Book - The Nervous System of Vertebrates (1907) 17
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Johnston JB. The Nervous System of Vertebrates. (1907) Blakiston's Son & Co., London.
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Chapter XVII. Correlating Centers In The Diencephalon (Continued)
4. The Olfactory and Gustatory Apparatus
The centers considered in the last chapter have to do with somatic sensory impulses and are all differentiated from the dorsal part of the mesencephalon and diencephalon. The centers now to be considered have to do with visceral sensory impulses and lie, with one exception, in the ventral part of the diencephalon. It was shown in Chapters IX and X that gustatory impulses are carried to the inferior lobes of the diencephalon by a tract from the superior gustatory nucleus and that olfactory impulses come to the same region in two or three isolated bundles of the tractus olfacto-hypothalamicus.
It is important to understand the exact limits of the hypothalamus in fishes and to determine the corresponding structures in higher vertebrates. In all fishes a pair of rounded lobes, the inferior lobes, containing a wide extension of the third ventricle project ventro-caudally behind the optic chiasma. In cyclostomes and selachians these lobes are relatively small and have a simple and primitive structure, but in ganoids and bony fishes they are greatly expanded and show a somewhat higher histological development. Caudo-ventrally the wall of the lobe is produced into a median thin-walled and vascular sac known as the saccus vasculosus. The caudal wall of the lobes above the opening of the saccus shows special characters and is known as the corpus mammillare. The relations of these structures may be seen in sagittal sections of the brain (Figs. 2, n, and Chap. XVIII).
The whole area which shows the structural characteristics of the inferior lobes and is related to olfactory and gustatory tracts may extend somewhat beyond the lobes proper, and the limits between this area and the thalamus have not been clearly appreciated. Thus, the nucleus of origin of the fasciculus longitudinalis medialis has been spoken of as lying in the hypothalamus (Edinger). If a section through the inferior lobes of the sturgeon brain be compared with one through the III nerve (Figs. 133, 116) it will be seen that the tectum mesencephali and lateral walls are the same in both, and also that the fasciculus longitudinalis medialis and the fundamental bundles of the lateral tracts extend forward into the thalamus. In front of the nucleus of the fasciculus longitudinalis medialis the nucleus of the tractus strio-thalamicus and the tract itself continue forward at about the same level. These several structures represent the brain base or stem. The greatly expanded inferior lobes ventral to them are anomalous structures. The characteristic structure of the inferior lobes extends up to the lateral border of the nucleus of the tractus strio-thalamicus, which belongs to the thalamus. The hypothalamic structure extends up lateral to this nucleus, however, upon the lateral surface of the thalamus and midbrain nearly to the ventral border of the tectum mesencephali (Figs. 133, 134, 135). This area is quite free from myelinated fiber tracts but is traversed by the myriads of fine fibers of the tractus lobo-bulbaris, to which it contributes additional fibers. The dorsal border of this area is defined by a dense bundle of this tract. In bony fishes the typical hypothalamic structure extends up in precisely the same way upon the lateral surface of the thalamus and midbrain and on account of the great size of the tectum mesencephali the base of the latter is crowded down into contact with this portion of thehypothalamus. In both the sturgeon and the bony fish the hypothalamic structure. is sharply limited dorsally by a groove and a septum of connective tissue extending deep into the brain substance. The limit between the thalamus and hypothalamus (Fig. 134) runs from a point near the base of the tectum mesencephali to the lateral border of the nucleus of the tractus strio-thalamicus. The structure of the hypothalamus is uniform throughout and is remarkably constant in the series of vertebrates. The fiber tracts connected with the hypothalamus are quite different from those connected with the thalamus. To what extent the structure of the hypothalamus extends up on the lateral surface of the thalamus in other classes than ganoids and teleosts is not known.
Fig. 133. Transverse section through the corpora mammillaria of the sturgeon.
Fortunately we have an account of the genetic relations of this hypothalamic structure which appears so anomalous in the adult brain. In early teleost embryos the brain consists of a number of ring-shaped neuromeres which have been described in Chapter III. As indicated in that chapter, the first neuromere gives rise to the forebrain, the second gives rise to the retina and comes to have the optic chiasma at its ventro-cephalic border (Chap. VIII). The third neuromere forms the thalamus, nucleus habenulae and epiphysis. The fourth and fifth neuromeres form the mesencephalon. One of the earliest differentiations of form to appear in the brain segments of bony fishes is an expansion of the ventral wall of the second neuromere, which begins to crowd back beneath the third neuromere (Fig. 19). This ventro-caudal projection from the second neuromere is the beginning of the hypothalamus. Its early appearance in the embryo is due both to its importance and to the fact that it is phylogenetically an old structure. The hypothalamic rudiment continues to expand until it forms the large inferior lobes beneath the third neuromere.
Fig. 134. Transverse section through the inferior lobes of the sturgeon.
The ventricle of the second neuromere is carried back with the outgrowth and fuses with that of the third neuromere along the line meso-ventral to the tractus strio-thalamicus and its nucleus.
The diencephalon therefore consists of the third neuromere and a part of the second, the remainder of the second being carried out into the retina. The third neuromere gives rise to the greater part of the thalamus, the nucleus habenulae and an epiphysis. The second neuromere forms the anterior part of the thalamus and the hypothalamus. The recognition of the two main divisions of the diencephalon and their genetic history leads the way to an understanding of the morphological relations of its special nuclei.
Fig. 135. Transverse section through the posterior commissure of the sturgeon.
The gross relations of the hypothalamus being established it remains to discuss its functional relations as indicated by the fiber tracts connected with it. The tracts which bring impulses to the hypothalamus are primarily the gustatory and olfactory tracts of the third order. Although the gustatory tracts have only recently been demonstrated they are probably not less fundamental than the olfactory tracts which have long been known. The olfactory tracts in fishes come to the hypothalamus either isolated or mingled with the tractus strio-thalamicus. On account of the latter fact the view has been advocated by some that a part of the olfactory impulses were delivered to nuclei in the thalamus proper. This is probably incorrect. When the medial olfactohypothalamic tract is mingled with the tractus strio-thalamicus there are always many fibers going ventrally from the latter tract into the inferior lobes and it is wholly probable that all olfactory impulses enter the hypothalamus (except those to the nucleus habenulae, see below). The tertiary gustatory tracts also enter the inferior lobes and both gustatory and olfactory tracts are widely distributed in the hypothalamus. In higher vertebrates nothing is known of the central gustatory tracts. The corpora mammillaria have long been known as the place of ending of the chief olfactory conduction path, the fornix. From the mammillary bodies the tractus mammillo-peduncularis goes caudally and is comparable with the tractus mammillo-bulbaris in fishes. Collateral branches from the fibers of this tract near their origin form the tractus mammillo-thalamicus to the nucleus dorsalis thalami (Fig. 136). The cephalic part of the hypothalamus, the tuber cinereum, has never been well understood in mammals.
Fig. 136. Scheme of the connections of the mammillary tracts, the nucleus habenulae, and the nucleus dorsalis thalami. From Cajal (Textura, etc.). A, corpus mammillare; B, nucleus dorsalis thalami; C, upper part of this nucleus; D, nucleus habenulae; E, corpus interpedunculare; F, dorsal nucleus of the tegmentum; J, optic chiasma; a, aqueduct of Sylvius; b, commissura habenularis; c, commissura posterior; d, tractus habenulo-peduncularis; e, peduncle of the corpus mammillare; /, bundle of Vicq d' Azyr; g, tractus mammillo-peduncularis ; h, olfactory tract of projection (tractus olfacto-hypothalamicus) ; i, stria thalami (tractus olf acto-habenularis) ; m, thalamo-cortical fibers; n, cortico-thalamic fibers ; o, p, fibers of the stria thalami which cross in the habenular commissure.
Fig. 137. Sagittal section of the tuber cinereum of the newborn rat. From Cajal (Textura, etc.). A, anterior or chief nucleus; B, posterior nucleus; C, internal nucleus of the corpus mammillare; D, optic chiasma; E, afferent tract from the forebrain (tractus olfacto-hypothalamicus) ; F, nucleus supraopticus; a and b, branches of bifurcation, and d, terminal branches of the afferent fibers.
The first important contribution toward a clear understanding of its relations has been made recently by Cajal. This author shows that the two nuclei of the tuber cinereum receive bundles of nonmyelinated and myelinated fibers which come from the septum pellucidum of the cerebrum, run close over the optic chiasma and end in these nuclei and in the mammillary bodies. These tracts are illustrated in Figures 137, 138, taken from Cajal's textbook. As the discussion in the next chapter will show, these fibers are homologous with the medial olfacto-hypothalamic tract of fishes and other vertebrates. They are tertiary olfactory fibers and it is evident that the whole hypothalamus receives such fibers in all vertebrates, although the caudal part, the coj^ora mammillaria, is especially developed as the end-nucleus of olfactory fibers of the fourth order, the fornix. Considering the conditions in fishes it is to be expected that a tertiary gustatory tract in mammals will be found ending in the tuber cinereum.
In fishes a tract from the tectum mesencephali enters the hypothalamus, the tractus tecto-lobaris. This tract would provide for the correlation of olfactory and visual or cutaneous impulses. The homologue of this tract in mammals has not been recognized, but it is known that fibers from other sources besides the olfactory systems described above end in the tuber cinereum.
Fig. 138. Sagittal section of the tuber cinereum of the rat of eight days. From Cajal (Textura, etc.). A, anterior or chief nucleus; B, posterior or accessory nucleus; C, internal nucleus of corpus mammillare; D, nerve tract coming from the septum pellucidum (tractus olfacto-hypothalamicus medialis); E, efferent fibers from the tuber which disappear in the central gray matter; F, efferent tract from the corpus mammillare; G, upper nucleus of the tuber; K, optic chiasma.
The tract going out from the corpora mammillaria, as already mentioned, is the same in all vertebrates. Tracts going from the tuber cinereum have been followed only into the central gray of the thalamus (Fig. 137). This is the direction taken by the tr. lobo-bulbaris in lower vertebrates. Another tract from the hypothalamus which is not known in mammals and but poorly understood in lower vertebrates, is an ascending tract to the forebrain, the tractus lobo-epislriaticus.
A second olfactory conduction path, already described for fishes, is found in mammals; namely, that by way of the nucleus habenulae. The tract from the forebrain to the nucleus habenulae comes partly from the secondary olfactory nucleus, the nucleus thaeniae or nucleus amygdalae, and in part from the hippocampus. Those from the hippocampus are a part of the fornix system.
Fig. 139. A scheme to show the embryological relations of the nucleus habenulae and the inferior lobes in fishes, i, 2, neuromeres.
The tract forms' a part of the striae medullaris and enters the nucleus habenulae, where it contributes to the habenular commissure as in lower forms. From the nucleus habenulae the tractus habenulo-peduncularis descends to the base of the mesencephalon, where the paired tracts form an intricate decussation and end in the corpus inter pedunclare.
The fact that the two olfactory conduction paths diverge so widely seems at first sight difficult to understand. The accompanying diagram will serve to show how this has come about. In Fig. 101 the olfactory paths are sketched into the outline of a sagittal section of a fish brain, and in Fig. 139 a reconstruction is given in which the several centers are drawn in their embryonic position. It will be seen that while in the adult the hypothalamus lies in an extreme ventral position, somewhat caudal to the nucleus habenulae, its embryonic and presumably early phylogenetic position was one neuromere cephalad from the nucleus habenulae and not so far ventrad as in the adult. On the other hand, while the nucleus habenulae in the adult lies at the dorsal border of the brain wall it was overtopped in primitive vertebrates by the somatic sensory center. When it is considered that the large center for the tractus strio-thalamicus and the central gray have separated the hypothalamus and nucleus habenulae, that the hypothalamus has protruded ventrally as the result of its expansion, and that the nuclei habenulae have been drawn dorsally by the habenular commissure connecting them, it becomes altogether probable that these two tertiary olfactory centers have been developed from the same column of indifferent material, namely, the substantia reticularis grisea of the second and third neuromeres. They are morphologically not dorsal and ventral structures, but represent the substantia reticularis of successive segments. Both receive identical tracts, including fornix fibers, which have been bifurcated by the mechanical shifting apart of their end-nuclei.
Fig. 140. Transverse section of the nucleus habenulae of the sturgeon.
An additional evidence in favor of this view is seen in the relation of the nucleus habenulae to the ventricle and the membranous roof. The nucleus habenulae in fishes (Fig. 140) is a lobe projecting into the ventricle, its cells either border upon the ventricle or are arranged in rows parallel with the ventricular surface, and the membranous roof passes beyond the nucleus habenulae and is attached to the dorsal border of the corpus geniculatum laterale. This shows clearly that the latter body forms the dorsal border of the brain wall. The nucleus habenulae has risen up mesial to it and pushed it outward. This will become clearer on comparison with the relations of the visceral and somatic sensory nuclei in the medulla oblongata. In Figure 141 are drawn two sections from the brain of a young freshwater dogfish (Amia), one through the medulla oblongata and one through the nucleus habenulae. It will be seen that in both the somatic sensory centers, which are morphologically dorsal, are turned out to the side and are overtopped by the visceral lobe and the nucleus habenulae respectively. The position of the nucleus habenulae is in everyway analogous to that of the vagal or facial lobe. In both cases a more median and ventral body has been hypertrophied and has pushed up until it has overtopped a more dorsal and lateral body. The nucleus habenulae has overtopped the geniculatum just as the vagal lobe or the facial lobe in bony fishes has overtopped the acusticum.
Fig. 141. Transverse sections through the brain of Amia at the level of the nucleus habenulae and of the facial lobe. Comm. sup. indicates the position in which the habenular commissure appears in the next adjacent section. The drawings were made from Golgi sections with the aid of the camera.
Fig. 142. Transverse section of the habenular nuclei in the dog. From Cajal (Textura, etc.). A, internal nucleus; B, external nucleus; C, stria thalami; D, tractus habenulo-peduncularis. Only the internal nucleus corresponds to the nucleus habenulae in the sturgeon (Fig. 140).
In mammals the nucleus habenulae is composed of a mesial portion similar in structure to the nucleus in fishes, and of a lateral portion which has a very different structure (Fig. 142). Two similar portions are to be recognized in the nucleus habenulae of some bony fishes also. Several fiber tracts are said to be connected with the nucleus habenulae in addition to those mentioned above but it is not known how the two portions of the nucleus are related to these tracts.
5. The Nucleus of the Tractus Strio-Thalamicus and the Substantia Reticularis Thalami==
The large nucleus of the tractus strio-thalamicus, as already stated, lies in the stem region of the thalamus. In bony fishes it is divided into the two large nuclei dor salts and v entrails (nucleus rotundus Fritsch). From these nuclei arise the large tractus thalamo-bulbares et spinales which constitute an important part of the fundamental bundles in the medulla oblongata. It is to be noticed that the striatum is not a secondary olfactory nucleus, but is a correlation center for olfactory, gustatory and perhaps other impulses, and that the tractus strio-thalamicus is a descending tract or motor tract. The pathway is broken once in the thalamus and the fibers arising here probably make connection with widely separated motor nuclei in the brain and spinal cord. These functional relations emphasize the statement made above that these tracts represent the stem or base of the interbrain and forebrain. Essentially the same motor conduction path is found in all vertebrates, although its functional relations may be somewhat modified in mammals on account of the cerebral cortex.
In all vertebrates there remains surrounding the ventricle a mass of central gray and in more highly specialized forms (teleosts, birds, mammals) special nuclei are present in addition to those mentioned. The functional relations of these are not yet sufficiently understood. It must be noted that some of these nuclei which constitute correlating centers of a higher order than those described above, by some authors have been assigned without reason to the hypothalamus. Thus the nucleus rotundus and even the corpus interpedunculare of bony fishes have been included in the hypothalamus. Equally unfounded is the assignment of the corpus Luysii, the field of Forel and adjacent centers in mammals to the hypothalamus. The term hypothalamus should be strictly limited to the infundibular and mammillary regions which have secondarily bulged ventrally and which serve as centers related to olfactory conduction paths.
6. The Saccus Apparatus
In all lower vertebrates the saccus vasculosus is a wide sac, sometimes much branched, with epithelial lining and with an extremely rich blood supply in its walls. This outgrowth of the brain wall comes into very close relation with the vestige of the hypophysis, the branching tubes of the saccus often being interdigitated with the epithelial sacs of the hypophysis. This fact has led many authors to ascribe to the hypophysis nervous structures which really belong to the saccus.
The saccus is a part of the brain wall and is composed of elements characteristic of the brain wall. The vascular plexus is of course of mesodermal origin. Within this the saccus consists of a layer of nerve fibers and an epithelium bounding the cavity which is a part of the brain ventricle. The epithelial lining is made up of supporting cells and nerve cells. The supporting cells form an internal limiting membrane as in the rest of the brain and extend through the fiber layer in which they form a supporting meshwork. These cells are therefore comparable with the ependyma cells of the brain. The nerve cells have been described in several fishes both in embryonic and adult stages (Lundborg, Johnston). They are rather large spindle-shaped cells whose inner ends bear a tuft of cilia projecting into the ventricle. Such cells are already present in the floor of the brain ventricle in Amphioxus in a position corresponding to that of the saccus ; a fact which indicates how ancient a structure the saccus is and that it functions in relation to the brain ventricle. In fishes the ciliated cells taper to a point at their outer ends and give rise to nerve fibers which help to form the fiber layer. These cells may be compared with typical primitive nerve cells elsewhere in the brain. The nerve fiber arises from the peripheral end of the cell as in the case of the neuroblasts in the embryo, and the cell-body retains the position in the epithelial lining which is characteristic of the germinal cells and many neuroblasts. Well developed nerve cells which retain their epithelial position are very numerous hi the brain of fishes (cf. p. 48). The saccus cells do not produce dendrites but instead bear cilia projecting into the ventricle. It must be supposed that these cells receive stimuli of some sort, whether vibratory or chemical, from the cerebro-spinal fluid.
Fig. 143. The efferent tract to the saccus vasculosus in the sturgeon. The drawing represents a sagittal section a little to one side of the median plane through the front part of the saccus and a part of the ventral wall of the inferior lobe. The course and main branches of three fibers are shown, from among the many impregnated in the section.
The impulses are carried by the neurites of the sense cells to the thalamus. The fibers form symmetrical tracts which in the sturgeon run up through the walls of the corpora mammillaria to end in a nucleus adjacent to the nucleus of the tractus striothalamicus in the extreme ventral part of the thalamus (Figs. 133, 134). In bony fishes the nucleus is situated lower down and a part of the tract decussates in the caudal wall of the corpora mammillaria (Goldstein). The tract in teleosts also arises from the ciliated cells of the saccus (Johnston) and a similar tract is present in selachians. The tract does not arise in the dorsal part of the thalamus and end in the saccus as Edinger described it, but arises in the saccus. The secondary connections of the endnucleus of this tract are not clear, but in teleosts a secondary tract has been traced caudally over the ansulate commissure.
Fig. 144. A general scheme of the saccus tracts as projected upon the median plane.
In addition to the tractus sacco-thalamicus a tract goes out from the hypothalamus to end in the saccus. This tract comes in the sturgeon from the region just behind the optic chiasma (Fig. 143) and in the salamander the origin of its fibers from cells in this position has been demonstrated (Bochenek). The fibers of this tract go to all parts of the saccus vasculosus and ramify richly among the cells of the epithelium. In the salamander the ciliated cells are said to be absent. A general scheme of the saccus apparatus is given in Fig. 144.
In mammals there is a sac with epithelial lining and dorsal to it a thick mass containing numerous cells of doubtful character and a rich plexus of nerve fibers (Berkeley, Cajal). From this plexus fibers pass into the epithelium to end freely among its cells (Cajal, Gemelli). The nerve plexus is connected with the brain by a large tract which runs along the raphe of the tuber cinereum. The tract takes origin from a nucleus situated directly over the optic chiasma. The tract and nucleus correspond in position to the efferent tract, and its nucleus in fishes. The epithelial sense cells and the tractus sacco-thalamicus have not been described in mammals.
In all classes of vertebrates this outgrowth of the brain wall is present and is provided with nervous elements. Although the structure has been very incompletely studied, enough is known from fishes, amphibia and mammals to indicate that the relations of the saccus are fairly constant in the vertebrate series. The only suggestion regarding its function is that it serves as an organ for controlling the character of the cerebro-spinal fluid. Its plentiful blood supply and its thin wall adapt it for secreting fluid into the brain ventricle. The existence of a double nerve supply, both centripetal and centrifugal, indicates that it does more than simply secrete. The ciliated cells must be regarded as sense cells and it is conceivable that they may be stimulated by changes of either pressure, density or chemical character in the cerebro-spinal fluid. In response to these stimuli the saccus may secrete some specific constituents of the ventricular fluid. The tract which ends in the saccus epithelium would arouse or control this secretive activity.
7. Mention may be made here of an epithelial structure resembling in some respects the saccus epithelium, which forms the base of the epiphysis in the roof of the diencephalon. This epithelium receives the free endings of nerve fibers and gives rise to fibers which go in various directions. Part of the in-coming and out-going fibers form a commissure over the base of the epiphysis (Holt, Johnston).
The fate of the four functional divisions in the midbrain and interbrain may now be reviewed in a few words. The motor columns are represented only by the nucleus of the III nerve and the thalamic nucleus of the fasciculus longitudinalis median's, which belong to the somatic motor division. The presence of efferent sympathetic fibers in the III nerVe in mammals indicates the presence in the midbrain of cells representing the visceral efferent column, but they have not yet been recognized as a definite column, unless the nucleus of origin of the mesencephalic root of the V nerve be that column. The somatic sensory division is represented by the tectum mesencephali, the corpus geniculatum laterale and mediale, and the nucleus of the medial lemniscus in the thalamus. With the exception of the mesencephalic root of the trigeminus, only secondary or tertiary tracts end in these nuclei. The tectum mesencephali is clearly the continuation forward of the primary cutaneous centers of the cord and medulla oblongata. It has been modified into a center which is chiefly secondary. In the diencephalon the corpora geniculata and the nucleus of the lemniscus are situated in the dorsal region and hold essentially the same relation to sensory tracts as do the several nuclei in the tectal region of the mesencephalon. In both midbrain and interbrain a relatively indifferent region has developed special nuclei for visual, cutaneous and auditory impulses. The fact is clear that the position of all the centers mentioned with relation to the axis of the brain and to other chief columns is the same as that of the somatic sensory columns in other segments, and the conduction paths in which these centers form stations are all somatic sensory conduction paths. The recent description of the brachium conjunctivum in mammals as a tract from the nucleus dentatus to the optic thalamus of the opposite side, brings the brachium into close comparison with the lemniscus system and adds an important fact to the grounds upon which the above interpretation of the diencephalon is based.
The visceral sensory division is not represented by any known special centers in the midbrain but in the interbrain it is largely developed and includes centers in gustatory pathways. The visceral sensory column (or the substantia reticularis belonging to it) has been distorted by the expansion and shifting of the centers lying in the two interbrain segments in such a way that a part of this column lies far dorsad (nucleus habenulae) and a part far ventrad (hypothalamus). This has been fully set forth above.
Finally, a considerable part of the adult midbrain and interbrain consists of substantia reticularis or nuclei derived from it, whose morphological and genetic relations to the primary functional divisions are unknown.
The Commissures of the Brain
The facts regarding the chief brain commissures which have been scattered through the foregoing pages may be brought together here for convenience of reference. It should be noted at the start that the fibercrossings in the lower vertebrates are for the most part mere decussations. The dorsal decussation of the spinal cord in higher vertebrates contains visceral sensory (sympathetic) fibers, collaterals from cutaneous and visceral fibers, and secondary fibers from both somatic and visceral sensory columns. At the junction of the spinal cord and brain, i.e. just behind the choroid plexus of the ventricle, in all vertebrates this decussation is greatly enlarged. This enlarged portion, known as the commissura infima (Figs. 81, 82, 83, 90, 92), is due chiefly to an increase of the visceral sensory fibers from the roots of the VII, IX and X nerves and of secondary visceral fibers arising from the nuclei of those nerves. Other fibers in this commissure come from the cells of the nucleus funiculi. The dorsal decussation of the cord is therefore mixed somatic and visceral in character. These two components must be rigidly distinguished if the dorsal decussations of the brain are to be understood.
The dorsal decussation of the medulla oblongata is not obliterated on account of the non-nervous roof, but its elements are crowded forward or backward. Behind the choroid plexus the commissura infima contains the visceral sensory elements proper to the segments of the VII, IX and X nerves. It is probable that the course of the root fibers of these nerves within the brain has been influenced by the crowding backward of their decussation and median nucleus by the choroid plexus. It is further probable that those fibers which take this caudal course are the more primitive components of these nerves, namely the general visceral fibers as distinguished from taste fibers. The point of special interest is that the concentration of the visceral decussation for the VII, IX and X nerves behind the choroid plexus precludes the expectation that the visceral elements of the first order will be found in the dorsal decussations farther forward. There are no visceral nerves anterior to N. VII.
The somatic sensory elements have behaved differently with reference to the IV ventricle. Instead of concentrating behind it they have concentrated hi front of it. In those vertebrates in which the cerebellum is most primitive (Petromyzon, Protopterus, Urodeles) a decussation constitutes a prominent part of it. This decussation consists of axones of granule cells situated in the cerebellum destined to the somatic sensory nuclei of the medulla oblongata. This is therefore to be considered as the homologue of the somatic sensory portion of the dorsal decussation of the spinal cord. It is an important decussation in all lower vertebrates.
A second prominent cerebellar decussation is found in fishes. This is situated in the velum medullare anterius or in the enlarged equivalent of the velum, the valvula cerebelli of ganoids and bony fishes. Instead of connecting the dorsal portions or lateral lobes of the cerebellum, this commissure connects two nuclei which in fishes lie in the lateral walls distinctly ventral to the somatic sensor}- centers, the superior secondary gustatory nuclei. The fibers of the secondary gustatory tract coming from the visceral sensory column end in part in the secondary gustatory nucleus of the same side and in part cross to the opposite side. The remainder of the decussation is formed of the neurites of the cells of these nuclei. The destination of these fibers is not certainly known, so that it is uncertain whether a true commissure is present. It is evident, however, that the inferior cerebellar commissure belongs to the visceral sensory division of the nervous system.
The dorsal decussation of the tectum mesencephali must be regarded as a somatic sensory decussation comparable with .the somatic sensory portion of the dorsal decussation in the spinal cord.
The posterior commissure has been discussed above (p. 265).
In the roof of the diencephalon two decussations are present, the well-known superior orhabenular commissure and a decussation closely related to the base of the epiphysis known only in a few forms. The habenular commissure contains decussating fibers from the olfactory nuclei of the forebrain (tractus olfacto-habenularis) and also probably true commissural fibers. It is to be compared with the inferior cerebellar commissure. Decussating fibers of the second and third order are present in each case and the nuclei in both cases are specialized parts of the substantia reticularis related to visceral functions.
The post-epiphysial decussation has been described only in ganoids (Johnston), bony fishes (Holt) and the horse (Favoro). It is poorly understood but deserves further study.
The anterior commissure, which will be described in the next chapter, is a dorsal decussation which, so far as at present known, is related in lower vertebrates to centers belonging to the visceral sensory system (olfactory and gustatory). In higher vertebrates two large commissures, one related to the olfactory cortex, the other to the somatic pallium, are developed from the anterior commissure.
The dorsal decussations may be summarized by saying that the mixed dorsal decussation of the cord has been differentiated in the brain into separate somatic and visceral sensory decussations. The commissura infima, the inferior commissure of the cerebellum, the habenular commissure and the anterior commissure represent the visceral portion. The superior commissure of the cerebellum, the dorsal decussation and posterior commissure in the mesencephalon (and the corpus callosum in the telencephalon) represent the somatic portion.
The ventral decussation of the spinal cord consists of internal arcuate fibers and of the neurites of heterolateral tract cells. It is in smaller part a decussation of secondary sensory tracts, in larger part belongs to the substantia reticularis. In the medulla oblongata the secondary sensory elements (lemniscus system) are more numerous and in the base of the mesencephalon a large number of fiber tracts decussate in the region of the nuclei of the III and IV nerves. These tracts are in part tertiary sensory tracts, in part descending tracts from somatic and visceral correlating centers going to make motor connections in the medulla oblongata and spinal cord. The complexity of these decussations is very great and the region is by no means well understood. For attempts at the analysis of the midbrain decussations the student must be referred to the papers by Edinger, Johnston, and Goldstein. The great number of decussating fibers in this region is due to the projection ventrad of the inferior lobes, which has crowded back the decussations from the segment of the diencephalon.
At the cephalic border of the hypothalamus another collection of decussating fibers is due in part to the same cause. These constitute the postoptic decussations. The optic chiasma has already been compared to the fibers of the lemniscus system in the ventral decussation of the medulla oblongata. The postoptic decussations are to be compared in a broad way with the other elements of the ventral decussation. They belong chiefly to the substantia reticularis, either of the hypothalamus or of the nuclei in the collicular region of the mesencephalon. These decussations have not yet been fully analyzed and the comparison of those in fishes and mammals is especially difficult. For the most recent treatment of these decussations the reader is referred to the papers of Myers, Kappers, and Goldstein. In Petromyzon the tractus lobo-epistriaticus, and in selachians the tractus olfactohypothalamicus lateralis decussate in the postoptic region.
Demonstration or Laboratory Work
- Study the general relations of the thalamus, hypothalamus, saccus and nucleus habenulae in haematoxylin preparations of the brain of a ganoid, bony fish or selachian, in the frog and in a mammal.
- Study the cells and fibers in the hypothalamus and nucleus habenulae in a fish brain by the method of Golgi. Study the general course of the chief fiber tracts in Weigert sections of the same brain.
Literature
Berkeley: The Nerve Elements of the Pituitary Gland. Johns Hopkins Hospital Reports, Vol. 4. 1895.
Bochenek, A.: Neue Beitrage zum Bau der Hypophysis cerebri bei Amphibien. Bull, internat. Akad. Sc. Cracovie. 1902.
Boeke, J.: Die Bedeutung des Infundibulums in der Entwickelung der Knochenfische. Anat. Anz., Bd. 20. 1901.
Boeke, J.: Ueber das Homologon des Infundibularorganes bei Amphioxus lanceolatus. Anat. Anz., Bd. 21. 1902.
Cajal, S. R. : Textura del sistema nervioso del Hombre y de los vertebrados.
Edinger, L.: Utersuchungen u.s.w. 2. Das Zwischenhirn. Abhdl. d. Senkenberg. Naturf.-Gesell. 1888.
Edinger, L.: Untersuchungen u.s.w. 4. Studien iiber das Zwischenhirn der Reptilien. Ibid. 1899.
Gemelli: Nuove Richerche sull Anatomia e sull Embriologia dell Ipofisis. Boll, della Soc. med.-chirurg. de Pavia. 1903.
Goldstein, K.: Vorderhirn und Zwischenhirn einiger Knochenfische, u.s.w. Arch. f. mik. Anat., Bd. 66. 1905.
Herrick, C. Judson: The Central Gustatory Paths in the Brains of Bony Fishes. Jour. Comp. Neur. and Psych., Vol. 15. 1905.
Hill, Charles: Developmental History of the Primary Segments of the Vertebrate Head. Zool. Jahrb., Bd. 13. 1899.
Holt, E. W. L.: Observations on the Development of the Teleostean Brain with especial reference to that of Clupea harengus. Zool. Jahrb., Bd. 4. 1890.
Johnston, J. B.: The Brain of Acipenser. The Brain of Petromyzon.
Kappers, C. U. A. : The Structure of the Teleostean and Selachian Brain. Jour. Comp. Neur. and Psych., Vol. 16. 1906.
Koelliker, A.: Gewebelehre. 6te. Aufl. Bd. 2.
Lundborg, H.: Die Entwickelung der Hypophysis und des Saccus vasculosus bei Knochenfischen und Amphibien. Zool. Jahrb., Bd. 7. 1895.
Johnston JB. The Nervous System of Vertebrates. (1907) Blakiston's Son & Co., London.
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