Book - The brain of the tiger salamander 21

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Herrick CJ. The Brain of the Tiger Salamander (1948) The University Of Chicago Press, Chicago, Illinois.

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Part I. General Description and Interpretation 1. Salamander Brains | 2. Form and Brain Subdivisions | 3. Histological Structure | 4. Regional Analysis | 5. Functional Analysis, Central and Peripheral | 6. Physiological Interpretations | VII. The Origin and Significance of Cerebral Cortex | VIII. General Principles of Morphogenesis Part 2. Survey of Internal Structure 9. Spinal Cord and Bulbo-spinal Junction | 10. Cranial Nerves | 11. Medulla Oblongata | 12. Cerebellum | 13. Isthmus | 14. Interpeduncular Nucleus | 15. Midbrain | 16. Optic and Visual-motor Systems | 17. Diencephalon | 18. Habenula and Connections | 19. Cerebral Hemispheres | 20. Systems of Fibers | 21. Commissures | Bibliography | Illustrations | salamander

Chapter XXI The Commissures

General Considerations

THE commissures are in two series, dorsally and ventrally of the ventricles. The fibers which cross the mid-plane are of two sorts: (1) some are strictly commissural, connecting corresponding regions of the two sides; (1) most of them are decussations, like the optic chiasma, connecting dissimilar regions. Those of the dorsal series include fibers of correlation which arise and terminate within the sensory zone and also connections between the sensory zone and other zones. Those of the ventral series are concerned in the main with motor co-ordination on the two sides of the bod3% some passing from sensory and intermediate zones to the motor zone, others lying wholly within the motor zone, and both sorts being accompanied by uncrossed fibers.

In addition to the crossed systems of correlation and co-ordination to which reference has just been made, it is a noteworthy fact that the main lines of fore-and-aft ascending and descending conduction in the brains of all vertebrates decussate, so that the adjusting centers for organs on the right side of the body are in the left side of the brain and vice versa. This applies especially to the apparatus of somatic adjustment, but not so generally to the visceral systems. Most of the fibers of the ascending secondary visceral-gustatory tract {tr.v.a.) and of the olfacto-hypothalamic tracts are uncrossed. The proprioceptive systems are both crossed and uncrossed.

The reason for the decussation of the major conduction pathways of the somatic sensori-motor systems has puzzled neurologists for many years, and fantastic theories have been expressed, some bearing the names of great masters — Wundt, Flechsig, Cajal, and others. This extensive literature has been reviewed by Jacobsohn-Lask ('24), who, instead of elaborating a theory in terms of the highly specialized human brain, like his predecessors, reviews the entire history of the evolution of the nervous system, from its first appearance in coelenterates, in relation to the bilateral symmetry of the body. None of these speculations have yielded satisfying conclusions.

A survey of the commissures and decussations of all vertebrate brains shows that some of them, like the posterior commissure and optic chiasma, hold constant positions throughout the series, while others vary widely in position and composition. It is evident that in the latter cases the site of crossing of a particular system of fibers is determined by the topographic arrangement of the parts to be connected. These arrangements are widely diversified in the lower groups of vertebrates and the crossing fibers tend to take the shortest available pathways. But when the course of a particular decussating tract has been established in an ancestral species, the original site of the decussation may be retained, despite great changes in the relative sizes of parts of the brain in phylogenetic descendants of the ancestral form, so that the tract may take a circuitous course to reach its decussation, as illustrated by some components of the postoptic complex in fishes. This conservatism may be accounted for by the fact that the decussating fibers may have collateral connections at any part of the course both before and after crossing. These considerations suggest that great caution should be observed in using the actual sites of crossing of the various systems of fibers as criteria of homology. Some of them are very stable; in other instances fibers with similar origins and terminations may cross in surprisingly different places, as illustrated by the various courses taken by fibers of the hippocampal commissure.

Primitively the roof plate of the brain, unlike the floor plate, was membranous, and the locations of commissures which invade it are determined by the functional requirements of the organs differentiated below it in the various species of animals. In the telencephalon the topographic arrangements of parts are extremely variable. In all cases there is a wide interruption of the commissures at the site of the stem-hemisphere fissure, paraphysis, dorsal sac, and their derivatives. In the diencephalon the habenular commissure is separated by the pineal recess from the commissura tecti, and the latter is continuous spinal ward with the posterior commissure and the com. tecti of the mesencephalon. In cyclostomes the middle part of the mesencephalic tectum is a membranous chorioid plexus, and the com. tecti is here interrupted.

At the posterior end of the tectum its commissure is continuous with a thin sheet of crossed and uncrossed fibers in the anterior medullary velum, and this, in turn, with the massive cerebellar commissures. Between the cerebellum and the calamus scriptorius the

THE COMMISSURES 291

roof is a membranous chorioid plexus except in some fishes, in which the large acousticolateral areas fuse over the ventricle, with some decussating fibers. In the region of the calamus there is a dorsal crossing of visceral sensory fibers (com. infima of Haller) and of somatic sensory fibers between the funicular nuclei. This dorsal commissure, reduced in size, is present through the entire length of the spinal cord.

The ventral series of commissures is concentrated in the anterior commissure of the telencephalon, the chiasma ridge of the diencephalon, the so-called ansulate commissure of the midbrain; and posteriorly of this it extends continuously as the ventral commissure of the rhombencephalon and spinal cord. The ventral decussations of Necturus from the tuberculum posterius to the spinal cord were described in 1930 ('30, p. 89) and the other commissures in papers before and after that date.

The arrangement of the telencephalic commissures of the Amphibia differs radically from that of all animals higher in the scale. As seen in median section (fig. 2), the anterior and hippocampal commissures do not cross in or above the lamina terminalis but in a high anterior commissure ridge which projects upward from the floor posteriorly of the interventricular foramina. This ridge is separated from the lamina terminalis by the wide precommissural recess, or aula, which is the vestibule of the interventricular foramina (fig. IB). Both pallial and subpallial parts of the hemispheres contribute fibers to the commissures in this ridge, and many fibers from the olfactory and hippocampal areas also cross in the habenular commissure.

The peculiar arrangement of the anterior and hippocampal commissures, the lamina terminahs, and the related chorioid plexuses of Ambly stoma has been described and illustrated ('35). It is explained by the topography at the di-telencephalic junction and the presence of unusually wide interventricular foramina. The explanation of this topography, in turn, is to be^ sought in the phylogenetic ancestors of existing Amphibia. In Protopterus as described by Rudebeck ('45), pallial formation is confined to the lateral wall of the hemisphere, and the hippocampal commissure passes down behind the foramen to cross in close association with the anterior commissure in essentially the same way as in amphibians. In view of the close similarity of development of the brains of amphibians and lungfishes, it is probable that the ancestral amphibian resembled Protopterus in this region.

The arrangement of myelinated fibers in all the commissures of Amblystoma as seen in the median plane is shown in figure 2C. The components of these commissures will now be summarized, with references to more detailed descriptions.

THE DORSAL COMMISSURES

Commissura hippocampi. — These fibers converge from all parts of the hippocampal area to its ventral border, from which they descend behind the foramen to cross in the dorsal part of the anterior commissure ridge (figs. 96-99). Many of these fibers join a mixed longitudinal fascicle, termed "fimbria," which borders the primordium hippocampi for its entire length at the ventromedial margin. Another fascicle, composed exclusively of commissural fibers, assembles at the ventrolateral border of the primordium. Most of these fibers are unmyelinated. Weigert sections show a few brilliantly stained myelinated fibers scattered among them.

Accompanying this commissural bundle as it leaves the hippocampus are other similar fibers of tractus cortico-thalamicus medialis and tr. cortico-habenularis medialis. Some of the latter cross in the habenular commissure and return to the hippocampal area of the opposite side, this being the com. pallii posterior (figs. 32, 34, 71, 72, 76).

Commissura hahenularum {com. superior). — The analysis of the stria medullaris as detailed in chapter xviii reveals the following components of the habenular commissure: (1) The most anterior member is the com. pallii posterior (fig. 20, no. 8). (2) Posteriorly of this the com. superior telencephali includes components 3, 4, 5, and 6 of the diagram, viz., tr. olfacto-habenularis anterior, tr. cortico-habenularis lateralis, and tr. amygdalo-habenularis. (3) Still more posteriorly are crossing fibers of uncertain connections, some of which may come from other components of the stria medullaris. There are doubtless strictly commissural fibers connecting the habenulae of the two sides. The commissural fibers from the hemisphere are accompanied by uncrossed fibers. It is possible that these tracts are accompanied by crossed and uncrossed fibers, which pass from the habenular nuclei forward into the hemispheres, but this has not been demonstrated. Commissura tecti dieiicephali.— Most of these fibers are commissural, connecting the two pretectal nuclei. Some tecto-habenular and habenulo-tectal fibers decussate here.

THE COMMISSURES 293

Commissura posterior. — This is the primary pathway from the anterior part of the optic tectum to the motor zone of the midbrain. Its decussating fibers appear very early in embryogenesis, accompanied by uncrossed fibers from the tectum and eminence of the posterior commissure. In the adult many of the commissural fibers end in this eminence. Crossed and uncrossed fibers spread widely in the posterior part of the thalamus, the nucleus of the tuberculum posterius, and the dorsal tegmentum. The largest fascicles connect with the big cells of the nucleus of Darkschewitsch (p. 217).

Commissura tecti mesencephali. — This thin sheet of crossing fibers is continuous between the posterior commissure and the anterior medullary velum. It contains thin and thick fibers (many of the latter myelinated) which spread widely through all layers of the tectum. Most of these seem to be commissural between the tecti of the two sides, but no satisfactory analysis has been recorded. Some fibers of the mesencephalic root of the V nerve apparently decussate here, but, if so, the number is small.

Commissures of the anterior medullary velum. — Most of the fibers in the velum are longitudinal — tr. tecto-cerebellaris — and some of these may decussate here. The most constant and noteworthy component is the decussation of the IV nerve roots ('36, p. 342; '42, p. 255). The velum contains cells and fibers of the mesencephalic V root, and some of these may decussate here. Our preparations give no clear evidence of crossed fibers of this root; if present, the number is certainly not large.

Cerebellar commissures. — Larsell's analysis of these commissures is confirmed. The two systems are quite distinct. (1) The com. cerebelli is related with the median body of the cerebellum, including decussating fibers of tr. spino-cerebellaris, sensory root fibers of the V nerve, secondary trigeminal fibers from the superior sensory V nucleus in the auricle, and commissural fibers between these nuclei and between the two sides of the corpus cerebelli. (2) The com. vestibulo-lateralis cerebelli is a more dispersed system of fibers related with the vestibular and lateral-line centers of adjustment in the auricles. It is composed of root fibers of the VIII nerve and secondary fibers of both vestibular and lateral-hne systems. There are doubtless also commissural fibers between the two auricles. None of these fibers make significant connections with the median body of the cerebellum through which they pass. Their terminal relations are with auricular tissue which is the primordium of the floccular part of the mammahan flocculonodular lobe.

Commu-sura infima Halleri. — This is a decussation of the fascicuH soHtarii at the calamus scriptorius, containing both root fibers and secondary fibers of the visceral-gustatory system and doubtless also commissural fibers between the two commissural nuclei of this system.

Commifisure of the funieular nuclei. — Intimately associated with the preceding are commissural fibers between the nuclei of the dorsal funiculi in the calamus region, with which decussating fibers are mingled. This dorsal commissure of somatic sensory fibers is extended, reduced in size, downward through the entire length of the spinal cord. There is some evidence that the visceral sensory com. infima is also represented in the cord (p. 125).

This completes the summary of the dorsal commissures. We now turn to the ventral series, beginning, as before, at its anterior end.

THE VENTRAL COMMISSURES COMMISSURA ANTERIOR

The complex com. anterior occupies the entire anterior commissure ridge except its dorsal border. Its largest components are the partial decussations of the medial forebrain bundles below and the lateral forebrain bundles above (figs. 25-28). Associated with these fibers are others, including the com. amygdalarum (which is part of the stria terminalis system, p. 256), some fascicles of the nervus terminalis, and a dispersed decussation between the olfactory fields of the anterior parts of the hemispheres ('396, fig. 21).

The crossing fibers of the anterior commissure ridge are enveloped by a thin layer of gray which expands laterally as the large bednuclei of the anterior commissure. The thin floor of the long preoptic recess between the anterior commissure ridge and the chiasma ridge contains the longitudinal fibers of tr. preopticus, some of which decussate here.

CHIASMA OPTICUM

All fibers of the optic nerves decussate in the chiasma opticum. The crossing occupies the anterior border of the chiasma ridge (figs. 2B and 2C). At the posterior border of the chiasma there is some mingling of optic fibers with those of the postoptic commissure, but in some of our Golgi preparations the optic fibers are electively impregnated and can be separated from the others ('41, '42).

THE COMMISSURES 295

COMMISSURA POSTOPTICA

The complex com. postoptica is represented in mammals by the supra-optic commissures, but its composition is so different in urodeles and mammals that exact homologies cannot be established. Further analysis of intervening species is requisite before these relationships can be clarified.

In Amblystoma this complex includes decussating fibers derived from the superior and inferior colliculi, the entire diencephalon, the amygdala, and the subpallial olfactory field of the cerebral hemisphere. Many of these tracts have collateral connections along their courses, both before and after crossing, and are accompanied by uncrossed fibers. There are also strictly commissural fibers connecting some of these regions (for evidence of such fibers in the frog see '25, p. 480). These decussating systems connect, after crossing, with extensive fields of the intermediate and motor zones, including the strio-amygdaloid area, preoptic nucleus, hypothalamus, ventral thalamus, geniculate neuropil, dorsal tegmentum, peduncle, isthmic tegmentum, and bulbar tegmentum. The posterior part of the postoptic commissure is comparable with the com. tuberis of some other vertebrates and consists mainly of decussating fibers from the ventral part of the hypothalamus to the peduncle and interpeduncular nucleus. The direction of conduction of most of these fibers has not been clearly determined, though most of the larger systems evidently converge from the sensory zone into the motor field.

The fibers of few of these components are assembled in wellorganized tracts ; most of them are so dispersed and commingled that analysis is very difficult. Some of them are myelinated, and these tend to be assembled in recognizable tracts. These myelinated tracts, as seen in Weigert sections, were the first to be described, but their distribution after crossing baffled analysis. These tracts are accompanied by much larger numbers of unmyelinated fibers in more dispersed arrangement. The courses of some of these have been revealed by elective Golgi impregnations, and other systems have been clarified by study of the sequence of their development as published in a series of papers from 1937 to 1941. In view of the complexity of these connections and the technical difficulties encountered in their study, it is not surprising that the earlier descriptions were incomplete and not free from error. It is believed that now it is possible to present an analysis of this complex which, though still incomplete, reveals its major features.

This generalized arrangement as seen in urodeles is probably primitive and may be taken as the point of departure in the study of the postoptic systems of more specialized brains of both fishes and higher vertebrates. These commissures have been analyzed in Necturus ('41a, p, 513), where they are still more generalized. Our present knowledge of these systems of Amblystoma was summarized on pages 219-28 of the paper of 1942, with diagrams illustrating the connections of the principal tracts. All known components are assembled in the following list. Here some of the earlier names of tracts have been replaced by more accurate terms; some others are retained, though now known to be inappropriate or inadequate. In this list there are included, first, the groups of fibers which descend to the chiasma ridge from the tectum and pretectal nucleus, followed by those descending from the thalamus, next, the systems arising in the hypothalamus, and, finally, a heterogenous group with hypothalamic connections. For additional details about some of these tracts in preceding chapters consult the Index.

1. Tr actus tecto-thalamicus et hypothalamicus cruciatus anterior (fig. 12, tr.t.th.h.c.a.). — This anterior tectal fasciculus is a mixture of myelinated and unmyelinated fibers from the dorsomedial part of the tectum opticum and the pretectal nucleus which descend across the thalamus in company with the more anterior fascicles of the optic tract. After partial crossing in the antero ventral part of the chiasma ridge, its fibers spread in the neuropil of the chiasma ridge and hypothalamus. Some of them may reach beyond this region. This cumbersome name was applied in my earlier papers to a mixed fascicle which had not been analyzed. It can now be replaced by the names of the several tracts of which it is composed. Some of these are uncrossed (notably tr. pretecto-thalamicus) , and some fibers of two of them decussate in the chiasma ridge, nos. 2 and 3 below. The arrangement of these components as seen in horizontal sections is shown in figures 25-36.

2. Tractus tecto-hypothalamicus anterior (tr.t.hy.a.). — This is the tectal component of the preceding fasciculus; it is evidently an optic pathway to the hypothalamus (p. 224). It passes through the pretectal nucleus and is accompanied by tecto-pretectal fibers. It probably is physiologically related with no. 3.

3. Tractus pretecto-hypothalamicus (fig. 15, tr.ptJiy.). — This tract as

THE COMMISSURES 297

it leaves the pretectal nucleus is accompanied by a large tr. pretectothalamicus (p. 234 and figs. 35, 36).

4. Tr actus tecto-ihalamicu.s ei hypothalamicus cruciatus posterior (fig. 12, tr.t.th.h.c.p). — This, like no. 1, is a mixed fascicle which has been analyzed. Both these names may now be discarded in favor of shorter terms — anterior and posterior tectal fascicles. The posterior fascicle arises chiefly from the nonoptic nucleus posterior tecti and the adjoining ventrolateral margin of the optic tectum, the latter region in Necturus receiving few terminals of the optic tract ('41a, p. 516). This fascicle is probably activated primarily by lemniscus, rather than optic, fibers or by a combination of the two. The tracts of which it is composed have collateral connections with the geniculate neuropil and ventral thalamus both before and after crossing. These fibers descend from the tectum parallel with those of the lateral optic tract and internally of them. So far as known they have a common origin in the tectum, but, after crossing, they take widely divergent courses. The tracts, consequently, are named according to their terminal distribution. The most important components are the following, nos. 5 and 6.

5. Tractus tecto-hypothal amicus posterior. — These are finer fibers which terminate in the postoptic neuropil and neighboring regions of the hypothalamus. They are more clearly seen in Necturus ('41a, p. 516) than in Ambly stoma.

6. Tractus tecto-tegmentalis cruciatus (fig. 12, tr.t.teg.c). — The course and distribution of the thicker fibers of the posterior tectal fascicle were not clarified until they were identified in larval stages, in which they were electively impregnated because they mature precociously. These were first recognized in early feeding larvae ('39, p. 106) and later in adult Necturus ('41a, p. 516) and Amblystoma ('42, p. 222). Some erroneous descriptions of these fibers in my earlier papers have been corrected ('39, p. 110).

The more heavily myelinated fibers of this tract decussate in the dorsal part of the chiasma ridge (figs. 2C, 12, 16, 25, tr.t.th.h.c.p.), and after crossing they enter tegmental fascicles of groups (8) and (6), and in smaller numbers they are dispersed in other fascicles. The dispersed fibers spread in the peduncle. Those which enter fascicles numbered (8) take a longer and more dorsal course, distributing to the dorsal, isthmic, and trigeminal tegmentum, some of them extending as far as the level of the V nerve roots. The entire course of these fibers can be followed in the horizontal sections, figures 25-35, where they are marked tr.t.th.h.c.p. before their decussation and (8)

beyond the crossing.

There are two strong systems of crossed tecto-peduncular and tecto-tegmental fibers, both of which are drawn in figure 12. The system just described descends chiefly from the nonoptic part of the tectum {tr.t.th.h.c.p.) and, after crossing, spreads in the peduncle by way of tegmental fascicles (6) and throughout the tegmentum by way of fascicles (8). The second system is tr. tecto-peduncularis cruciatus (tr.t.p.c), which arises in the optic part of the tectum, crosses in the commissure of the tuberculum posterius, and then spreads out in the peduncle. This well-myelinated tract is accompanied by similar fibers from the pretectal nucleus and dorsal thalamus. These two tectopeduncular systems evidently have quite different physiological significance.

Attempts to analyze the postoptic components arising in the thalamus were unsuccessful until the sequence of development of these fibers was revealed by embryological studies. These findings were then confirmed by elective Golgi impregnations of older larvae and adults. Some errors in the earlier descriptions have been corrected, and now it is possible to give a fairly complete account of both the crossed and the uncrossed fibers which diverge from the thalamus. Since the direct and crossed fibers are evidently intimately related physiologically, both series are included in the following description. Efferent fibers from the thalamus are arranged in two sharply contrasted series, which arise, respectively, from the dorsal thalamus and the ventral thalamus.

The efferent series from the dorsal thalanuis includes uncrossed fibers to the tectum, habenula, cerebral hemisphere, ventral thalamus, hypothalamus, and peduncle which need not be further considered here; but some of the other uncrossed tracts, which evidently are in reciprocal physiological relation with the crossed tracts, should be specifically mentioned. Decussating fibers from the dorsal thalamus are in two groups. The first includes thick myelinated fibers of tr. thalamo-peduncularis cruciatus (tr.th.p.c), which, as already mentioned, joins tr. tecto-peduncularis cruciatus {tr.t.p.c.) to decussate in the commissure of the tuberculum posterius as described under that caption below. The second group is a much larger number of thin unmyelinated or lightly myelinated fibers which decussate in the postoptic commissure and will next be described.

THE COMMISSURES 299

7. Tractus thalamo-hypothalamicus et peduncnlaris cruciatus {tr.ih.h.p.c.). — In the earlier descriptions of both Aniblystoma and Necturus this name was given to a large collection of unmyelinated and lightly myelinated fibers which descends in disj^ersed arrangement from the dorsal thalamus to the postoptic commissure. Their distribution beyond the decussation could not be clearly followed, and some of those descriptions now require correction. As elsewhere pointed out ('42, p. 223), this name should now be discarded because at least three quite distinct tracts are represented here and relatively few of these fibers have any connection with the peduncle. The three tracts represented in this complex (nos. 8, 9, and 10) have a common origin in the dorsal thalamus, chiefly its middle sector, and, after crossing, take widely different courses. Their decussation is posterior to that of the tectal components of the commissure in a band which is narrow dorsally and spreads ventrally through a wide area of the neuropil of the chiasma ridge (fig. 2C, tr.th.h.d.c).

8. Tractus thalamo-hypothalamicus dorsalis cruciatus (tr.th.h.d.c). — These fibers, most of which are unmyelinated, cross in the middle region of the chiasma ridge (figs. 2C, 15, 25) and then spread in the hypothalamus. In figures 27-33, 95, 102, and 103 the symbol tr.th.h.d.c. refers to the mixture of fibers of tracts 8, 9, and 10 in their descending course from the thalamus to the commissure. The fibers of nos. 9 and 10 cross dorsally of those of no. 8, though there is mingling of the fibers of the three tracts with one another and with those of surrounding decussations.

The remaining fibers of this complex, after crossing, are distributed to the tegmentum in two tracts which take parallel courses, one superficially, the other at the border of the gray. These are designated components A and B, respectively. In figures 15 and 21 the components A and B are not separately designated.

9. Tractus thalamo-tegmentalis dorsalis cruciatus A {tr.th.teg.d.c.A.). — This tract was first described in the early feeding larva ('39, p. 116) and later in the adult ('42, p. 224). After decussation, these unmyelinated fibers ascend from the chiasma ridge parallel with the course of the descending uncrossed limb of this commissure and more superficially. Their further course is shown in figures 26-34, here marked A . In the dorsal tegmentum they turn spinal ward along the ventrolateral border of the tectum. Here they lie close to the pial surface and immediately ventrally of the lateral optic tract (fig. 94, A). In this part of their course they join an uncrossed tract with similar origin from the dorsal thalamus and similar distribution in the tegmentum — tr. thalamo-tegmentalis rectus (figs. 15, 31-34, 94, tr.th.teg.r.). This latter tract arises from both dorsal and ventral thalamus, but only the dorsal component of it is under consideration here (fig. 21, tr.th.teg.d.r.). In some Golgi preparations there is evidence that axons from the dorsal thalamus may divide, with branches entering both the uncrossed and the crossed thalamo-tegmental tracts ('42, p. 224). Evidently, the crossed and uncrossed tracts are reciprocally related physiologically.

10. Tractus thalamo-tegmentalis dorsalis cruciatus B. — This is a deep component of the same system as the preceding, receiving nearly all the myelinated fibers of no. 7 of this list. In the chiasma ridge these fibers separate from the others of the group and cross at the dorsal border of the postoptic commissure (marked B in figs. 26 and 95). Beyond the decussation they scatter widely, and most of them enter the well-myelinated tegmental fascicles of group (8), within which they may descend as far as the V nerve roots.

The two crossed tracts to the tegmentum, nos. 9 and 10 of this list, seem to have the same origin and about the same field of distribution, except that one of them {A) terminates in the superficial tegmental neuropil and the other {B) arborizes in the deep neuropil. The physiological properties of these zones of neuropil evidently are different.

11. Tractus thalamo-tegmentalis ventralis cruciatus (figs. 2C, 17, tr.th.teg.v.c). — These thick fibers (many of them well myelinated) converge into the postoptic commissure from all parts of the ventral thalamus. In this part of their course they are not fasciculated but are scattered among other similar fibers so that their courses could not be followed until they were studied embryologically. They mature early and may be impregnated with reduced silver at stages when few other fibers respond to this treatment. We also have good elective Golgi impregnations of them in later larval stages ('39, pp. 98, 120; '396, p. 546; '42, p. 225).

These fibers cross in the posterodorsal part of the chiasma ridge, mingled with those of other systems. The thickest of the myelinated fibers are crowded together at the dorsal margin of the postoptic complex. After crossing, they spread widely in the peduncle and tegmentum. Most of those from the posterior part of the ventral thalamus enter ventral tegmental fascicles of group (4), and some of these may descend in the f . longitudinalis medialis. Many fibers from the

THE COMMISSURES 301

middle and anterior parts of the ventral thalamus reach the tegmentum through fascicles of groups (6) and (8) . These crossed fibers are accompanied by many others that take similar courses without decussation.

The hypothalamic components of the postoptic commissure include, in addition to the terminals of extrinsic tracts already described, a few well-defined tracts and several other less-well-known connections.

12. Tractus hypothalamo-peduncularis et tegmentalis (figs. 18, 23, tr.hy.ped.; fig. 21, tr.hy.ieg.). — This is the most noteworthy component originating within the hypothalamus. Its fibers assemble from the whole of the ventral part of the hypothalamus and comprise the main pathway from this region to the motor field of the peduncle and tegmentum. Some of them decussate in the posterior part of the chiasma ridge ; others are uncrossed ; still others decussate in the commissure of the tuberculum posterius. They connect by terminals or collaterals with the dorsal part of the hypothalamus, ventral part of the peduncle (including the neuropil of the area ventrolateralis pedunculi), and isthmic tegmentum. The last-mentioned group includes thick fibers, some of which are well myelinated, which enter ventral tegmental fascicles (4); and some of these may take long courses in the f. longitudinalis medialis (for further description see p. 280, and '42, p. 226).

13. Olfactory projection tract (fig. 19, oLp.tr.). — This name has been given to a thin strand of unmyelinated fibers which pass in both directions between the strio-amygdaloid area and a specific nucleus at the posterior border of the chiasma ridge ('21, p. 247; '27, p. 304; '36, fig. 5). Some of these fibers decussate here.

14. Tractus pedunculo-hypothalamicus. — A large component in the posterodorsal part of the postoptic commissure was provisionally given this name, though the exact connections of its fibers could not be determined ('42, p. 227).

15. Medial forebrain bundle. — The fibers of the medial forebrain bundle are interlaced with all components of the postoptic commissure laterally of the chiasma ridge. Many of these fibers of both descending and ascending systems enter the postoptic neuropil and participate in its formation. This implies a transfer of more or less of this activity to the opposite side of the brain, but no details of specific decussational or commissural pathways have been revealed.

Postoptic neuropil. — In the mid-plane, the postoptic decussations occupy most of the chiasma ridge, and these fibers are enveloped on all sides except ventrally by a gray layer, the bed-nucleus of the postoptic commissure. This nucleus is expanded laterally. From this gray layer, richly arborized ependymal elements and dendrites of neurons are spread among the decussating fibers. Similar long dendrites enter it from all surrounding parts, including the nucleus magnocellularis, from which tr. hypophysius arises ('42, fig. 51). The entire chiasma ridge is also permeated with dense axonic neuropil which is continuous with that of surrounding parts. This neuropil receives terminals and collaterals of axons of hypothalamic neurons, medial forebrain bundles, tractus preopticus, and most of the decussating systems. The chief outflow from it seems to be by axons of the nucleus of the postoptic commissure directed into tr. hypothalamo-peduncularis. Other fibers enter the medial forebrain bundle (for further details see '42, p. 219; Necturus, '336, p. 251; 'Mb, p. 383).

This neuropil is clearly one of the major adjusting centers of the urodele brain. Situated in the center of the great olfacto-visceral field, its connections indicate that it may be activated from every correlation center of the cerebrum. Undoubtedly it plays an important part in all general visceral activities. It is equally evident that there is no provision here for localization of specific functions. This is probably the undifferentiated primordium from which some of the specialized hypothalamic nuclei of mammals have been elaborated.

COMMISSURA TUBERCULI POSTERIORIS

The commissure of the tuberculum posterius was defined in 1917 (]). 224) as the ventral mesencephalic decussations between the infundibulum and the fovea isthmi. Posteriorly of the latter the ventral tegmental commissure extends backward without interruption through the rhombencephalon and spinal cord. In Necturus there is a very short interruption of the ventral commissural system at the fovea isthmi ('30, p. 89), but in most urodeles this gap does not appear.

The composition and arrangement of these ventral commissures are very diversified in different vertebrates. The hypothalamic connections at the anteroventral end of the commissure of the tuberculum posterius of Amblystoma are in some other vertebrates widely separated as the retro-infundibular decussations, or decussatio hy

THE COMMISSURES 303

pothalamicus posterior. The remainder of the commissure of the tubercuhim and the anterior part of the ventral tegmental commissure comprise the ansulate commissure of the literature.

In Amblystoma I have recognized four chief components of the commissure of the tubercuhim posterius ('36, p. 305 and fig. 2), arranged as shown in sagittal section in figures 81, 104; in horizontal sections in figures 28, 29, 30, 31 (component 4 being here marked, tr.t.b.c.l.); and in transverse section in figure 94 (components 2 and 3 shown here, components 1 and 4 being shown in a neighboring section, '36, fig. 11). The connections of these components are as follows :

1. This is the partial decussation of unmyelinated fibers from the dorsal (mamillary) part of the hypothalamus to the interpeduncular nucleus ('36, p. 338 and figs. 3, 8, 11-13). Most of them enter ventral tegmental fascicles of group (2) as marked tr.mam.inp., on figures 19, 27-30, 71, 81, 92, 103.

2. This component contains partly myelinated fibers from the dorsal hypothalamus to the peduncle and tegmentum, accompanied by many uncrossed fibers and by fibers conducting in the reverse direction from the peduncle to the hypothalamus (p. 278; figs. 8, 21, tr.mam.feg., 23, tr.mam.ped.). Most of them spread in the alba of the peduncle or enter ventral tegmental fascicles of group (3). Another system is tr. hypothalamo-peduncularis et tegmentalis from, the ventral part of the hypothalamus to the peduncle and tegmentum (fig. 21, ir.hy.teq. and fig. 23, tr.hy.ped.). There is interchange of all these fibers with those of component 1 and with other tegmental fascicles. They are marked tr.mam.ped., tr.mam.teg., and (3) on figures 18, 21, 23, 27-31, 79, 82, 92, 94, 103.

3. The third component contains heavily myelinated fibers from the tectum, pretectal nucleus, and dorsal thalamus to the peduncle, accompanied by many uncrossed fibers — tr. tecto-peduncularis cruciatus {tr.t.p.c.) and tr. thalamo-peduncularis cruciatus (tr.th.p.c), as elsewhere described (p. 223; '42, p. 267). These are shown on figures 12, 15, 18, 22, 29-36, 94, 103.

4. The dorsal component of this commissure is larger than the others and is composed chiefly of tecto-bulbar and tecto-spinal fibers from the anterior part of the tectum opticum, marked tr.t.b.c.l. on the figures. Its fibers after oblique decussation enter ventral tegmental fascicles of group (1) (see p. 277 and figs. 12, 27-36, 93, 94).

In addition to these four well-defined components, there are unmyelinated fibers which descend from the tectum and dorsal tegmentum through the gray and deeper layers of the alba to the region of the nucleus of the III nerve (fig. ^2, tr.t.ped.). Some of these tectopeduncular fibers decussate in the ventral commissure both before and behind the fovea isthmi. In Necturus some fibers of the nervus terminalis probably decussate in the commissure of the tuberculum posterius (McKibben, '11), and this may be true in Amblystoma.

COMMISSURA VENTBALIS

The ventral decussations of Necturus were analyzed in the paper of 1930 (p. 89), and those of Amblystoma are similar. For the details the reader is referred to that description and to a later contribution on larval Amblystoma ('396).