Talk:Book - The Nervous System of Vertebrates (1907) 16
CHAPTER XVI. CENTERS OF CORRELATION (Continued). THE MESENCEPHALON AND DIENCEPHALON[edit] I. THE CUTANEOUS APPARATUS.
In describing the centers and fiber tracts connected with the general and special cutaneous nerves (Chapters VI and VII) it was shown that the largest secondary tract from the primary centers goes in the form of internal arcuate fibers to the opposite side of the brain and passes forward to end in the roof of the mesencephalon. This tract is called in lower vertebrates the tractus bulbo-tectalis, in mammals the lemniscus. The tract has a wider distribution in the mesencephalon and diencephalon in mammals than in lower vertebrates. All the fibers which have this general course may be known as the lemniscus system. The evolution of this system and of its end nuclei will be considered here.
The first important point to notice is that the lemniscus system in mammals is divided into two chief parts, the medial and the lateral lemniscus. Each of these parts is believed to contain descending fibers in addition to the ascending ones. It must be emphasized that the ascending fibers alone are of interest in the present connection. The medial lemniscus arises in small part from the dorsal horns of the cord and chiefly from the nuclei of the dorsal funiculi and from the nucleus of the spinal V tract. It is therefore a general cutaneous conduction path. Some ascending fibers from the nuclei of the vestibular nerve probably join the medial lemniscus and others probably run separately to similar destinations in the diencephalon. The lateral lemniscus arises from the various nuclei of the cochlear nerve and is therefore an auditory conduction path. The chief place of ending of the lateral lemniscus is the posterior corpora quadrigemina, that of
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the medial lemniscus is the anterior corpora quadrigemina and certain nuclei in the thalamus. The separation of the lemniscus system into two parts is the result of a differentiation in the mesencephalon and diencephalon the main features of which can be outlined, but the details of which offer most interesting subjects for investigation.
The mesencephalon in primitive vertebrates was a segment of simple tubular form comprising two primary neuromeres similar to the cerebellar neuromere or even to a segment of the spinal cord. In typical fishes the roof of the mesencephalon is greatly enlarged and is as highly specialized as the cerebellum. This is due to the entrance into the mesencephalic roof of secondary tracts (i) from the general and special cutaneous centers of the medulla oblongata and spinal cord, and (2) from the retina. These tracts (tractus bulbo-tectalis and tractus opticus) have been described and their morphology discussed in previous chapters. In lowly fishes the centers for these tracts are not isolated but are more or less confused or intermingled in the brain roof. Indeed the differentiation of special centers within an indifferent region is in progress in existing fishes.
In typical fishes the midbrain roof is an arch whose bases rest on the ventro-lateral walls of the brain and whose keystone is formed by a thin lamina (roof plate of His) traversed by the dorsal decussation. Enlargement of the centers in the roof is secured by the widening of the arch and expansion of the ventricle. The thickness of the walls varies much less in different fishes than the extent of the roof. In most fishes there is formed in this way a dorsal expansion of the iter known as the optic ventricle or optocoele. In Fig. 127 are shown outline sections through the tectum of a cyclostome, a selachian, a bony fish, an amphibian and a mammal to illustrate the changes of form which this region undergoes. The first important fact is that the expansion of the optic ventricle takes place by bulging the side walls of the arch without separating its bases. From this it results that the ventro-lateral part of the tectum remains beneath the side of the expanded optic ventricle and is overhung by the bulging dorso-lateral portion of the tectum. The portion of the original roof which now lies beneath the optic
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ventricle is much thicker than the rest and is known as the collieular region. The portion which roofs over the optic ventricle
Rad. esenc.
olliculus
Lemniscus system
Arcuate fibers
Decuss. and A Nuc. tr. hab. pedunc
Rad. mesenc. V
olliculu
Lemniscus system
_r. tecto-cerebel tecto-cereb. tr. lobo-bulbaris C
Rad. mesenc. V, / Cor P- ^ uad - ant
Lernniscus system
III
Cereb.ped.
Fig. 127. Outline transverse sections through the mesencephalon of various vertebrates to illustrate the changes of form of the tectal region. A, a cyclostome; B, a selachian; C, a ganoid; D, a bony fish; E, an amphibian; F, man (after Koelliker). r, nucleus ruber; 5. n., substantia nigra.
is known as the tectum opticum. (It must be remembered always that the tectum opticum is only a part of the tectum mesencephali.)
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Although differentiation between these centers is not complete in fishes it may be said in general that the optic tract predominates in the tectum opticum, while the collicular region is especially related to the tractus bulbo-tectalis. That the specialization of these centers is not complete is clear from the fact that hi blind fishes and those from which the eye has been experimentally removed, the tectum opticum is not atrophied but only certain layers of its cells disappear (Ramsey). Evidently the secondary cutaneous tracts enter the tectum opticum as well as the colliculus, but the optic tract is more nearly confined to the tectum opticum. The descending fibers from both the tectum opticum and the colliculus run in the same tracts so that they are all grouped under the name of tractus tecto-bulbaris.
In the amphibia a marked change hi the form of the mesencephalon is seen (Fig. 127 E). The form of the transverse section is more nearly that of a simple tube. The wall is relatively thick and there is only a slight indication of a special optic ventricle. In selachians, ganoids and bony fishes there is an increasing expansion of the tectum opticum and thickening of the massive colliculus. The size of the tectum opticum is directly affected by the size and importance of the eyes, while the great volume of the colliculus is due to the great development of the acustico-lateral system. In amphibia the reduction of the roof is due to a decrease in both optic and cutaneous tracts which end in it. In most amphibia the eyes are less important than in most fishes and the number of optic fibers which enter the tectum is much farther reduced by the greater development of the optic centers in the thalamus. On the other hand, the cutaneous tracts are greatly reduced by the complete disappearance of the lateral line organs in the adult. The amphibia are believed to have descended from fish-like ancestors whose affinities are with the lower orders of existing fishes. The amphibian brain shows resemblances to that of both selachians and cyclostomes. The reduction of the midbrain roof has left it in a primitive form. There is no clear distinction between tectum opticum and colliculus, but the greater thickness of the wall in the caudal part of the roof indicates the beginning of formation of the posterior
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corpora quadrigemina. It is probable that the roof in amphibia is an indifferent center for cutaneous, auditory and optic stimuli in which special centers for optic and auditory tracts are beginning
Fig. 128. Transverse section of the diencephalon of the rat at the level of the body of Luys. From Cajal (Textura, etc.). A, optic tract; .Band C, nuclei of the corpus geniculatum laterale; D, sensory nucleus (end nucleus of lemniscus); E, nucleus of Luys; F, zona incerta; /, posterior commissure; N, nucleus triangularis.
to be formed. In its caudal part the roof is comparable to the colliculus of fishes rather than to the tectum opticum; in its cephalic part the reverse is true.
The form of the midbrain roof in reptiles and mammals is in
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essentials similar to that in amphibia. The wall has thickened and the anterior and posterior corpora quadrigemina have differentiated. The anterior corpus quadrigeminum serves as the place of ending of optic and secondary cutaneous tracts and is comparable to the cephalic part of both tectum opticum and colliculus in fishes. The posterior corpus quadrigeminum serves as the chief place of ending of the secondary auditory paths and is roughly comparable to the caudal part of the colliculus in fishes. In birds a large tectum opticum has been developed on account of the large size of the eyes and the mesencephalon of birds is more like that of bony fishes than that of other lower vertebrates.
The lateral lemniscus arises, as has been said, from the nuclei of the cochlear nerve. A large part of the fibers come as internal arcuate fibers from the nuclei of the opposite side, the remainder as direct fibers from the nuclei of the same side. The lateral lemniscus passes forward into the mesencephalon, bends upward and separates from the medial lemniscus, and the greater part of it enters the posterior corpus quadrigeminum. A part of the fibers pass forward to end in the anterior quadrigeminum and a part go on to end in the medial corpus geniculatum or adjacent nuclei in the thalamus. From the posterior corpus quadrigeminum fibers go to the medial corpus geniculatum and from this center auditory impulses are forwarded to the cerebral hemispheres. From the posterior quadrigeminum other fibers go down through the lateral lemniscus to end in the various cochlear nuclei (recurrent fibers).
The medial lemniscus consists of both crossed and uncrossed fibers from the centers for cutaneous nerves. The tract receives fibers also from many other sources not related to the cutaneous nerves. As the tract passes forward fibers go from it to end in the gray matter of the medulla oblongata, pons, isthmus, midbrain and hypothalamus. It is shown, however, that the great majority of the fibers which arise in the cutaneous nuclei pass forward to the thalamus and it is believed that the other fibers which enter and leave the tract do not belong to the lemniscus system proper. The secondary cutaneous fibers end in the so-called nucleus ventralis of the thalamus. It must be noticed that this nucleus
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is by no means situated in the ventral part of the thalamus (as often stated) but lies mesial to the corpus geniculatum laterale in the dorsal half of the thalamus (Fig. 128). It would be better to call it as Cajal does the sensory nucleus of the thalamus, especially as the term nucleus ventralis is used for a different center in fishes. From this sensory nucleus impulses are forwarded to the cerebral cortex and descending fibers from the cortex also end in this nucleus. In the cat (Tschermak) a part of the fibers of the me
Fig. 129. Transverse section of the dorso-mesial portion of the posterior corpus quadrigeminum of the newborn dog. From Cajal (Textura, etc.). A, peripheral fiber layer; B, cells of the second layer; C, stellate and fusiform cells of the third layer; D, fibro-cellular layer; E, central gray matter; R, commissure; F, plexus of second and third layers.
dial lemniscus are found by the degeneration method going through the thalamus to reach the cortex directly. There is some evidence that such bulbo-pallial fibers exist also in man. In view of the ending of secondary cutaneous fibers in the midbrain in lower vertebrate sit may be expected that some of the fibers of the
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medial lemniscus in mammals should stop in the corpora quadrigemina. The large branch of the medial lemniscus which runs to the anterior quadrigemina may be made up of such fibers. The structure of the corpora quadrigemina is shown in Figures 129 and 130. The lemniscus system in lower vertebrates is related to an
Fig. 130. Transverse section of the anterior corpus quadrigeminum of the rabbit of eight days. From Cajal (Beitrage u.s.w.). A, surface at the median line; B, superficial gray layer; C, layer of opticus fibers; D, transverse fiber layer; a, border cells; b and c, horizontal spindle cells; e, vertical spindle cells; /, g, various cell types of the gray layer; h, j, spindle cells of the opticus layer; M, L, cells of the fiber layer; n, ending of opticus fiber.
indifferent tectum mesencephali. It has been differentiated into a medial general cutaneous tract and a lateral auditory tract. The auditory tract has claimed the caudal portion of the tectal region. The cutaneous tract, while it is probably related to the
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anterior corpus quadrigeminum, has chiefly shifted its place of ending to the dorsal part of the thalamus. There is evidence that a few fibers of both auditory and cutaneous tracts may go directly to the cerebral hemisphere.
2. THE VISUAL APPARATUS.
It was seen in the last section that optic tract fibers in all vertebrates end in the tectum opticum. Although this is apparently the earliest place of ending of optic tract fibers a second important end nucleus appears as low as the selachians. This is the corpus geniculatum later ale situated in the lateral wall of the thalamus. This nucleus increases in importance in the series of vertebrates and forms the largest optic center in the mammalian and human brain. In addition, the pulvinar of the thalamus receives optic tract fibers in mammals. Some authors have described as places of ending of optic tract fibers a nucleus named by Edinger the corpus ectomammillare (reptiles, Edinger; teleosts, Goldstein) and the ganglion isthmi in birds. The relations of the ganglion isthmi of birds require further study. The nucleus in amphibia and fishes to which Edinger has given the same name is of a very different character. The identity of the corpus ectomammillare and the proof that optic fibers end in it are not clear from the descriptions that have been given.
In selachians the optic tracts after their decussation in the chiasma pass up on the lateral surface of the thalamus where they cover superficially a layer of cells which is homologous with the corpus geniculatum laterale of higher forms. Only collaterals of the optic fibers are said to end in this nucleus. The main body of the optic tract ends in the tectum opticum. In some bony fishes the corpus geniculatum is more highly developed and receives a larger number of optic fibers. This is true also in Atnia (Fig. 141). In some bony fishes and in the sturgeon the corresponding cells are closely related to the nucleus anterior thalami and receive the endings of a small part of the optic tract. In amphibia, reptiles and birds a well developed corpus geniculatum laterale is found and a large part of the optic tract ends in
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it. In birds on account of the large size and importance of the eyes both the tectum opticum and optic thalami are well developed. In mammals and man the number of uncrossed fibers in the optic tract increases greatly. The corpus geniculatum laterale (Fig. 131) becomes the most important center for optic impulses
Fig. 131. Lower part of the corpus geniculatum laterale of the newborn cat. From Cajal (Beitrage u.s.w.). A, B, C, D, endings of opticus fibers.
and some fibers end in the center of similar structure situated in the pulvinar of the thalamus. The relations in mammals show that with the increasing size of the corpus geniculatum there has gone hand in hand a process of differentiation of function between
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it and the tectum opticum. In man the anterior corpus quadrigeminum is small and poorly developed as compared with that of lower animals. The descending tract from it is relatively small and represents only a part of the tractus tecto-bulbaris of lower vertebrates. The bundles of the two sides descend over the central gray of the midbrain and form a decussation in its ventral wall called by Forel the " fountain-like" decussation. As the tracts pass on toward the medulla oblongata they give collaterals and terminals to the nuclei of the eye muscle nerves. This is the chief connection of the tract going out from the anterior quadrigeminum and it shows that the chief function of that nucleus is to direct eye muscle reflexes to visual stimuli. The existence of fibers from the anterior quadrigemina to the cerebral cortex has not been clearly demonstrated and the complete loss of the anterior quadrigemina does not affect light or color vision.
The corpus geniculatum, on the other hand, sends a large tract to the occipital region of the cortex and the geniculatum is the chief intermediate center in visual perception. The connection of the geniculatum and pulvinar with the occipital cortex is clearly shown by the secondary degeneration of these nuclei after destruction of the visual cortical area. The tract has been followed clearly in animals by the method of Golgi (Cajal) and in the newborn babe the tract from the geniculatum to the cortex is medullated while adjacent tracts are not, so that Weigert staining gives a clear picture of this conduction path (Flechsig). There is evidence that in lower vertebrates fibers pass from the tectum opticum and the geniculatum to the forebrain and it is probable that this tract was one of the first to stimulate the development of the neopallium. Little is certainly known of the relations of the geniculatum in lower vertebrates, although connections with various parts of the brain have been described (Catois, Edinger).
The existence of the corpus geniculatum laterale in selachians shows that it is far from being a new or phylogenetically young optic center. It is equally true, however, that it does not become important relative to the tectum opticum until the cerebral hemispheres are developed. Through the vertebrate series there has been a gradual shifting cephalad of the endings of optic tract
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fibers. In fishes they are distributed throughout the whole length of the tectum mesencephali and a few end in the thalamus. In mammals the caudal half of the tectum no longer receives optic tract fibers and most of them (in man 80 per cent.) end in the thalamus. This shifting of optic tract fibers is part of the process of specialization of the mesencephalon described in the last section. The caudal part comes into the service of the secondary auditory tracts, the cephalic part becomes a visual reflex center for eye movements, while the conduction paths for general cutaneous and visual perception both leave the tectum mesencephali and end in thalamic nuclei. All these changes have resulted in greater compactness and directness in the arrangement of centers and tracts. Generally speaking, each conduction path in mammals is the shortest and most direct that could be evolved out of the unspecialized centers and tracts which already served the same set of functions in lower vertebrates. The caudal part of the tectum is claimed by the auditory paths which come from behind, the cephalic part by the optic path which comes from in front. The secondary general cutaneous path shifts from the tectum to the thalamic nuclei, which are nearer the cerebral cortex to which the impulses of general sensation are destined. Similarly the visual impulses destined for the occipital cortex are transferred to the corpus geniculatum instead of going to the more distant corpus quadrigeminum anterior. The shifting forward (prosencephalization) of visual centers is explained by the advantage of gaining shorter paths to the cerebral cortex, for the reason that visual impulses are significant in higher mammals chiefly for space and color perception of the surroundings. In fishes visual impulses arouse reflex movements of the body for the capture of food, avoidance of obstacles, etc. In mammals the reflexes which depend solely on visual impulses are few (lid reflex, etc.). Visual impulses are for the most part carried to the pallium where they are associated with impulses of other sorts (tactile, auditory) for the formation of a complex perception of the situation. Under the influence of this percept voluntary impulses arouse movements adapted to meet the situation. This may be taken as a characteristic illustration of the difference between the brain
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of man and that of a fish. Whereas the sensory impulses in a fish are correlated by the relatively slightly organized centers of the mesencephalon and call forth simple, direct and often very quick responses; the center for correlation in man is removed to the highly organized cerebral cortex and the responses lose something in quickness but gain vastly in precision and in the completeness with which they are adapted to the several factors of the situation.
There should be added to this account some mention of the centrifugal fibers in the optic tracts which end in the retina. The presence of such fibers is demonstrated by the Golgi method which shows their origin, course and their endings in the internal molecular layer of the retina (Figs. 71, 72.). Their presence and course is also shown by the methods of primary and secondary degeneration. In fishes in which one eye has long been lost the optic tract of the opposite side degenerates with the exception of these fibers, which persist and are stained by the Weigert method. In mammals, following section of the optic tract there occurs secondary degeneration of cells in the anterior quadrigeminum, and in the dorsal part of the geniculatum laterale and pulvinar. These findings in mammals agree with those in fishes by the Golgi and degeneration methods, where the centrifugal fibers arise from the tectum opticum and geniculatum (Catois). The significance of these fibers is not understood but their presence in all vertebrates seems to show that they have some constant function. It has been suggested that they are examples of fibers which pass forward from the cutaneous center of one segment to that of a center farther forward, and hence homologous with the fibers of the lemniscus system.
3. CENTERS RELATED TO THE POSTERIOR COMMISSURE.
The posterior commissure, although it has long been used as one of the most prominent and constant landmarks in the brain, is still very imperfectly understood. In Petromyzon (Fig. 132) the fibers of the commissure arise from cells widely scattered through the dorso-caudal part of the thalamus and the cephalic part of the tectum opticum and the collicular region. The fibers
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are very fine until they approach the point of crossing, when they thicken into relatively coarse fibers. After crossing they bend ventro-caudally and spread widely through the wall of the midbrain, mingling with the fiber tracts descending to the medulla oblongata. In mammals the commissure arises from a nucleus situated at the cephalic border of the anterior corpora quadrigemina, between the tractus habenulo-peduncularis, the nucleus ruber and the root of the III nerve (Koelliker, p. 445). In other vertebrates the origin of the commissure has not been clearly distinguished. It is variously described as arising in, ending in or serving as a true commissure for a nucleus lying cephalad from the nucleus of origin of the III nerve. The same nucleus is described
Fig. 132. Nucleus of posterior commissure in Lampetra. vent., the aqueduct of Sylvius below, the optic ventricle above.
as the nucleus of origin of the fasciculus longitudinalis medialis, and the fibers of the posterior commissure itself are said to form that fasciculus. These vague and conflicting statements are due to imperfect methods. Those who have worked by the Weigert technique have been misled by the fact that the fibers of the commissure do not become myelinated until they approach the point of crossing. In Scyllium the fibers approach the median plane from the lateral direction just as in Petromyzon and presumably arise from a nucleus situated dorsally as in Petromyzon. The same is true in amphibia. The nucleus praetectalis of Edinger
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which has been described in fishes by various authors lies in the region of junction of thalamus and tectum mesencephali and probably is at least a part of the nucleus of the posterior commissure. After crossing, the limbs of the commissure bend downward and backward through the central gray more or less parallel with the tractus habenulo-peduncularis and are lost among the longitudinal and decussating tracts in the region of the III nucleus and the ansulate commissure. Most authors agree that the tract is related to the nucleus which gives rise to the fasciculus longitudinalis medialis and to the nuclei of the eye muscle nerves. The commissure contains many fibers which arise from or end hi the tectum opticum and its whole nucleus of origin in fishes is closely related to the tectum. All the evidence goes to show that the system of the posterior commissure is closely related to the optic centers.
DEMONSTRATION OR LABORATORY WORK.
1. Trace the lemniscus systems in Weigert sections of the brain of a bony fish, frog and a mammal. Note especially the relations of the thalamic centers.
2. Study in Golgi sections the structure of the tectum mesencephali (corpora quadrigemina) and the corpus geniculatum laterale in the brain of a selachian, frog and the mouse or rat.
3. In the Golgi and Weigert sections used above search carefully for the origin and destination of the fibers of the posterior commissure.
LITERATURE.
Barker, L. F.: The Nervous System. Chapter LIII.
Cajal, S. R.: BeitragezurStudium der Medulla oblongata. Leipzig. 1896.
Cajal, S. R. : Textura del sistema nervioso del Hombre y de los vertebrados. Madrid. 1904.
Catois, E. H.: Recherches sur Phistologie et 1' anatomic microscopique de 1'encephale chez les poissons, Bull, de Sci. de la France, Tome 31. 1901.
Edinger, L.: Vorlesungen liber den Bau der nervosen CentralorganeLeipzig. 1904.
Johnston, J. B.: The Radix mesencephalica trigemini. Ganglion isthmi. Anat. Anz., Bd. 26. 1905.
Kappers, C. U. A.: Teleostean and Selachian Brain. Jour. Comp. Neur. and Psych., Vol. 16. 1906.
Koelliker, A.: Gewebelehre, 6 Aufl. Bd. 2.
268 NERVOUS SYSTEM OF VERTEBRATES.
Myers, B. D.: The Chiasma of the Toad and of some other Vertebrates. Zeit. f. Morph. u. Anthrop. Bd. 3. 1901.
Ramon, P.: Investigaciones sobre los centres 6pticos, etc. Zaragoza. 1890.
Ramon, P.: El encephalo de los reptiles. Zaragoza, 1891.
Ramon, P.: El encephalo del cameleon Rev. trim, micrografica, tomo I. 1896.
Ramon, P.: Centres opticos de las aves. Rev. trim, micrografica, tomo I. 1898.
Ramsey, E.: The Optic Lobes and Optic Tracts of Amblyopsis speleus de Kay. Jour. Comp. Neur., Vol. n. 1901.
Van Gehuchten, A.: La structure des lobes optiques chez Pembryon de poulet. La Cellule. 1892.
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