheres. The complete or primary fissures are infoldings of the walls of the cerebral hemispheres, and involve their entire thickness. They consequently give rise internally to certain prominences upon the wall of each lateral ventricle—namely, the lateral choroid plexus, hippo
 
 
 
1594 A MANUAL OF ANATOMY
 
 
campus, calcar avis, and eminentia collaterals. The primary fissures are as follows:
 
 
Lateral. Calcarine (anterior part).
 
 
Choroidal. Parieto-occipital.
 
 
Hippocampal. Collateral (central part).
 
 
With the exception of the lateral fissure, already described, the other complete fissures appear on the medial surface of the vesicle of the cerebral hemisphere.
 
 
The choroidal fissure is not really a fissure, but merely a groove or sulcus produced by an infolding of the vesicular wall, which is here composed entirely of ependyma. It commences above and behind the interventricular foramen of the corresponding side, and it terminates close to the tip of the temporal pole, where the inferior or descending horn of the lateral ventricle ends. Between these two points it passes backwards, downwards, and then forwards into the future temporal lobe in a crescentic manner, so as to embrace the stalk of the cerebral hemisphere. After the ependymal infolding has taken place, vascular mesenchyme dips in between its two folds, and so a plica chorioidea is formed. From this choroidal fold the lateral choroid plexus of the corresponding side is formed. This plexus projects into the lateral ventricle, but is excluded from the ventricular cavity by the ependyma of the wall, previously infolded. When the lateral choroid plexus is withdrawn, the thin ependymal covering of the plexus comes away with it, or is broken down. Under "these circumstances the choroidal fissure is really a fissure, inasmuch as the lateral ventricle now opens upon the surface through it. The internal prominence produced by the choroidal fissure is the lateral choroid plexus covered by ependyma, this prominence being verv conspicuous.
 
 
The other complete fissures will be found described in connection with the cerebral hemispheres. All the primary fissures are formed before the fourth month.
 
 
The incomplete fissures are very numerous. The first two to make their appearance are the calloso-marginal fissure, or sulcus cinguli, on the medial surface of the cerebral hemisphere, and the central sulcus, or fissure of Rolando, on the external surface. These, along with the other incomplete fissures, will be found described in connection with the cerebral hemispheres. - These are developed in the later foetal months, and inconstant tertiary fissures appear for years after birth.
 
 
The cavity of the vesicle of the cerebral hemisphere forms on either side the lateral ventricle, which is very much curtailed by the thickening undergone by the vesicular walls, and the internal prominences produced by the complete fissures. As the frontal lobe undergoes development the body of the ventricle extends forwards into it, and so the anterior horn of the ventricle is formed. As the occipital lobe becomes developed at a later period the body of the ventricle extends backwards into it, and so the posterior horn is formed. Meanwhile the interventricular foramen on either side, originally large, is being gradually much diminished in size.
 
 
Basal Ganglia. —The basal ganglia of each cerebral hemisphere are as follows:
 
 
Corpus striatum.
 
 
Claustrum.
 
 
Amygdaloid nucleus.
 
 
ihey are all developed from the deep part of the much thickened cortical substance which forms the floor of the lateral fossa. The claustrum and amygdaloid nucleus remain of small size, but the nucleus caudatus of the corpus striatum forms a conspicuous prominence in the lateral ventricle as it bulges into that cavity.
 
 
Commissures. —The commissures are as follows:
 
 
1. Corpus callosum. 4. Anterior.
 
 
2. Fornix. 5. Posterior.
 
 
3. Hippocampal. 6. Middle.
 
 
 
7. Habenular.
 
 
 
THE NERVOUS SYSTEM
 
 
 
1595
 
 
 
At an early period in the development of the cerebral hemispheres the interhemispherical (great longitudinal) fissure leads directly down to the roof of the diencephalon. At a later period the roof of the diencephalon is separated from the great longitudinal fissure by two commissures placed one above the other. The upper commissure is the corpus callosum, and the lower one is the fornix. These may be looked on as drawn back from the lamina terminalis by the growing vesicles, although this is not quite an accurate statement of their formation.
 
 
 
 
Fig. 984.—Schematic Figure to show the General Formation of
 
 
the Main Commissures.
 
 
The thalamus (OT) is supposed to be cut away in part, exposing the region of the corpus striatum; this has grown up "(producing the curved form of ventricle already described), and the choroidal fissure (, ch.f .) is therefore a curved line. Fibres of the internal capsule ( i.c.) also come through the mass of the corpus striatum in a curved line, and lie between the thalamus and lenticular part of corpus striatum. Hippocampal formations lie on the other side of the choroidal fissure, and association fibres here make the fimbria ( fimb .) and fornix (/). Commissural fibres cross the middle line and are cut at h (hippocampal commissure) and ac (anterior commissure) ; these are in the lamina terminalis. As the neopallium grows, its commissure, the corpus callosum, begins to form; it is at first in the lamina terminalis (cc.), but with increasing growth it extends forwards (A) and backwards (P). The rostrum, shown by the dotted line, is subsequently formed by fusion of the walls of the two'vesicles, the septum lucidum being that part of the wall between this and A. The backward extension carries with it the hippocampal commissure; these backward movements, involving also the hippocampus, are associated with great growth of the front part of the brain.
 
 
Fornix—Anterior Commissure— These appear in the third month. In Fig 984 is shown the inner aspect of a cerebral vesicle in which the lamina terminalis is visible. Thickenings begin in this from fibres crossing between the olfactory and insular regions', these make the anterior commissure. . A little later fibres extend from each hippocampus to the sides of the lamina, where they turn into the side walls of the telencephalon, and reach much later the basal laminae of the thalamencephalon, constituting the anterior
 
 
 
 
 
 
 
 
1.596 A MANUAL OF ANATOMY
 
 
pillars of the fornix. The rest of the fornix is carried back with the growing vesicle.
 
 
Corpus Callosum. —Some time later, as the neopallium grows, its commissural fibres begin to become evident as the corpus callosum. These at first make use of the path already utilized by the earlier commissural fibres, and are found crossing at and above the upper part of the lamina terminalis as far back as the level of the interventricular foramen, as seen in the figure. As the neopallium grows, however, its commissural fibres become too numerous to be confined to this area, and their line of crossing extends backwards and forwards. The hinder extension necessarily lies with the fornix above the choroidal fissure, while the forward extension is between the two anterior expansions of the vesicles or hemispheres. These anterior fibres of the corpus callosum come from the frontal lobe above the anterior horn of the ventricle, hence that part of the wall of the hemisphere below these fibres is the medial wall of this horn. The hemispheres become approximated and fused below this small area of medial wall as the result of growth of neopallium round it, and commissural fibres now find their way through the line of fusion below the medial wall of the anterior horn. This makes the rostrum, and its continuity in front and above with the original forward extension of the corpus callosum cuts off the two areas of medial wall from the rest of this wall in the longitudinal fissure; they now form the two layers of the septum lucidum, and the cavity between them is only the corresponding part of the space of the fissure, similarly cut off. The fusion between the medial walls is not confined to the region just considered, but is found behind this in the neighbourhood of the callosal fibres and the fornix; this explains the posterior extension of the region of the septum lucidum.
 
 
The hippocampal commissure appears on the back part of the ventral aspect of the plate formed by the fused areas of the cerebral hemispheres, to which position it has been carried by the backgrowth of the corpus callosum. Its fibres pass across from one hippocampus to the other, and they correspond to the region known as the lyra or psalterium.
 
 
The posterior commissure is formed in connection with the back part of the roof of the diencephalon behind the pineal diverticulum.
 
 
The so-called middle commissure is not a commissure properly so called, but is formed by the fusion over a limited area of the grey matter of the medial surfaces of the thalami, and properly termed connexus thalami.
 
 
The habenular commissure is produced by the decussating fibres of the thalamic stria?, these fibres, as they decussate, forming the dorsal part of the pedicle of the pineal body.
 
 
Meninges of the Encephalon. —The walls of all the cerebral vesicles are invested by mesoderm, and this tissue becomes differentiated into the three meninges—namely, the dura mater, arachnoid, and pia mater.
 
 
Choroid Plexuses. —The choroid plexuses of the two lateral, third, and fourth ventricles are developed as infoldings of the ependymal walls of the ventricles. Vascular mesenchyme (mesoderm) dips in between the two layers of each infolding, and in this manner plicce c-horioidece are formed. These choroidal folds give rise to the choroid plexuses, which as they project into the ventricles carry the ependymal walls, already infolded, before them.
 
 
Tela Chorioidea. —The vascular mesoderm ( pia mater) investing the neural tube is converted into a double layer interposed between fore-brain and cerebral vesicles as a result of the backgrowth of the latter. It is clear that this velum interpositum extends to the interventricular foramen, where its two layers are continuous, and where vessels of the cerebral layer can join those of the earlier one. The dotted lines in Fig. 982 show the position of these two layers on section; it can be seen that the ‘ cerebral ’ layer reaches out to the thin medial wall of the vesicle, and its marginal vessels can invaginate this to form the choroid plexus of the lateral ventricle, while the deeper layer rests on the roof of the third ventricle, and makes its choroid plexus. The continuity of the two layers at the foramen explains why the lateral vein runs there to join the internal cerebral vein, which is in the lower layer.
 
 
 
THE NERVOUS SYSTEM
 
 
 
1597
 
 
 
Development of the Peripheral Nervous System.
 
 
The peripheral nerves are arranged in two groups—namely, spinal, which are derived from the spinal cord; and cranial, which arise from the brain.
 
 
The spinal nerves are composed of two kinds of fibres—efferent, centrifugal, or motor; and afferent, centripetal, or sensory.
 
 
A motor spinal nerve-fibre arises as the axon of a neuroblast or nerve-cell in the mantle layer of the neural tube (see Development of the Spinal Cord).
 
 
A sensory spinal nerve-fibre is developed from a cell of a spinal ganglion, and these ganglia are developed from the corresponding neural crest.
 
 
Neural Crests. —The neural or ganglionic crests, right and left, are ridges of ectodermic cells which lie on either side of the neural tube. They are derived from a single crest of ectoderm, which is formed by the fusion of the ectoderm over each neural fold, this single crest being situated medially on the dorsal aspect of the neural tube along che line of fusion of the neural folds to close the tube. Subsequently the medial crest divides into right and left halves, which cover the dorsolateral aspects of the neural tube.
 
 
Each neural crest becomes broken up into a number of segments, or ganglia, there being four pairs for the head region, and thirty-one pairs for the region of the trunk.
 
 
Spinal Ganglia. —The spinal ganglia are arranged in thirtyone pairs, right and left.
 
 
Each cell of a ganglion acquires two poles — afferent or centripetal, and efferent or centrifugal—and at this stage it is consequently a bipolar cell.
 
 
The centripetal or proximal pole, which is the axon of a ganglionic cell, grows into the dorsal part of the wall of the neural tube and forms part of the dorsal or posterior root of a spinal nerve. Within the marginal layer of the neural tube the centripetal pole or nerve-fibre divides into tw r o branches, ascending and descending, which give off collaterals and terminate in arborizations. The centrifugal or distal pole joins the ventral or anterior nerve-root of the same segment of the spinal cord on the distal side of the ganglion to form a spinal nerve.
 
 
Though the cells of a spinal ganglion are originally bipolar, they become m the course of growth unipolar, the single pole having a T-shape. d his is brought about by an excessive growth of one wall of the bipolar cell, which biings the two original poles into contact, when they fuse, and are now connected with the cell by one stalk or pole, which divides into a centripetal and a centrifugal process.
 
 
Whilst the fibres of the dorsal or posterior roots of the spinal nerves grow into the mantle layer of the neural tube from the cells of the spinal ganglia, the fibres of the ventral or anterior roots arise within the mantle lacei as the axons of its neuroblasts or nerve-cells. The fibres of the anterior roots therefore grow out from the neural tube.
 
 
Cranial Nerves. _The development of the cranial nerves, with the exception
 
 
of the olfactory and optic nerves, corresponds for the most part with the development of the spinal nerves. The motor cranial nerve-fibres arise as the axons of nerve-cells of the brain, and groiv into the brain, whereas the sensory cranial nerve-fibres grow into the brain from the cells of the cephalic ganglia.
 
 
 
Neural Crest (Ectoderm)
 
 
 
 
Fig. 985. —Development of the Neural or Ganglion Crest (Keibel and Mall) (after Von Lenhossek and Koi.lmann).
 
 
 
 
 
 
 
1598
 
 
 
A MANUAL OF ANATOMY
 
 
 
Cephalic Ganglia. —The cephalic ganglia, like the spinal ganglia, are developed from the neural crests, and they constitute four pairs of ganglionic groups— namely, trigeminal, acoustico-facial, glosso-pharyngeal, and vagal. They are all comparable to the spinal ganglia.
 
 
The trigeminal ganglion is connected with the sensory root of the fifth cranial nerve. The centripetal poles of its nerve-cells pass inwards into the brain, forming the large sensory root of the nerve, and the centrifugal poles of its cells pass peripherally, forming the ophthalmic, superior maxillary, and sensory part of the inferior maxillary nerves. The trigeminal ganglion is thus clearly comparable to a spinal ganglion. The small motor root of the fifth nerve is homologous to the motor or anterior root of a spinal nerve, inasmuch as its fibres arise as the axons of nerve-cells within the brain.
 
 
The acoustico-facial ganglion resolves itself into facial and acoustic parts.
 
 
The facial ganglion, known as the geniculate ganglion, is connected with the genu of the facial nerve in the aqueduct of Fallopius. The centripetal poles of the nerve-cells of this ganglion form the sensory root of the facial nerve—the pars intermedia of Wrisberg —which passes inwards to the fasciculus solitarius and upper part of the glosso-pharyngeal nucleus. Many of the centrifugal poles of the cells issue from the ganglion as the chorda tympani nerve, which passes to the anterior two-thirds of the tongue as a nerve of special sense.
 
 
The acoustic ganglion is represented by the vestibular and cochlear ganglia in connection with the auditory nerve.
 
 
The vestibular ganglion is connected with the vestibular division of the auditory nerve in the internal auditory meatus. As in the other ganglia, the centripetal poles of the cells of this ganglion form the centripetal fibres of the vestibular nerve, which pass inwards to the brain. The centrifugal poles of the cells leave the ganglion, and form the peripheral part of the nerve as regards its distribution.
 
 
The cochlear ganglion, known as the ganglion spirale, is connected with the cochlear division of the auditory nerve, and is situated in the spiral canal of the modiolus. Its nerve-cells are related to nerve-fibres, as in the case of the vestibular ganglion.
 
 
The glosso-pharyngeal ganglion, which is broken up into a superior (jugular) ganglion and an inferior (petrous) ganglion, is comparable to a spinal ganglion. The centripetal poles of the nerve-cells, which issue from the ganglion, represent the centripetal sensory fibres of the glosso-pharyngeal nerve, passing into the brain. The centrifugal poles of the nerve-cells, issuing from the ganglion, represent the peripheral sensory fibres of the nerve.
 
 
The vagal ganglion, which is broken up into the upper ganglion (of the root) and the lower ganglion (of the trunk), is disposed towards the sensory fibres of the vagus nerve, as in the case of the other ganglia.
 
 
MENINGES OF THE ENCEPHALON.
 
 
Dura Mater.
 
 
The dura mater is a strong fibrous membrane which surrounds the encephalon, and is composed of two layers—outer and inner. The outer or endosteal layer serves as the internal periosteum or endocranium of the cranial bones, and the inner or sustentacular layer supports the encephalon. It is more firmly adherent to the bones forming the base of the skull than to those over the cranial vault, and it is also firmly attached along the course of the sutures. At the various openings it is prolonged outwards, blending with the sheaths of the transmitted nerves, and also becoming continuous with the external periosteum or pericranium. At the sphenoidal fissure it passes into the orbit
 
 
 
THE NERVOUS SYSTEM
 
 
 
1599
 
 
 
to form the orbital periosteum. At the lower margin of the foramen magnum the two layers of which the dura mater is composed separate. The external layer blends with the periosteum of the occipital bone around the margin of the foramen magnum. The internal layer is prolonged into the spinal canal, and forms the theca of the spinal cord. The outer surface of the dura mater is rough and flocculent, owing to fibrous processes which connect it to the inner surfaces of the cranial bones. Its inner surface is smooth and covered by endothelium. Superiorly, on either side of the superior longitudinal sinus there are several small granular nodules, called arachnoid granulations, which are best marked in old age. They indent the parietal bone, and protrude into the superior longitudinal sinus, carrying with them prolongations from the endothelial lining of the sinus, which separate them from the blood.
 
 
The bodies are enlargements of the villi of the arachnoid membrane (see p. 1609).
 
 
Structure. —The dura mater consists of fibrous and elastic tissues arranged as parallel bundles.
 
 
The intracranial dura mater differs from the dura mater of the spinal cord in the following respects: (1) it consists of two layers— outer or periosteal, and inner or sustentacular—whereas the spinal dura mater has only one layer, representing the sustentacular layer; (2) it furnishes certain processes or septa, which project into the cranial cavity, and separate parts of the encephalon from each other, whereas the spinal dura mater sends no septa into the spinal cord; and (3) it contains venous sinuses, which are absent in the spinal dura mater, or are represented by the extradural venous plexuses (see p. 1434) •
 
 
Subdural Space.—This is the interval between the dura mater and the arachnoid membrane. There is really no space, but simply sufficient interval to contain a minute quantity of serous fluid for lubricating purposes. The dura mater and the arachnoid are therefore practically in contact with each other.
 
 
Blood-supply. —The cranial dura mater is supplied by the meningeal arteries, which are extradural and supply the inner table of the cranial bones. These vessels are very numerous, and the chief are as follows on either side, from before backwards: (1) anterior meningeal, two in number, from the anterior and posterior ethmoidal arteries; (2) meningeal, from the cavernous part of the internal carotid artery; (3) small meningeal, from the middle meningeal, or from the first part of the maxillary artery; (4) middle meningeal, from the first part of the internal maxillary; (5) meningeal branches of the ascending pharyngeal artery, (6) posterior meningeal branch of the occipital artery; and (7) posterior meningeal, from
 
 
the vertebral artery. . , . ,
 
 
The anterior meningeal branch of the anterior ethmoidal artery arises from that vessel as it accompanies the nasal nerve on the cribriform plate of the ethmoid bone, and it takes part in the supply of the dura mater of the anterior
 
 
fossa
 
 
The anterior meningeal branch of the posterior ethmoidal artery arises from that vessel after it has entered the cranial cavity through a minute foramen between the cribriform plate of the ethmoid and the sphenoid. It has a limited distribution to the dura mater in this region.
 
 
 
i 6 oo
 
 
 
A MANUAL OF ANATOMY
 
 
 
The meningeal branch of the internal carotid artery arises from the cavernous part of that vessel, and enters the middle fossa to supply the dura mater.
 
 
The small meningeal artery is usually a branch of the middle meningeal, but it may arise from the first part of the maxillary artery. It enters the cranial cavity through the foramen ovale, and supplies the adjacent dura mater and the trigeminal ganglion.
 
 
The middle meningeal artery, as stated, is a branch of the first part of the maxillary artery. Its diameter is that of the foramen spinosum, through which it enters the cranial cavity. After passing into the cranium it divides into two branches, anterior and posterior. The anterior branch passes forwards, outwards, and upwards in a groove on the upper surface of the great wing of the sphenoid bone to the inner aspect of the antero-inferior angle of the parietal bone, where there is a groove, or sometimes a short canal. The position of the middle meningeal artery at this level is ascertained by taking a point on the exterior of the skull ii inches behind the zygomatic process of the frontal bone and inches above the zygomatic arch. From this point the artery ascends in a
 
 
 
Frontal
 
 
Diploic
 
 
Vein
 
 
 
 
T rx.
 
 
Anterior Temporal Diploic Vein
 
 
 
Posterior Temporal Diploic Vein
 
 
 
Occipital Diploic Vein
 
 
 
Fig. 986.— The Veins of the Diploe.
 
 
 
The outer tables of the cranial bones have been removed.
 
 
 
branching meningeal groove near the anterior border of the parietal bone as high as the superior longitudinal sinus. In this part of its course it furnishes numerous branches forwards and backwards.
 
 
The posterior branch passes backwards on the squamous part of the temporal bone, and then on to the inner aspect of the inferior border of the parietal bone, where there is a meningeal groove about the centre. From this point it ascends in a branching groove as high as the superior longitudinal sinus, giving off branches forwards and backwards.
 
 
The distribution of the middle meningeal artery extends as high as the superior longitudinal sinus forwards on to the frontal bone, and backwards on to the occipital bone. Besides supplying the dura mater and the inner table and diploe of the bones, the vessel furnishes the following branches: (1) Ganglionic to the trigeminal ganglion; (2) a petrosal brcinch, which passes through the hiatus (hallopii) to supply the facial nerve in its canal, and anastomose with the stylo-mastoid branch of the posterior auricular artery; and (3) an orbital branch, which enters the orbit through the sphenoidal fissure, and anastomoses with the ophthalmic artery.
 
 
 
 
 
 
THE NERVOUS SYSTEM
 
 
 
1601
 
 
 
The meningeal branches of the ascending pharyngeal artery, which is a branch of the external carotid, are three in number: One passes through the foramen lacerum into the middle fossa; another passes through the jugular foramen into' the posterior fossa; and the third passes through the anterior condylar foramen, likewise into the posterior fossa.
 
 
The posterior meningeal branch of the occipital artery passes through the jugular or mastoid foramen into the posterior fossa.
 
 
The posterior meningeal branch of the vertebral artery passes through the foramen magnum into the cerebellar fossa of the occipital bone.
 
 
As a rule, only one of these various posterior meningeal arteries is large enough to carry injection. .#•
 
 
Meningeal Veins. —The sinuses or veins with the middle meningeal artery are two in number. They pass through the foramen ovale, and terminate in the pterygoid plexus of veins. The other meningeal veins are disposed in one of two ways. Some of them accompany the corresponding arteries and terminate in extracranial veins; whilst others end in the various intracranial venous sinuses, in part directly and in part by means of venous lacunae.
 
 
Veins of the Diploe. —These vessels are situated in the cancellous tissue between the outer and inner plates of the cranial bones, and are exposed after removal of the outer plate. They are destitute of valves, and are arranged in the form of a network, from which the blood is returned by four diploic veins on either side—namely, frontal, anterior temporal, posterior temporal, and occipital. These terminate partly in extracranial veins, and partly in the intracranial venous sinuses and meningeal veins.
 
 
The frontal diploic vein passes downwards and escapes through an opening in the outer plate of the frontal bone at the supra-orbital notch, where it joins the communicating vein which passes between the supra-orbital and ophthalmic veins. It receives radicles from the frontal air-sinus of the same side.
 
 
The anterior temporal diploic vein is confined to the back part of the frontal and anterior part of the parietal regions, and descends to terminate in two ways. It partly joins one of the extracranial deep temporal veins by passing through an opening in the outer plate of the great wing of the sphenoid, and in part it ends in a meningeal vein, or in the spheno-parietal, or it may be the cavernous
 
 
sinus. „ , . , - ,
 
 
The posterior temporal diploic vein is confined to the parietal region, and descends to the postero-inferior angle of the parietal bone, where it pierces the inner plate of that bone, and terminates in the transverse sinus..
 
 
The occipital diploic vein occupies the occipital region, and pierces the inner table of the occipital bone, to terminate in the transverse or sigmoid sinus.
 
 
Nerves of the Dura Mater— The dura mater receives nerves from (i) the sympathetic plexuses which accompany the arteries; (2) the three divisions o the fifth cranial nerve; (3) the ganglion of the root of the va gus nerve , an (4) the hypoglossal nerve. Headache is said to be due to irritation of these nerves, especially to one of the branches of the fifth, known as the nervus tentorn.
 
 
Processes of the Dura Mater.— It has been already explained that the dura mater is composed of two layers outer or periosteal, and inner or sustentacular. The processes are formed by the inner or sustentacular layer, and are four in number the faix cerebri, tentorium cerebelli, falx cerebelli, and diaphragma sellse.
 
 
The falx cerebri is an extensive sickle-shaped process, which occupies the great longitudinal fissure, where it lies between the two cerebral hemispheres. Anteriorly it is almost pointed, and is attached to the crista salli of the ethmoid bone. Posteriorly it is broad, and is attached to the upper surface of the tentorium cerebelli along the median line the straight sinus being situated at the place of junction The supenor border is convex, and is attached m the median line to the frontal,
 
 
IOI
 
 
 
1602 A MANUAL OF ANATOMY
 
 
parietal, and occipital bones, extending upon the latter bone only as low as the internal occipital protuberance. The superior sagittal sinus is situated along this border. The inferior border is concave and free. It overhangs the upper surface of the corpus callosum, from which it is separated by a slight interval. The inferior sagittal sinus is situated within this border. The lateral surfaces face the medial surface of the cerebral hemispheres.
 
 
The tentorium cerebelli is an extensive crescentic sheet which covers the cerebellum. Superiorly it supports the posterior parts of the cerebral hemispheres, and is elevated along the median line, whence it slopes towards the attached borders. Anteriorly it presents a free,
 
 
Falx Cerebri
 
 
9
 
 
I
 
 
 
 
Fig. 987.—The Falx Cerebri, Tentorium Cerebelli, and Venous Sinuses
 
 
of the Dura Mater (Left View).
 
 
i- Superior Sagittal Sinus 4. Transverse Sinus
 
 
2. Inferior Sagittal Sinus 5. Superior Petrosal Sinus
 
 
3. Straight Sinus 6. Internal Jugular Vein
 
 
sharp, concave border, which forms, with the dorsum sellae of the sphenoid bone anteriorly, an oval opening, called the foramen ovale tentorii , within which the mesencephalon is placed. Posteriorly and laterally the tentorium cerebelli is convex, and is attached as follows, from behind forwards: (i) to the horizontal ridge on the inner surface of the tabular part of the occipital bone, where the process contains the transverse sinus; (2) to the inner aspect of the postero-inferior angle of the parietal bone, where the process also contains this sinus; and (3) 1° the superior border of the petrous part of the temporal bone, where the process contains the superior petrosal sinus. Close to the apex of th e pars petrosa the outer and anterior borders of the tentorium
 
 
 
 
 
 
 
 
 
 
THE NERVOUS SYSTEM
 
 
 
1603
 
 
 
cerebelli cross, the outer border passing inwards to be attached to the posterior clinoid process of the sphenoid bone, and the anterior border passing forwards to be attached to the anterior clinoid process of that bone.
 
 
In all the carnivora the tentorium is ossified.
 
 
 
The falx cerebelli extends forwards into the posterior cerebellar notch, where it lies between the two cerebellar hemispheres. It is falciform in outline. Superiorly it is attached to the posterior part of the inferior surface of the tentorium cerebelli in the median line. Posteriorly it is attached to the internal occipital crest, and this portion bifurcates interiorly, the two divisions being connected to the lateral margins of the vermiform fossa. Along this posterior attachment it contains the occipital sinus and its two divisions. Anteriorly it ends in a free border.
 
 
In the ornithorhynchus and many cetacea the falx is ossified.
 
 
 
The diaphragma sellse is a small circular fold horizontally placed, which forms a roof for the sella turcica or hypophysial fossa of the sphenoid bone, and almost entirely covers the hypophysis. At its centre there is a small opening for the passage of the infundibulum.
 
 
Venous Sinuses of the Dura Mater.—These are blood-channels or spaces situated between the two layers of the dura mater, and lined with endothelium. They are as follows:
 
 
 
Superior sagittal sinus. Inferior sagittal sinus. Straight sinus.
 
 
Transverse sinuses (two). Occipital sinus. Cavernous sinuses (two).
 
 
 
Spheno-parietal sinuses (two). Circular sinus.
 
 
Superior petrosal sinuses (two). Inferior petrosal sinuses (two). Basilar sinus.
 
 
Petro-squamous sinuses (two).
 
 
 
The superior sagittal sinus is situated in the median line within the superior convex border of the falx cerebri. It extends from the crista galli of the ethmoid bone to the internal occipital protuberance, where as a rule it turns sharply to the right, and opens into the right transverse sinus. It sometimes, however, turns to the left, and opens into the left transverse sinus. Its posterior extremity is dilated, and forms the confluens sinuum, which usually occupies a depression on the right side of the internal occipital protuberance, and is connected with the dilatation at the posterior extremity of the straight sinus by a transverse vessel. It increases in size as it passes backwards, and its shape is triangular in section, the base being directed towards the cranial vault. The apex is directed downwards, and in this region the sinus is crossed by a number of delicate fibrous bands. Opening into the sinus on either side there are venous spaces, called lacunae laterales, which are situated within the dura mater, and projecting into these lacunae from below, or into the sinus itself, there are seveial arachnoid granulations, covered by the endothelial lining. The sinus receives the superior cerebral veins and some of the meningeal veins of the falx
 
 
 
A MANUAL OF ANATOMY
 
 
 
1604
 
 
cerebri. The former open into it from behind forwards, so far at least as the more posterior vessels are concerned, so that the blood-flow in these veins is opposed to the current of blood in the sinus, which is from before backwards.
 
 
The superior sagittal sinus sometimes communicates anteriorly with the veins of the roof of the nose through the foramen ccecum, and
 
 
 
Frontal Air-Sinus
 
 
 
 
Anterior Fossa
 
 
/
 
 
 
Cerebellar Fossa
 
 
 
Accessory Nerv Hypoglossal Nerve
 
 
 
Spinal Cord ' Occipital Sinus
 
 
 
Optic Nerve ^ Ophthalmic Artery
 
 
 
Third Nerve
 
 
Fourth Nerve
 
 
Region of Meckel’s Cave
 
 
Fifth Nerve
 
 
Facial Nerve’
 
 
Auditory Nerve
 
 
Glosso-pharyngeal Nerve
 
 
Vagus Nerve
 
 
 
Transverse Sinus
 
 
 
Cerebral Fossa
 
 
^ Confluens Sinuum
 
 
 
Border of Small Wing ✓ ' of Sphenoid
 
 
 
Ant. Pt. of Circ. Sinus ^ (Ant. Intercav. Sinus)
 
 
Hypophysis
 
 
 
Cavernous Sinus Sixth Nerve
 
 
 
Middle Fossa
 
 
 
Basilar Sinus Sup. Petrosal Sinus - Inf. Petrosal Sinus
 
 
Sigmoid Sinus
 
 
 
Fig. 988.—Ihe Internal Uase or- the Skull, showing the Cranial
 
 
Nerves and Venous Sinuses.
 
 
 
it communicates with the veins of the scalp by means of an emissary vein, which passes through the parietal foramen of each side, when that is present.
 
 
The inferior sagittal sinus (vein) is of small size, and is situated in the lower free border of the falx cerebri over its posterior two-thirds. Its direction is backwards, and it opens into the front part of the straight sinus at the anterior margin of the tentorium cerebelli. It is circular, and increases in size as it passes backwards. Its tributaries are derived
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE NERVOUS SYSTEM
 
 
 
1605
 
 
from the lower part of the falx cerebri, and from the adjacent parts of the medial surfaces of the cerebral hemispheres.
 
 
The straight sinus is situated at the junction of the falx cerebri with the tentorium cerebelli, and is formed by the union between the inferior longitudinal sinus and the great (internal) cerebral vein at the anterior margin of the tentorium. It is triangular in section, and increases in size from before backwards. Its course is downwards and backwards in the median line to the left side, as a rule, of the internal occipital protuberance, where it becomes slightly dilated, and is continued into the left transverse sinus. Its terminal dilatation is connected with the confluens sinuum by a transverse vessel. In those cases where the superior longitudinal sinus passes into the left transverse sinus the straight sinus passes into the right transverse sinus. In addition to the inferior longitudinal sinus and the great cerebral vein, the straight sinus receives some of the superior cerebellar veins and tributaries from the falx cerebri and tentorium cerebelli.
 
 
The transverse sinuses extend on each side from the internal occipital protuberance to the postero-lateral compartment of the jugular foramen, through which it passes, to terminate in the bulb of the internal jugular vein. The sinus pursues a curved course. It passes outwards in the transverse groove on the inner surface of the tabular part of the occipital bone as far as the inner surface of the postero-inferior angle of the parietal bone. Here it ascends for a little in a groove, and, having described a sharp curve, it passes downwards and medially in the sigmoid groove on the inner surface of the mastoid portion of the temporal bone. Finally, it turns forwards in the groove on the upper surface of the jugular process of the occipital bone to the jugular foramen. As far as the postero-inferior angle of the parietal bone the transverse sinus is contained within the attached border of the tentorium cerebelli, and just before leaving this it receives the superior petrosal sinus. The right transverse sinus is, as a rule, formed by the superior longitudinal sinus, and in these circumstances is larger than the left, which is usually formed by the straight sinus. The reverse, however, may be the case. Before leaving the torcular the two sinuses communicate freely.
 
 
The transverse sinus of each side, in addition to the superior petrosal sinus, receives tributaries from the posterior part of the cerebrum and the superior and inferior surfaces of the cerebellum. It also receives the posterior temporal and occipital diploic veins, and the petrosquamous sinus, when that sinus is present. Near its termination it communicates with the marginal sinus of the same side. The sinus communicates with extracranial veins by means of two large emissary veins, posterior condylar and mastoid, which pass through the corresponding foramina. These foramina, however, being inconstant, the
 
 
emissary veins are sometimes wanting.
 
 
The name ‘ transverse ’ is strictly applicable only to that part ot the sinus which is contained within the attached border of the tentorium cerebelli , and extends from the internal occipital protuberance to the
 
 
 
i6o6
 
 
 
A MANUAL OF ANATOMY
 
 
 
postero-inferior angle of the parietal bone. Beyond this latter point the sinus is known as the sigmoid sinus.
 
 
The occipital sinus is situated within the falx cerebelli along its attachment to the internal occipital crest. It is of small size, and is formed inferiorly by the union of the two marginal sinuses (inferior occipital) which lie on either side of the vermiform fossa and foramen magnum, where they communicate with the posterior intraspinal veins and the terminal part of each lateral sinus. Superiorly it opens into the confluens sinuum. In certain cases the marginal or inferior occipital sinuses remain separate, and then each opens into the corresponding transverse sinus. The occipital sinus receives tributaries from the falx cerebelli and the inferior surface of the cerebellum, and it establishes a communication between the beginning and end of the transverse sinuses.
 
 
The cavernous sinuses are so named because the interior of each is broken up by fibrous filaments, which impart to it a reticular appearance. Each sinus is situated on the side of the body of the sphenoid bone, and extends from the inner extremity of the sphenoidal fissure to the apex of the petrous part of the temporal bone. Anteriorly it receives the ophthalmic vein or veins from the orbit, and posteriorly it terminates by dividing into the superior and inferior petrosal sinuses. In addition to the ophthalmic vein or veins, the cavernous sinus receives the spheno-parietal sinus and some of the inferior cerebral veins. It communicates with the angular vein of the face through the superior ophthalmic vein; with its fellow of the opposite side through the circular sinus; with the transverse sinus by the superior petrosal sinus; with the internal jugular vein by the inferior petrosal sinus; with the pterygoid plexus through the inferior ophthalmic vein, and by an emissary vein which passes through the foramen ovale, or through the foramen Vesalii ’; and with the pharyngeal plexus by an emissary vein which passes through the foramen lacerum medium. The internal carotid artery, with the cavernous sympathetic plexus, lies within the sinus, and the sixth cranial nerve (Fig. 989) is close to the outer side of the artery. In connection with the roof and outer wall of the sinus there are the third and fourth, as well as the ophthalmic and superior maxillary divisions of the fifth cranial nerves.
 
 
The spheno-parietal sinus is of small size, and is situated on the inferior surface of the lesser wing of the sphenoid bone. It generally begins in connection with the anterior temporal diploic vein, and ends in the anterior part of the cavernous sinus.
 
 
The circular sinus fills any small part of the hypophysial fossa which
 
 
is not occupied by the gland; on each side it opens freely into the cavernous sinus.
 
 
The superior petrosal sinus is situated along the superior border of the petrous part of the temporal bone, and lies within the attached margin of the tentorium cerebelli. It begins at the back part of the cavernous sinus, and, having passed laterally and backwards, it opens into the transverse sinus as that is about to enter the sigmoid groove
 
 
 
/
 
 
 
THE NERVOUS SYSTEM 1607
 
 
of the pars mastoidea. It receives tributaries from the cerebellum and tympanum.
 
 
The inferior petrosal sinus occupies the groove at the junction of the basilar process of the occipital bone with the petrous part of the temporal bone. It begins at the back part of the cavernous sinus, and, passing backwards and slightly laterally, it leaves the cranial cavity through the antero-medial compartment of the jugular foramen, to terminate in the bulb of the internal jugular vein. It receives tributaries from the inferior surface of the cerebellum, and from the internal ear.
 
 
 
 
Fig. 989._Right Internal Carotid put in Position on Base of Skull and
 
 
Crossed by Sixth Nerve.
 
 
Greater superficial petrosal nerve is also seen entering foramen lacerum.
 
 
The basilar sinus is really a plexus of veins situated within the dura mater over the basilar process of the occipital bone, extending as low as the anterior margin of the foramen magnum, where it communicates with the anterior intraspinal veins. It connects the anterior ends of the inferior petrosal sinuses with each other.
 
 
The petrosquamous sinus is situated along the junction of the petrous and squamous parts of the temporal bone. Its direction is backwards, and it opens into the transverse sinus as that is entering the
 
 
siermoid fossa of the pars mastoidea.
 
 
Each petro-squamous sinus represents the continuation of the transverse sinus in early life, to terminate in the primitive jugular vein. Often there are no traces of the petro-squamous sinus.
 
 
 
 
 
 
 
 
i6o8
 
 
 
A MANUAL OF ANATOMY
 
 
 
Emissary Veins. —These are vessels which pass through foramina in the cranial wall, and establish communications between the intracranial venous sinuses and the extracranial veins. The principal emissary veins are mastoid, nasal, posterior condylar, parietal, and occipital. In addition to these there are emissary veins, which pass through (i) the foramen ovale, (2) the foramen Vesalii (when present), and (3) the foramen lacerum and the carotid canal.
 
 
The transverse sinus in many cases has two emissary veins, mastoid and posterior condylar.
 
 
The mastoid emissary vein is of large size. It passes through the mastoid foramen, and usuallv opens into the posterior auricular vein.
 
 
The posterior condylar emissary vein passes through the posterior condylar foramen, and opens into the suboccipital plexus, from which the blood is carried away by the vertebral and deep cervical veins.
 
 
The mastoid and posterior condylar veins are not constant.
 
 
The superior sagittal sinus may have three emissary veins—parietal and occipital and nasal.
 
 
The parietal emissary vein passes through the parietal foramen, and opens into the occipital plexus, or into radicles of the superficial temporal vein.
 
 
The occipital emissary vein passes from the confluens sinuum to the occipital plexus, being transmitted through a minute foramen which pierces the external occipital protuberance.
 
 
The nasal emissary vein passes through the foramen caecum; like all other emissary veins it is often absent. It has been seen already that the cavernous sinus communicates with (1) the angular vein of the face, (2) the pterygoid plexus, and (3) the pharyngeal plexus. The marginal or inferior occipital sinus communicates with the posterior intraspinal veins, and the basilar sinus communicates with the anterior intraspinal veins.
 
 
 
Arachnoid Membrane.
 
 
This is a very delicate membrane which loosely surrounds the encephalon, and is situated between the dura mater and pia mater. It does not dip into the fissures, except in the case of the great longitudinal fissure, its general course being over the gyri and other eminences and depressions of the encephalon. It is conspicuous at the base of the encephalon in the region of the interpeduncular space, pons, and medulla oblongata. Its outer surface is practically in close contact with the inner surface of the dura mater, the extremely slight interval containing a very little lubricating serous fluid, and being known as the subdural space. Between the arachnoid and the pia mater there is the interval known as the subarachnoid space. The membrane furnishes sheaths to the various cranial nerves.
 
 
Subarachnoid Space.—This space lies between the arachnoid and the pia mater. It is crossed by delicate trabeculae of connective tissue, which pass between the two membranes, in a reticular manner. The meshes of this reticulum contain the cerebro-spinal fluid.
 
 
The subarachnoid space communicates with the ventricles of the brain by one main opening, the median aperture of the ventricle or foramen of Magendie, which is situated in the median line of the roof of the ventricle a little above the lower angle.* In some situations— as, for example, over the gyri—the arachnoid and pia mater are in close
 
 
The modern view is that there are also foramina at the lateral angles of the
 
ventricle, lateral apertures.
 
 
 
/
 
 
 
THE NERVOUS SYSTEM
 
 
 
1609
 
 
 
contact, but in other localities the two membranes are more or less widely separated by intervals, called cisternas. The most important of these are the cisterna magna, cisterna pontis, cisterna basalis, and cisterna venae magnae.
 
 
The cisterna cerebello-medullaris (or magna) lies between the posterior part of the inferior surface of the cerebellum and the medulla oblongata. It is of large size, the arachnoid, which here passes from cerebellum to medulla, being widely separated from the pia mater. It is continuous through the foramen magnum with the posterior part of the subarachnoid space of the spinal cord.
 
 
The cisterna pontis is situated on the ventral aspect of the pons. Interiorly it is continuous with the anterior part of the subarachnoid space of the spinal cord, and in the region of the medulla oblongata it is continuous otherwise with the previous cisterna and interpeduncularis. It contains the basilar artery.
 
 
The cisterna basalis or interpeduncularis is situated in front of the pons, in which situation the arachnoid extends over the interpeduncular space from one temporal lobe to the other. It contains the arteries which form the circulus arteriosus. This cisterna is prolonged outwards on each side into the stem of the lateral fissure, each of these prolongations containing the middle cerebral artery. Anteriorly it extends in front of the optic commissure into the great longitudinal fissure over the upper surface of the corpus callosum, this prolongation containing the anterior cerebral arteries.
 
 
The cisterna venae magnae lies just behind the entrance of the great transverse fissure, between the splenium and the corpora quadrigemina, where the great cerebral vein comes out.
 
 
The subarachnoid fluid can be drained away in two directions. It can enter the lymph-spaces of the cranial nerves upon which the arachnoid is prolonged outwards in the form of sheaths; and it can enter the lacunae laterales, and through the intervention of the Pacchionian bodies make its way into the superior longitudinal sinus.
 
 
Structure of the Arachnoid Membrane. —The arachnoid consists of fine fibrous tissue arranged in interlacing bundles, the intervals between these bundles being occupied by delicate cellular membranes. Several such layers, intimately blended together, form the membrane.
 
 
Beneath the arachnoid, and constituting a part of it, there is a reticulum of subarachnoid trabeculce. These trabeculae consist, as in the case of the arachnoid proper, of fine fibrous tissue, but the intertrabecular spaces, instead of being occupied by cellular membranes, contain cerebro-spinal (subarachnoid) fluid. The trabecular reticulum connects the arachnoid with the subjacent pia mater. The superficial surface of the arachnoid is covered with a delicate layer
 
 
of endothelium.
 
 
Arachnoid Granulations— These are small granular bodies which are situated along the course of the superior longitudinal sinus, into which some of them project. They are seldom met with in adults in other sinuses— e.g., the lateral and straight sinuses. Each body is a villous projection of the arachnoid membrane, with which it is connected by a narrow pedicle. Some bodies project into the superior
 
 
 
i6io
 
 
 
A MANUAL OF ANATOMY
 
 
 
longitudinal sinus; others project from below into the lacunae laterales. In all cases the bodies pierce the dura mater and carry before them the lining of the sinus. Superficially the bodies give rise to the depressions on the internal surface of the parietal bone near the superior border. Each body contains a prolongation of the subarachnoid space and reticulum. This is surrounded by a prolongation of the arachnoid membrane, and external to this is the endothelial lining of the sinus or of the lacuna. The granulations probably are channels through which the subarachnoid fluid can be drained away from the subarachnoid space into the lacunae laterales, and thence into the superior sagittal sinus, as well as into the other sinuses— e.g., the transverse and straight sinuses. They are rarely met with in children under twelve, and then most commonly in the transverse sinus.
 
 
Pia Mater.
 
 
The pia mater is the most internal covering of the encephalon. It is a very vascular membrane, which invests and is closely adherent to the entire surface. From its internal surface delicate processes pass into the cerebral substance, which represent the minute bloodvessels, surrounded by pia-matral sheaths. The pia mater not only invests the external surface, but also dips into the sulci, and covers the opposed surfaces of the gyri. It also furnishes sheaths to the various cranial nerves, which blend with their perineurium. It gives rise to two weblike expansions—namely, the tela chorioidea inferior and tela chorioidea superior.
 
 
The tela chorioidea inferior is situated in the lower part of the roof of the fourth ventricle, and from it are derived the choroid plexuses of that ventricle. The tela chorioidea superior (or velum interpositum) is an invagination of the pia mater through the transverse fissure beneath the splenium of the corpus callosum. It lies underneath the body of the fornix, and its lower surface is covered by the ependymal lining of the third ventricle, the latter forming the roof of that cavity. The tela chorioidea superior furnishes the choroid plexuses of the two lateral and third ventricles.
 
 
The pia mater of the encephalon differs from the pia mater of the spinal cord in being thinner and less adherent to the nervous substance. The greater thinness is due to the fact that it is destitute of the outer layer which characterizes the spinal pia mater.
 
 
Structure. —The pia mater of the encephalon consists of a single layer of areolar tissue, which contains a great many small bloodvessels, these being derived from the larger vessels lying in the subarachnoid space.
 
 
The Cranial Nerves.
 
 
The cranial nerves are arranged in twelve pairs. They have received numerical names according to the order in which they leave the cranial cavity from before backwards, and they also have descrip
 
 
THE NERVOUS SYSTEM
 
 
 
1611
 
 
 
tive names. The different pairs of nerves are as follows, in order from before backwards:
 
 
 
First, or olfactory.
 
 
Second, or optic.
 
 
Third, or oculo-motor.
 
 
Fourth, or trochlear (pathetic). Fifth, or trigeminal (trifacial). Sixth, or abducent.
 
 
 
Seventh, or facial.
 
 
Eighth, or auditory.
 
 
Ninth, or glosso-pharyngeal. Tenth, or vagus.
 
 
Eleventh, or accessory. Twelfth, or hypoglossal.
 
 
 
The cranial nerves are connected to certain parts of the encephalon, and these connections constitute their superficial or apparent origins. The fibres, however, can be traced to certain collections of grey matter, which are called nuclei. From the deep positions occupied by these nuclei they constitute the deep origins of the nerves.
 
 
First or olfactory nerve consists of the olfactory filaments or nerves, which are about twenty in number.
 
 
The olfactory nerves are non-medullated. They arise as the axons of the olfactory cells of the olfactory mucous membrane of the nasal fossa; and enter the cranial cavity through the foramina of one half of the cribriform plate of the ethmoid bone. Thereafter they enter the grey matter on the ventral or inferior aspect of the olfactory bulb, and terminate in arborizations which intermingle with the arborizations formed by the dendrites of the mitral cells situated in the granular layer of the bulb (see p. 1570).
 
 
Second or Optic Nerve.—This nerve arises from the brain by means of the optic tract, the deep connections of which have been already described (p. 1545). Each optic tract passes forwards and inwards to the optic commissure or chiasma, which is situated in front of the interpeduncular space. 1 he optic nerve of each side arises from the anterior part of the optic chiasma. It courses forwards and outwards to the optic foramen, through which it passes into the orbit, piercing the dura mater, and receiving a sheath from it, as well as from the arachnoid membrane. Having reached the back part of the eyeball, it pierces the sclerotic and choroid coats ^ inch to the nasal or inner side of the axis of the eyeball, and terminates in an expansion which forms the most internal layer of the retina, called the nerve-fibre or optic layer.
 
 
Neither this nor the preceding is, strictly speaking, a nerve at all.
 
 
Third or Oculo-motor Nerve.—The fibres of this nerve arise from the oculo-motor nucleus, which is situated in the grey matter of the ventral aspect (floor) of the aqueduct on a level with the upper quadrigeminal body, and extends superiorly for a short distance on to the lateral wall of the third ventricle. The nucleus is intimately related to the medial longitudinal bundle, by means of which it is connected with the trochlear and abducent nuclei. All three nuclei receive collaterals from the bundle; and in this manner a functional association between these nuclei is maintained, and harmonious action is insure on the part of the muscles which are supplied by the nerves arising from them. It consists of several groups of cells. As many as seven groups are ascribed to each oculo-motor nucleus by Perlia, which correspond
 
 
 
1612
 
 
 
A MANUAL OF ANATOMY
 
 
 
to the seven muscles supplied by the oculo-motor nerve, and are disposed symmetrically. In addition to these, there is a medially-placed group, the cells of which furnish fibres to both oculo-motor nerves. The fibres which arise from the individual groups of each nucleus are regarded as supplying particular orbital muscles. Certain of the oculo-motor fibres of one side arise from the nucleus of the opposite side, the fibres from either side decussating at the median line. Moreover, each oculo-motor nerve is said to receive fibres from the abducent
 
 
 
Olfactory Bulb
 
 
 
 
Lateral Fissure Gyrus Rectus
 
 
 
Ant. Perforated Substance
 
 
 
Tuber Cinereum
 
 
 
Corpus Mamillare Crus Cerebri
 
 
 
Post. Perforated Substance
 
 
 
Pons
 
 
 
Olfactory Tract
 
 
 
Optic Nerve
 
 
 
'.Medulla Oblongata
 
 
 
Spinal Cord
 
 
 
Optic Chiasm a
 
 
 
Optic Tract Third Nerve
 
 
 
Fourth Nerve
 
 
 
Fifth Nerve Sixth Nerve Facial Nerve
 
 
Sensory Root ol VIJ.
 
 
Auditory Nerve Glosso-pharyngeal Nerve Vagus Nerve ' Accessory Nerve
 
 
Hypoglossal Nerve
 
 
 
Fig. 990. —The Base of the Encephalon, and the - Cranial Nerves. 1, frontal lobe (orbital surface); 2, temporal lobe; 3, cerebellum.
 
 
 
nucleus of the opposite side, which ascend in the posterior longitudinal bundle and cross to the other side.
 
 
Two views are entertained in regard to the nerve-supply of the medial rectus muscle. According to one view, the muscle of one side is supplied by those fibres which have crossed from the oculo-motor nucleus of the opposite side. The other view is that the muscle of one side is supplied by those fibres which have crossed from the abducent nucleus of the opposite side. According to this latter view,
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
THE NERVOUS SYSTEM
 
 
 
1613
 
 
 
the nerve-fibres which supply the lateral rectus muscle of one side and those which supply the medial rectus muscle of the opposite side arise from the same nucleus—namely, the abducent nucleus—and vice versa.
 
 
Probably the whole of the oculo-motor nucleus is not in series with the medial somatic group to which the fourth, sixth, and twelfth nuclei belong, but that some of it corresponds to the more lateral group containing the seventh, ninth, and tenth nuclei. Fibres from this part probably go to the ciliary muscle and iris.
 
 
 
 
Fig qqi._Scheme showing the Different Cell-groups which constitute,
 
 
'according to Perlia, the Nucleus of Origin of the Third Cranial
 
 
OR OCULO-MOTOR NERVE (FROM TESTUT, AFTER PERLIA).
 
 
 
1. Posterior Dorsal Nucleus 1'. Posterior Ventral Nucleus
 
 
2. Anterior Dorsal Nucleus 2'. Anterior Ventral Nucleus
 
 
 
3
 
 
4
 
 
5
 
 
 
Central Nucleus ,
 
 
Jucleus of Edinger and Westphal Lntero-medial Nucleus
 
 
 
6. Antero-lateral Nucleus
 
 
7. Trunk of Oculo-Motor Nerve
 
 
8. Crossed Fibres
 
 
9. Nucleus of Origin of Fourth Nerve
 
 
g'. Intercrossing of Fourth Cranial Nerves
 
 
10. Third Ventricle M. Middle Line.
 
 
 
The old view, that the orbicularis oculi is supplied from this nucleus, by the medial longitudinal bundle and the facial trunk, is no longer held by anatomists.
 
 
Course of the Fibres o£ the Third Nerve.— The fibres pass forwards from their origin through the tegmentum, the red nucleus, and the medial portion of the substantia nigra, and afterwards make their superficial appearance at the oculo-motor sulcus on the medial aspect of
 
 
the crus cerebri. .
 
 
The third or oculo-motor nerve supplies the following seven muscles.
 
 
the levator palpebne superioris; the superior, inferior, and internal
 
 
 
 
 
 
A MANUAL OF ANATOMY
 
 
 
1614
 
 
recti; the inferior oblique; the sphincter pupillae; and the ciliary muscle.
 
 
Fourth (Trochlear, or Pathetic) Nerve. —The fibres of this nerve arise from the trochlear nucleus, which is situated in the grey matter of the ventral aspect (floor) of the aqueduct on a level with the upper part of the lower quadrigeminal body. The nucleus is intimately related to the medial longitudinal bundle, by means of which it is connected with the oculo-motor nucleus.
 
 
The fibres are at first directed laterally and backwards, and then medially to the upper part of the superior medullary velum, which they enter. Here the nerve crosses to the opposite side, decussating
 
 
 
 
Fig. 992.—Deep Origins of Third, Fourth, and Fifth Cranial Nerves.
 
 
with its fellow, after which it emerges from the upper end of the superior medullary velum close below the lower quadrigeminal body, and by the side of the frenulum veli. After this the nerve turns over the superior peduncle of the cerebellum, and is then directed forwards, round the outer aspect of the crus cerebri, between which and the temporal lobe it makes its superficial appearance.
 
 
The fourth nerve supplies the superior oblique muscle of the eyeball.
 
 
Fifth Cranial, Trigeminal (or Trifacial) Nerve. —The fifth cranial nerve resembles a spinal nerve in having two roots—sensory and motor
 
 
the former being large, and having a ganglion, called the trigeminal ganglion.
 
 
 
 
 
 
 
 
 
 
 
 
 
THE NERVOUS SYSTEM
 
 
 
1615
 
 
Sensory Root. —The fibres of this root are derived from the central poles of the bipolar cells of the trigeminal ganglion. After entering the pons each fibre divides into two branches, ascending and descending, as in the case of the fibres of the dorsal or sensory root of a spinal nerve. The terminal nuclei of these ascending and descending sensory fibres are two in number—upper and lower.
 
 
♦The upper sensory nucleus is situated in the outer portion of the dorsal part of the pons, where it lies close to the lateral side of the pontine or principal motor nucleus of the nerve. The ascending sensory fibres, after a short course, enter this nucleus and terminate in arborizations around its cells.
 
 
The lower sensory nucleus, continuing the line of the upper sensory nucleus, is an upward prolongation of the substantia gelatinosa from the tubercle and funiculus gelatinosus in the medulla oblongata. The nucleus is traceable as low as the dorsal grey horn of the spinal cord on a level with the second cervical spinal nerve, where it is close to the substantia gelatinosa. The descending sensory fibres, which are numerous, and constitute the spinal root of the fifth nerve, pass downwards through the pons and medulla oblongata into the spinal cord as low as the level of the second cervical spinal nerve. They are accompanied by the lower sensory nucleus, and at different levels they enter this nucleus and terminate in arborizations around its cells.
 
 
The disposition of the fibres and cells within the spinal root is of a reversed order—that is, the ophthalmic nerve is associated with the lower part of the spinal root, above this the maxillary, with the mandibular at the upper end.
 
 
The ascending or mesencephalic nucleus of the fifth extends along the grey matter on the side of the aqueduct as far as the level of the lower part of the upper corpus quadrigeminum. Its lower limit is lateral to the substantia coerulea in the upper part of the fourth ventricle. The mesencephalic root has only been recognized as sensory within the last few years, and there is reason to suppose that it receives proprioceptive impulses from certain muscles.
 
 
Motor Nucleus. —This nucleus is placed in the lateral part of the tegmental region of the pons, deep to the floor of the upper or pontine portion of the fourth ventricle, and immediately medial to the upper sensory nucleus of the nerve. Its fibres run ventro-laterally to emerge as the small motor root of the nerve.
 
 
Most of the axons of the cells of the terminal sensory nuclei pass inwards to the raphe and cross to the opposite side. They then become longitudinal and ascend in company with the medial lemniscus or chief sensory tract, their destination being the thalamus of the side to which they have crossed. They thus constitute a trigemino-thalamic ascending tract. From these fibres collaterals are furnished to (1) the facial nucleus, and (2) the ventral vago-glosso-pharyngeal nucleus, or nucleus am
 
 
i6i6
 
 
 
A MANUAL OF ANATOMY
 
 
 
biguus, from the cells of which latter the efferent or motor fibres of the pneumogastric or vagus nerve arise.
 
 
A few of the axons, however, enter the pontine or chief motor nucleus, and also the mesencephalic sensory nucleus, of the nerve, and terminate in arborizations around its cells.
 
 
The large sensory and small motor roots appear close together, on the lateral aspect of the ventral surface of the pons, the motor root lying above and slightly internal to the sensory root. The sensory root enters and the motor root leaves the pons.
 
 
Distribution. —The fifth cranial nerve has an extensive distribution by means of its three divisions—ophthalmic, superior maxillary, and inferior maxillary.
 
 
Ophthalmic Nerve (Sensory). —(i) The front part of the cranium; (2) the integument of (a) the upper eyelid, and ( b ) the root and tip of the nose; (3) the anterior part of the nasal mucous membrane, and the conjunctiva; (4) the eyeball; and (5) the lacrimal gland.
 
 
Maxillary Nerve (Sensory). —(1) The integument of the zygomatic and anterior part of the temporal regions; (2) the integument of (, a) part of the lower eyelid, ( b) the side of the nose, (c) the upper lip, and (d) that part of the face between the lower eyelid and the upper lip; (3) the upper teeth, and the mucous membrane of the upper gum;
 
 
(4) a large part of the nasal mucous membrane; (5) the mucous membrane of the maxillary air-sinus (or antrum of Highmore); (6) the mucous membrane of (a) the naso-pharynx, and ( b ) the soft and hard palate and the tonsil.
 
 
Mandibular Nerve (Sensory and Motor). —The sensory distribution
 
 
of this nerve is as follows: (1) the integument of (a) the temporal region, ( b ) the outer surface of the pinna, and (c) the external auditory meatus;
 
 
(2) the integument of the lower lip, and that which covers the mandible; a recurrent branch runs along the petro-squamous suture, supplying the mucous membrane of the tympanum and of the mastoid antrum;
 
 
(3) the temporo-mandibular joint; (4) the parotid salivary gland;
 
 
(5) the mucous membrane lining the buccinator muscle, and the integument covering that muscle (by means of the long or sensory buccal nerve ); (6) the mucous membrane (fungiform and conical papillae) of the anterior two-thirds of the tongue (common sensation); (7) the submandibular and sublingual salivary glands; and (8) the pulps of the lower teeth, and the mucous membrane of the lower gum.
 
 
The motor distribution of the mandibular nerve is as follows: (1) The muscles of mastication—namely, (a) the masseter, ( b ) the temporal, and (c) the pterygoid muscles; (2) the mylo-hyoid muscle and anterior belly of the digastric; (3) the tensor tympani muscle by means of a branch from the otic ganglion; and (4) the tensor palati muscle through the otic ganglion.
 
 
Sixth or Abducent Nerve. —The fibres of this nerve arise from the abducent nucleus, which is situated in the dorsal part of the pons close to the median line. It lies above the striae acusticae on the floor of the
 
 
 
THE NERVOUS SYSTEM
 
 
 
1617
 
 
fourth ventricle subjacent to the eminentia teres. The fibres emerge from the inner part of the nucleus, and pass through the lower part of the pons in a forward and slightly downward and lateral direction to the lower border of the pons just lateral to the pyramid of the medulla oblongata, where the nerve makes its superficial appearance.
 
 
The abducent nucleus receives collaterals from the medial or posterior longitudinal bundle, and a functional connection is thereby established between that nucleus and the oculo-motor nucleus. The medial rectus muscle of one side and the lateral rectus of the other side are thus associated muscles.
 
 
The sixth nerve supplies the lateral rectus muscle of the eyeball.
 
 
 
 
Fig. 993. —Deep Origins of Sixth, Seventh, and Eighth
 
 
Cranial Nerves.
 
 
Seventh or Facial Nerve.— The facial nerve is composed of two parts. One of these consists of efferent or motor fibres, and is known as the facial nerve proper. The other part, of small size, consists of afferent or sensory fibres. The facial nerve proper arises from the facial nucleus , which is situated deeply in the dorsal part of the lower portion of the pons. The fibres of the nerve pursue an intricate course before appearing superficially. They at first pass backwards and inwards to the floor of the fourth ventricle. Here they turn upwards, lying close to the median line in the form of a single bundle. The nerve then makes a sharp bend laterally, and passes forwards through the pons in a downward and outward direction to its place of emergence.
 
 
T02
 
 
 
 
 
 
 
 
 
 
 
 
i6i8
 
 
 
A MANUAL OF ANATOMY
 
 
 
In its course within the pons the nerve is intimately related to the dorsal aspect of the abducent nucleus.
 
 
1 he intrapontine part of the facial nerve proper is intimately related to the following structures:
 
 
1. The abducent nucleus. 4. The spinal root of the fifth
 
 
2. The superior olive. nerve.
 
 
3. The corpus trapezoides. 5. The medial or posterior
 
 
longitudinal bundle.
 
 
The motor facial nucleus receives fibres from the following sources: (1) The corpus trapezoides, being thereby brought into connection with the cochlear division of the auditory nerve; (2) the spinal root of the
 
 
 
sup?- cerebellar^ peduncle.
 
 
 
emmeniia, teres.
 
 
nucleus of"., sixth n.
 
 
medial longitudbundle.
 
 
stalk of olive"' mediul fillet -
 
 
 
\- -V-inferiori cerebellar -ymiddle j peduncle.
 
 
_\_lai- vestibular
 
 
nucleus.
 
 
 
-/-spinal root of V f - h n.
 
 
 
facial nucleus
 
 
-facial nerve
 
 
^vestibular part '"of 8 f h nerve.
 
 
s . "'-corpus trapezoides
 
 
"'Supr. olivary nucleus
 
 
abducens nerve pyramidal tract.
 
 
Fig. 994.— Diagrammatic Section through the Pons, to show Deep Origins of Sixth (Red) and Seventh (Black) Cranial Nerves.
 
 
 
fifth cranial or trigeminal nerve, which is the sensory nerve of the face; and (3) the pyramidal tract of the opposite side, being thereby brought into connection with the precentral motor area of the cerebral cortex.
 
 
All the foregoing fibres terminate within the nucleus in arborizations around its component cells.
 
 
The sensory portion of the facial nerve arises from the central poles of the bipolar cells of the geniculate ganglion on the facial nerve in the facial canal. This ganglion resembles the ganglion of the fifth nerve and the spinal ganglia, and most of the peripheral poles of its bipolar cells give rise to the chorda tympani nerve. The pars intermedia passes from the facial canal into the internal auditory meatus, after leaving which it runs to the lower border of the pons, where it lies between the
 
 
 
 
 
 
 
 
 
THE NERVOUS SYSTEM
 
 
 
1619
 
 
 
facial nerve proper and the auditory nerve. The nerve then enters the medulla oblongata, and passes downwards to the upper part of the nucleus of the fasciculus solitarius (see Glosso-pharyngeal Nerve), and its fibres terminate in arborizations around the cells of the upper part of that nucleus. In this situation it is closely associated with the terminal afferent or sensory fibres of the glosso-pharyngeal nerve.
 
 
The facial nerve proper emerges from the brain at the lower border of the pons in front of, and internal to, the auditory nerve; and the sensory part enters between the facial nerve proper and the auditory nerve.
 
 
 
 
Fig. 995.—Terminal Nuclei of the Vestibular Nerve, with their Superior Connections (Schematic) (L. Testut’s * Anatomie Humaine ’).