Book - The Nervous System of Vertebrates (1907) 2

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Johnston JB. The Nervous System of Vertebrates. (1907) Blakiston's Son & Co., London.

   1907 The Nervous System of Vertebrates: 1 The Study of the Nervous System | 2 General Morphology of the Nervous System | 3 Development of the Nervous System | 4 Nerve Elements and Their Functions | 5 The Functional Divisions of the Nervous System | 6 Somatic Afferent Division. General Cutaneous Subdivision | 7 Somatic Afferent Division. Special Cutaneous Subdivision | 8 Somatic Afferent Division. The Visual Apparatus | 9 The Visceral Afferent Division | 10 The Olfactory Apparatus | 11 The Somatic Motor Division | 12 The Visceral Efferent Division | 13 The Sympathetic System | 14 Centers of Correlation | 15 The Cerebellum | 16 Centers of Correlation. The Mesencephalon and Diencephalon | 17 Correlating Centers in the Diencephalon (Continued) | 18 The Evolution of the Cerebral Hemispheres | 19 The Neopallium | Figures
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Chapter II. General Morphology of the Nervous System

In any vertebrate animal two chief parts of the nervous system are distinguished, the central system consisting of the brain and spinal cord and the peripheral system. The peripheral system includes the nerves which connect the central system with various parts of the body, the ganglia of those nerves, sense organs, and the sympathetic system. The central nervous system is situated dorsal to the alimentary canal and is surrounded by more or less strong skeletal structures which constitute the greater part of the skull and spinal column.

In the trunk region the central nervous system consists of a rounded cord which is enclosed within the neural arches of the vertebrae. In fishes this spinal cord extends the whole length of the spinal column. In higher forms the caudal portion is less developed, the cord as a whole grows less rapidly than the trunk and remains in the adult shorter than the spinal column, ending in the lumbar region. Beyond this a slender thread continuing into the tail represents the caudal portion of the spinal cord of fishes. In all vertebrates having well developed limbs the two regions of the spinal cord with which the nerves of the limbs are connected are somewhat thicker than the rest of the cord. These thickened portions are known as the thoracic and lumbar enlargements. The cord is usually more or less flattened dorso-ventrally and upon the dorsal and ventral surfaces in the middle line are to be seen longitudinal grooves, the dorsal and ventral fissures. The ventral fissure is usually a deep furrow r . If the cord be cut across and the cut surface' examined with a hand lens, the greater depth of the ventral fissure will be evident and in the median plane there will be seen a narrow opening. This is the central canal wiiich extends throughout the length of the cord. The material surrounding the central canal and filling up the inner portion of the cord is very soft and somewhat gray in color, while the outer part is more firm and white. The gray matter is composed chiefly of cells, the white matter chiefly of fibers. The whiteness is due to the sheaths covering the fibers. The distinction between cellular and fibrous parts is not apparent in the cord of such an animal as Petromyzon whose nerve fibers are not provided with such sheaths. The gray matter has in cross section somewhat the form of the letter H, the central canal being found in the crossbar of the H. In either half of the cord a part of the gray matter extends dorsal to the central canal and a part extends ventral to it. These parts are called respectively the dorsal and ventral horns of the gray matter. In mammals and man a smaller projection of the gray matter laterally is spoken of as the lateral horn.

Each lateral half of the cord has connected with it the roots of the peripheral nerves. In all vertebrates there is in each typical segment of the body a dorsal root connected with the dorsal surface of the cord and a ventral root connected with the ventral surface. In the lowest vertebrates (Amphioxus and Cyclostomes) the dorsal and ventral roots of the same side alternate with one another along the cord. This is due to the fact that the dorsal nerve is destined to go in larger part to the skin while the ventral nerves go to the muscles of the trunk. Since the muscles are arranged in simple transverse segments following one another, the dorsal nerve is placed in the interval between two segments where it passes in the intermuscular septum of connective tissue to the skin. The ventral nerve, on the other hand, is situated opposite the middle of the muscle segment and is distributed directly to the muscle. Since part of the muscle lies above the spinal cord the ventral root divides into a dorsal and a ventral ramus. The dorsal root, after emerging from the spinal column becomes thickened by a collection of ganglion cells. This thickening is called the spinal ganglion. Beyond the ganglion the nerve divides into dorsal and ventral rami, which go to the skin.

In true fishes and in all higher vertebrates there has come about a shifting of parts such that the dorsal and ventral roots of a given segment come to lie nearly in the same transverse plane of the body and the two roots unite at or just beyond the ganglion of the dorsal root (Fig. i, B). The dorsal rami of the two roots unite to form a common dorsal ramus and the ventral rami unite to form a common ventral ramus. From the ganglion or from the common ventral ramus just beyond the ganglion, there arises a branch which enters a ganglion of the sympathetic system, by way of which the viscera are brought into connection with the central nervous system. The branch to the sympathetic system is known as the ramus communicant.

In the head region in all true vertebrates the central nervous system becomes considerably enlarged to form the brain. The nerves connected with the brain are called cranial nerves. There is no sharp limit between spinal cord and brain or between spinal and cranial nerves, as there is none between trunk and head. The brain presents in the adult five constant and well marked portions which will serve as a guide in the description of the nervous system of the head. These are known as secondary segments of the brain and are named from behind forward: myelencephalon, metencephalon, mesencephalon, diencephalon and telencephalon (Fig. 2).


FIG. 1. Outlines of the spinal cord with the dorsal and ventral nerve roots; A, in Petromyzon; B, in a mammal, d. /., dorsal fissure; d. h., dorsal horn; d. ., dorsal nerve; /. h., lateral horn; v. /., ventral fissure; v. h., ventral horn; v. n., ventral nerve.

The mydencephalon, or medulla oblongata, appears as a gradual enlargement extending forward from the spinal cord. As the spinal cord merges into the brain the dorsal portions of the cord spread apart and the dorsal fissure seems to widen out into a broad thin membranous roof of the central canal. This canal at the same time becomes enormously enlarged and is known as the fourth ventricle of the brain. Its roof is called the plexus chorioideus of the fourth ventricle.


FIG. 2. The brain of a selachian, Heptanchus. A, dorsal view; B, lateral view. The roots of the nerves are drawn in black as they appear after painting with osmic acid. The left half of the roof of the fourth ventricle is removed.

If the plexus be removed and the brain examined from within as well as from without, certain ridges and grooves will be seen which indicate a division of the myelencephalon into longitudinal zones (Fig. 3). The ventral portion of the myelencephalon appears to be a continuation of the ventral part of the spinal cord with slight modification. In the floor of the fourth ventricle is to be seen a deep median groove bounded by two narrow but usually high ridges. The fiber bundles which make up these ridges bear important relations to the motor nerve roots. The ventral horns of the gray matter of the cord continue into the myelencephalon and are marked by two grooves which bound laterally the two ridges just mentioned. In the lateral zones there is much greater change from the cord to the brain. The greater size of the myelencephalon is due chiefly to the greater volume of these lateral parts and to their bulging laterally. From the internal surface of the lateral wall there projects into the cavity a ridge constituted chiefly of a thickening of the gray matter. It extends from the caudal end of the myelencephalon to near the cephalic end, stopping abruptly opposite the seventh or facialis nerve. This ridge has been known as the lobus vagi or the lobus facialis, owing to the relation which it bears to the vagus and facialis nerves. The ridge in cyclostome fishes is small, in selachians of moderate size, in ganoids (Fig. 3) relatively large and in some bony fishes enormous (Fig. 4, L. vg.). In some bony fishes, as in Carpiodes whose brain is shown in Figure 4, it is the caudal part of this ridge which is greatly enlarged. In other cases the cephalic portion is so large that it overtops all other parts of the medulla oblongata, and the ridges of the two sides may fuse in the median plane into a single mass, the lobus impar. As will be seen later (p. 1 59) this ridge is the place of ending of the sensory fibers of the visceral surfaces in the head and of the organs of the sense of taste. The ridge would therefore be appropriately named the lobus visceralis. In amphibians, reptiles, birds and mammals the part of the brain corresponding to this lobe, although recognizable microscopically, is much smaller than in fishes and is not to be seen as a projecting ridge. The name visceral sensory column will be used for this whole structure. When it forms a projecting ridge the term lobus visceralis may be used for the whole, or lobus vagi and lobus facialis for its two parts.


FIG. 3. The medulla oblongata and cerebellum of the lake sturgeon (Acipenser rubicundus), to show the longitudinal zones. A, dorsal view with the choroid plexus removed. B, C and D, sketches of sections at the levels indicated by the reference lines. The dark area with light circles is the continuation of the ventral horn of the cord. The dark area with rectangular spaces is the continuation of the lateral horn. The area with oblique lines is the visceral sensory column (lobus visceralis). The area with vertical lines is the somatic sensory column.


FIG. 4. Two views of the brain of the buffalo fish, Carpiodes veli/er. (Raf .) ; (i) from above, (2) from the right side. Twice the natural size. From C. Judson Herrick after C. L. Herrick.

The vagal lobes (L. vg.) are very large and, with the overhanging cerebellum, completely conceal the facial lobe. In the upper figure the cerebellum appears as a nearly rectangular body in front of the vagal lobes and in front of this is the roof of the mesencephalon. The optic lobes are pushed wide apart by the enormous valvula cerebelli within and the shaded area in the figure represents a membranous portion of the roof connecting the optic lobes. In front of this is the basal ganglion of the forebrain, the membranous roof of which has been cut away. The olfactory bulbs are cut away. In the lower figure the large inferior lobes are seen below the optic lobes and behind the latter the cerebellum is produced ventrally as the superior secondary gustatory nucleus. Immediately behind this and above the VIII nerve is the lobus lineae' lateralis. The nerves of the trigemino-facial root complex are marked Vi. d, Vi. v, 2 and VII, the auditory nerve VIII, and the glossopharyngeus and vagus IX and X.

The upper part of the lateral wall of the myelencephalon is formed by a thick ridge of gray matter which is continuous caudad with the dorsal horn of the spinal cord and cephalad with the cerebellum. This ridge is more or less prominent in all lower vertebrates (see Figs. 2, 3) and its equivalent in mammals is to be found in the acustic nuclei and the restiform bodies. The ridge has been known as the tuberculum acusticum on account of its relation to the eighth cranial or auditory nerve. Since the ridge is the place of ending of all the cutaneous nerves of the head and since the term tuberculum acusticum is applied to a restricted portion of this region in mammals, it would be better to call this the somatic sensory column. In selachians and ganoids and to a less extent in cyclostome fishes a short part of this lobe projects prominently dorsad just behind the cerebellum. Since this is related wholly to nerves which supply the so-called lateral line organs, it is called the lobus lineae lateralis.

Most of the cranial nerves are connected with the myelencephalon. In this region the dorsal and ventral nerves remain separate from one another throughout life. Of the dorsal nerves the most caudal which can be recognized in all vertebrates corresponds to the tenth cranial or vagus nerve of human anatomy. This arises by numerous small roots some of which are sensory, others motor. The motor are situated slightly ventral to the sensory. The roots all unite into one large trunk which descends to the dorsal border of the second gill slit in fishes and there bears a ganglion (Fig. 5). From this ganglion a main trunk continues caudally and bears a ganglion over each gill slit. From each ganglion two rami arise. The first runs ventrally in front of the slit and is called the ramus praetrematicus. It gives the large ramus pharyngeus to the mucosa of the roof of the pharynx, and then supplies the gill filaments and the mucosa of the lateral and ventral walls of the pharynx. The other is the ramus posttrematicus which runs ventrally behind the gill slit and supplies the muscles of the branchial arch, gill filaments, the mucosa and taste buds, and in cyclostomes the overlying skin. Beyond the last gill slit the vagus trunk is continued caudad as the ramus intestinalis. In many forms especially among bony fishes the position of the roots and ganglia of the vagus with reference to the gills becomes modified and is not so simple as indicated in the diagram. Figures 51, 63, 79 will show the arrangement in a cyclostome, a bony fish and an amphibian.


FIG. 5. Simple diagrams of the branchial nerves of lower vertebrates as seen from the left side. A, in a cyclostome; B, in a true fish. In B the trigeminus nerve is not shown.

The more caudal motor roots of the vagus series supply certain muscles* connected with the shoulder girdle (trapezius musculature). Owing to the disappearance of the gills in higher vertebrates and the consequent reduction of the more cephalic motor roots, these more caudal roots become more prominent higher in the scale of vertebrates. They have been set apart as an independent nerve under the name of the eleventh cranial or spinal accessory nerve.

A short distance cephalad from the vagus appears the ninth cranial or glossn pharyngeus nerve. It arises by a sensory and a motor root, bears a ganglion over the first gill slit, and gives rise to pharyngeal, pretrematic and posttrematic rami as in the case of each of the vagus ganglia. The pharyngeal ramus extends into the palate and is known as the ramus palatinus IX, and the posttrematic ramus is known as the ramus lingualis IX because it continues into the tongue.

Above the glossopharyngeus, sometimes in front of and sometimes behind it, arises the nervus lineae later alls. It is an independent sensory root which usually joins the trunk of the vagus and runs for some distance with it, then continues separately beneath the skin as the nerve of the special sense organs of the lateral line. This nerve enters the dorsal somatic sensory lobe and cephalad from it two or three other roots enter the same lobe. The most caudal of these is the eighth cranial or auditory nerve. In front of this there are in cyclostomes, selachians and ganoids, two roots, the more dorsal connected with the lobus lineae lateralis and the more ventral connected with the tuberculum acusticum, which together supply the lateral line organs of the head. In bony fishes and aquatic amphibia only the more ventral root is present. In terrestrial vertebrates the roots which supply lateral line organs on both the trunk and head disappear because the sense organs are useful only in aquatic life. Of this group of nerves only the auditory remains in higher vertebrates. The nerves which supply the lateral line organs of the head enter into close relations with the facialis or trigeminus nerve and their rami have been variously named as rami of the latter nerves. Three chief rami are formed, a supraorbital, an infraorbital and a mandibular, which will be more fully described in a later chapter (see Chap. VII).


FIG. 6. A diagram of the lateral line canals and pit organs together with the nerves which supply them in a ganoid fish (Amia caVvd). After E. Phelps Allis. The canals are shaded with cross lines and the canal organs are shown as black discs in the course of the canals. The pit organs are shown as rows of black dots. Only the peripheral nerve trunks are shown, the ganglia and roots being omitted.

Ventrad or ventro-cephalad from the auditory root are the two roots, a sensory and a motor, which constitute the seventh cranial or jacialis nerve. The two roots enter a ganglion from which arise the ramus palatinus to the mucosa of the roof of the mouth and a ramus hyoideus which is joined for some distance with the mandibular ramus of the lateral line nerves in the common ramus hyomandibularis, and eventually supplies the hyoid muscles and the mucosa of the floor of the mouth. The ramus hyoideus is a posttrematic ramus, since it runs behind the spiracular cleft when that is present, and there is often a pretrematic ramus running in front of the cleft and arising in common with the ramus palatinus.

The distribution of the organs of the sense of taste innervated by the X, IX and VII nerves is of great importance. In fish-like vertebrates the taste buds are found in the mouth and branchial cavities. They are also distributed more or less widely on the outside of the head and in extreme cases, as in some bony fishes, on the fins and over almost the entire body. They have usually no regular arrangement in rows and differ from the lateral line organs in that they usually project above the surface and are never depressed in pits or canals. In terrestrial forms the taste organs are confined to the mouth cavity.

From the cephalic end of the somatic sensory column of the myelencephalon the fifth cranial or trigeminus nerve takes its origin by a more dorsal sensory and a more ventral motor root. The roots enter a ganglion which is partly divided into two portions, a dorso-cephalic and a ventro-caudal portion. From the dorsocephalic ganglion a large nerve runs forward through the dorsal part of the orbit and supplies the skin of the snout and dorsal surface of the head. This is the nervus ophthalmicus projundus and its ganglion may be known as the profundus ganglion. The ventro-caudal ganglion is properly known as the trigeminal ganglion. From it arise two large rami: the ramus maxillaris which supplies the skin beneath the eye and the lining of the front part of the roof of the mouth; and the ramus mandibularis which supplies the skin over the lower jaw and the muscles which move the lower jaw. These two rami are apparently comparable to the pretrematic and posttrematic rami of the branchial nerves, the mouth taking the place of a branchial cleft. The nerve complex consisting of the ophthalmicus profundus and trigeminus proper is fairly constant in its relations and size throughout the vertebrate series because of the constancy of the sensory area supplied by it, the skin of the anterior part of the head.

Two ventral nerves are connected with the myelencephalon, the so-called hypoglossus and the abducens. The hypoglossus or twelfth cranial nerve arises by a variable number of roots in cephalo-caudal succession in about the region of junction of brain and spinal cord. These roots unite into a common trunk or plexus which supplies the muscles of the tongue. In lower vertebrates it is evident that the roots are the equivalent of several ventral segmental nerves and form a simple continuation forward of the series of ventral nerves of the trunk. The number of such nerves present is greater in the more primitive forms. While in higher vertebrates an interval representing several segments intervenes between the roots of the hypoglossus and abducens, in such forms as Chimaera (Fig. 7), Heptanchus and related forms (Fig. 2), and in Petromyzon dorsatus (Fig. 51) only one or two ventral nerves are wanting in the series between the abducens and the ventral spinal nerves. In Bdellostoma the series of ventral nerves is quite complete except the eye-muscle nerves, which are wanting. The abducens or sixth cranial nerve also arises by several rootlets which may extend from the level of the VII nerve root back nearly to the level of the IX nerve. The nerve formed by these rootlets supplies the rectus externus muscle of the eye-ball.


FIG. 7. A sketch of the brain of Chitnaera monslrosa from the left side to show especially the position of the nerve roots. The roots of the nerves were blackened by osmic acid and in this way one or more roots of the abducens and hypoglossus, not to be seen in ordinary dissections, were brought to view. The nerve roots are indicated by the usual Roman numerals. The hypoglossal and spinal roots are numbered w, x, y, z, a, b, c, 4, 5, 6, after Fiirbringer's scheme.

The metencephalon is a short segment of the brain which ventrally appears to be merely a continuation forward of the myelencephalon, but dorsally is sharply distinguished by the possession of a massive roof instead of a choroid plexus. This massive roof is the cerebellum. It is large in all true fishes and in some selachians it is the most prominent part of the whole brain. In bony fishes it projects inward also, encroaching upon the fourth ventricle and largely filling the cavity of the mesencephalon. In cyclostomes, dipnoans and amphibians the cerebellum is very small, but in reptiles, birds and mammals it becomes progressively larger and more important. Its size is evidently correlated with the activity of the animal and with the number and importance of the cutaneous sense organs. In all vertebrates the cerebellum consists fundamentally of an arch of gray matter covered externally by a fiber layer which forms a commissure dorsally. The two pillars of the arch are continuous with the somatic sensory column of the medulla oblongata. In mammals and man the ventral wall of the metencephalon is greatly thickened and forms a ventral protuberance known as the pons Varolii.

That portion of the dorsal wall of the brain which connects the cerebellum with the mesencephalon is thin in most vertebrates and is known as the velum medullare anterius. This velum undergoes various modifications which are discussed in a later chapter (see p. 171). The fourth cranial or trochlearis nerve, which crosses with its fellow in the velum and emerges from the brain between the cerebellum and mesencephalon, is reckoned with the cerebellar segment. It supplies the superior oblique muscle of the eye-ball.

The mesencephalon or midbrain is perhaps the most constant portion of the brain in vertebrates. Its ventral and lateral walls are always massive, its cavity a narrow canal, the aqueduct of Sylvius. Its dorsal wall is less thick and is divided by a longitudinal furrow into lateral portions which are known as the optic lobes, because they serve as the place of ending of the fibers of the optic tract coming from the retina. In mammals the lateral lobes become divided by transverse furrows into anterior and posterior parts and the four bodies thus formed receive the name of corpora quadrigemina. The optic lobes vary in size in different classes of vertebrates, being noticeably larger in those animals in which the eyes are especially large and important (bony fishes, some selachians, birds, etc.). In mammals, however, the size of the eyes does not greatly affect the size of the corpora quadrigemina (compare Chapter XVI). The cephalic border of the roof of the mid-brain is marked by the posterior commissure. The lateral and ventral walls of the mid-brain are in general comparable to the same portions of the medulla oblongata. From the ventral surface of the mesencephalon arises the third cranial or oculomotor nerve which supplies four of the eye muscles, rectus superior, rectus inferior, rectus internus and obliquus inferior. The diencephalon or interbrain, although the smallest of the secondary segments of the brain, is one of the most interesting and important and presents many points of morphological significance and many variations in different classes of vertebrates. When seen from the side (Figs. 2, 7), it appears as a wedge-shaped segment, the edge of the wedge being upward. The short dorsal wall is mostly membranous, but is thickened at one point by the so-called superior commissure which connects the two small knob-like thickenings of the dorsal border of the lateral wall, the nuclei habenulae. These two bodies which are constantly present and of great significance in the vertebrate brain, are usually of unequal size on the right and left sides. Just behind the superior commissure there arises from the dorsal surface of the diencephalon a small sac or tube which in cyclostomes, many fishes and reptiles extends through the cranium to end beneath the skin on the dorsal surface of the head. This sac in the forms mentioned bears some resemblance in structure to an eye and in several cases is probably functional as a light-percipient organ.

FIGS. 8, 9 and 10. The outline of the brain and brain ventricles of several vertebrates as seen from above. The relative size of the brains is ignored in the figures but the form of . the brain and ventricles is accurately drawn from dissections or microscopic sections.


FIG. 8. A, the brain of a cyclostome fish, Lampetra Wilderi.

B, the brain of a selachian, Mustelus canis. The outline of the ventricle in the optic lobes and cerebellum is drawn in dotted lines.

C, the brain of a young specimen of a bony fish, Coregonus albus. On the left side is shown in dotted line the form of the optic ventricle, on the right side the outline and cavity of the inferior lobe of the diencephalon.

D, the brain of a tailed amphibian, Necturus maculatus.


FIG. 9. A diagram of one side of the forebrain of Mustelus cants to show what is believed to be the primitive relations of the wall^and ventricle. FIG. 10. The outline of the ventricles in man.

It is hence called the pineal or parietal eye. It will be seen later (Chapter VIII) that there are in vertebrates two of these rudimentary eyes one behind the other. In amphibia, birds and mammals the parietal eye is quite rudimentary and in man is known as the pineal gland or body. The anterior border of the roof of the diencephalon is marked in lower vertebrates and in the embryos of all vertebrates by a deep transverse fold called the velum transversum (Fig. n). The velum forms the cephalic wall of a larger or smaller median sac of the choroid roof of the diencephalon, which may be called simply the dorsal sac. In mammals a median sac occupying nearly the same topographical position bears the name of paraphysis. It is nervous in character and its significance will be treated later (compare Chapter XVIII).

The lateral walls of the diencephalon are known as the optic thalami. They are thick and in lower vertebrates are traversed by the optic tracts on their way to the optic lobes, while in higher forms a large part of these tracts end in the optic thalami themselves. The ventral wall of the diencephalon in lower vertebrates is expanded and is divided by a median ventral furrow into lateral halves, known as the inferior lobes. These lobes are relatively small in cyclostome fishes and become progressively larger in selachians, ganoids and bony fishes. In vertebrates above the fishes the region corresponding to the inferior lobes is less expanded and in mammals it forms a funnel-shaped body with apex ventrad, which is called the injundibulum. (Compare Figs. 2, 7, n.)

An evagination of the caudal wall of the inferior lobes or inf undibulum in all vertebrates forms a pair of bodies projecting somewhat laterally and caudally which bear the name of mammillary bodies (corpora mammillaria, Figs. 2, n). Between and ventral to these the floor is thin and is produced caudally and ventrally into a thin-walled sac which is supplied richly with blood spaces. It is hence called the saccus vascidosus. It is present in all vertebrates but is much larger in the true fishes than elsewhere. Connected with the ventro- cephalic surface of the saccus is a glandular body properly known as the hypophysis (see p. 66). The two together constitute the pituitary body. The so-called optic or second cranial nerve is connected with the ventral wall of the diencephalon and marks the cephalic border of the inferior lobes. These are not true nerves but central brain tracts, and will hereafter be called the optic tracts. In all vertebrates except bony fishes the two tracts as they enter the diencephalon form a deeussation which is more or less completely hidden in the wall of the brain. This is known as the optic chiasma. In bony fishes the chiasma is carried out from the brain wall and in many cases the two tracts cross one another at a considerable distance from the brain wall, on their way to the eyes.

The telencephalon or forebrain in the different classes of vertebrates presents great differences in both size and structure. The forebrain of some primitive selachians (e.g. Heptanchus, Fig. 2) consists clearly of paired lateral lobes which are somewhat elongated. At its anterior end each lobe is produced forward and laterally as a slender cylinder which becomes enlarged as the olfactory bulb. This lies in contact with the inner surface of the olfactory sac. From the epithelium of the olfactory sac, nerve fibers pass through the wall and directly into the olfactory bulb. These fibers constitute the olfactory or first cranial nerve. The slender portion connecting the bulb with the forebrain proper is the olfactory tract. Dorsally a membranous roof (tela chorioidea) connects and covers the lateral lobes, enclosing the forebrain ventricle. The ventricle in its caudal part is common to both lobes and in its cephalic part divides in Y-shape and continues through the olfactory tracts to the olfactory bulbs. The common cavity together with that of the diencephalon is usually known as the third ventricle of the brain. The lateral or olfactory portions are to be compared broadly with the lateral ventricles of the mammalian brain (see Chapter XVII) and the point of their separation from the third ventricle is the foramen of Monro. It will appear later that the third ventricle extends a short distance forward beyond the opening into the lateral ventricles and these openings are therefore true lateral structures. It is necessary, therefore, to speak of paired foramina of Monro. The caudal border of the roof of the forebrain is marked by the velum transversum. In front of this the roof is produced dorsally into a tube or sac which varies in size and is more or less complexly branched or folded in different vertebrates. This is properly called the paraphysis and is not homologous with the structure of the same name in mammals mentioned above (Figs, n, 36, 150, 152, 158). The latero-ventral walls of the forebrain are thick and are loosely spoken of as the corpora striata. The thin portion connecting these in the mid- ventral line is known as the lamina terminalis and is thickened at one place by the fibers of the anterior commissure. In many other selachians (e.g. Squalus, Fig. n, Scyllium, Raja) the forebrain is shorter and more compact and massive. The olfactory bulbs are larger and the olfactory tracts usually shorter. The lateral lobes are shorter, thicker, and more rounded. The brain is wider between the olfactory tracts and the massive nervous structure extends up on the dorsal surface farther, so that the choroid roof is shorter than in Heptanchus. So too, the median ventricle is shorter and the lateral ventricle is relatively more important. (Compare Figs. 2, n.)

FIG. 11 . The mesial surface of the right half of the brain of a selachian, Sqitalits acanthias, to show the secondary segments of the brain, the longitudinal zones of the medulla oblongata, the form of the ventricle and the position of the inferior lobes, dorsal sac, paraphysis, etc. The olfactory bulb is not drawn (compare Fig. 147).

FIG. 12. A sketch of the brain of a cyclostome fish, Lampetra Wilderi, as seen from the left side. d. i, first dorsal spinal nerve; 7V. /. /., roots of the lateral line nerves; Vm., two motor roots of the trigeminus, the smaller of which innervates an eye-muscle; Vs., sensory root of the trigeminus.

The cyclostome forebrain (Fig. 12), although outwardly bearing a resemblance to the massive selachian type, in reality owes its form to the pressure which is exerted upon it by the great buccal funnel in front. The forebrain of ganoids and bony fishes resembles the elongated type of selachian brain, but is more slender and is simpler because the olfactory structures are less highly developed. The tela chorioidea is more extensive and the lateral lobes are smaller and more compact. The olfactory tract varies in length. The lamina terminalis is nearly horizontal (Figs. 148, 139).

The forebrain of dipnoans and amphibians differs from the compact type in selachians in two ways. First, the olfactory tract is absent as an external feature and the bulb is connected directly with the lateral lobe. Second, the lateral lobes are separated from one another in front (Compare Fig. 8, B, D, and Fig. 9) the median ventricle is still shorter and the lateral ventricles are relatively still more important. The lateral lobes have complete nervous walls and the tela chorioidea covers only the median ventricle. In reptiles, birds and mammals the lateral lobes are fundamentally of the amphibian type but become larger and greatly modified in connection with the development of the cerebral hemispheres. (See Chapter XVII.)

In most vertebrates only one pair of nerves is connected with the forebrain. These are the olfactory nerves which were mentioned above. The olfactory nerve always arises from the sense cells of the nasal epithelium, has no ganglion in its course and enters the olfactory bulb. In addition to the olfactory there is found in many selachian and in some ganoid and dipnoan fishes another pair of nerves connected with the forebrain, whose presence is of the greatest importance in the study of the morphology of the nervous system. This new nerve arises from the upper or lower surface of the forebrain near the median line, bears a ganglion which is often lodged in the angle between the olfactory tract and the forebrain, passes forward along the olfactory tract and over the olfactory bulb, divides into branches and is distributed to the epithelium of the nasal sac (Fig. 2). This nerve differs from the olfactory nerve in that its fibers are myelinated, that it has a ganglion and that it enters the forebrain proper and not the olfactory bulb. Because of its attachment to the front end of the brain, this nerve has been called the nervus terminalis.

The figures in this chapter are intended rather to illustrate the typical form of the vertebrate brain and to bring out features of special morphological significance than to show the varying forms of the brain in different classes. For this the reader should refer to the figures in the larger text-books of zoology and comparative anatomy.

Demonstration of Laboratory Work

  1. Dissect the nervous system of the dogfish or skate, the frog, and of a mammal (rabbit, cat, dog or man).
  2. Compare the brains of representatives of all classes of vertebrates (Petromyzon; Squalus acanthias or Raja; Ameiurus, Catostomus or other bony fish; frog; lizard or turtle; fowl; mammal). Especial attention should be given to the relative size of the different parts and the correlation of these with the activities (habits) of the animals and the number and importance of the sense organs. The brains should be dissected with care in such ways as to expose the ventricles and all of the points mentioned in the text.


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His, W.: Zur allgemeinen Morphologic des Gehirns. Arch. f. Anat. u. Physiol, Anat. Abth., 1892.

Merkel, Fr. : Ueber die Endigungen der sensiblen Nerven in der Haut der Wirbelthiere. Rostock, 1880.

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Stannius, H.: Handbuch der Anatomic der Wirbelthiere. 2te. Aufl. Berlin, 1854.

Wiedersheim, Robert: Vergleichende Anatomic der Wirbelthiere. 6te. Aufl. Leipzig, 1906.

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   1907 The Nervous System of Vertebrates: 1 The Study of the Nervous System | 2 General Morphology of the Nervous System | 3 Development of the Nervous System | 4 Nerve Elements and Their Functions | 5 The Functional Divisions of the Nervous System | 6 Somatic Afferent Division. General Cutaneous Subdivision | 7 Somatic Afferent Division. Special Cutaneous Subdivision | 8 Somatic Afferent Division. The Visual Apparatus | 9 The Visceral Afferent Division | 10 The Olfactory Apparatus | 11 The Somatic Motor Division | 12 The Visceral Efferent Division | 13 The Sympathetic System | 14 Centers of Correlation | 15 The Cerebellum | 16 Centers of Correlation. The Mesencephalon and Diencephalon | 17 Correlating Centers in the Diencephalon (Continued) | 18 The Evolution of the Cerebral Hemispheres | 19 The Neopallium | Figures

Johnston JB. The Nervous System of Vertebrates. (1907) Blakiston's Son & Co., London.

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