Book - The brain of the tiger salamander 17

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

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

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 XVII Diencephalon

GENERAl FEATURES

In the following analysis tlie structures at the di-telenceplia ic junction are omitted, since they are fully described in the next chapter. The retina and the optic nerve are integral parts of the diencephalon. The retina itself is more than a simple receptor, for it contains elaborate apparatus for analysis and synthesis of v'sual excitations (Polyak, '41).


The diencephalon combines characteristics of the sensory, intermediate, and motor zones, and the tissues involved are so closely interwoven that toi)ographic subdivisions into zones is arbitrary and misleading. The lai-gest number of afl'erent fibers enter by the optic nerve and are distributed to the thalamus, pretectal nucleus, and hypothalanms, in addition to their mesencephalic connections. The nervus terminalis has widely spread terminals in the preoptic nucleus and the hypothalamus. The parietal nerve contains a few fibers of uncertain connections and functions. The hypo])hysial nerve carries many fibers to the ])ars nervosa of the hypophysis ami a smaller number to the j)ars distalis.


Between the central stations of these peripheral nerves and interpenetrating them is correlating tissue of intermediate-zone type. As repeatedly emphasized in several preceding contexts, the diencephalon as a whole (except for the retina) is a transitional sector of the brain, inter})olated between the olfactory field anteriorly and all other sensori-motor fields posteriorly. This apparatus of correlation is supplementary to the more direct paths of through traffic which organize the basic patterns of the stable action system characteristic of the species. In urodeles the most direct descending olfactory connections are by way of the tractus olfacto-peduncularis for somatic responses and to the hy})othalamus for visceral responses. Additional diencephalic olfactory connections are ancillary to these. The apparatus of other stereotyped patterns of sensori-motor adjustment is similarly organized in the stem below the diencephalic level. These


DIENCEPHALON 231

systems all have a collateral discharge into the thalamus, which is probably concerned with conditioning and other types of "inflection" of the stable components of behavior. In the most primitive vertebrates this primordial thalamic apparatus of higher synthesis and conditioning is small and at a low grade of differentiation. In Amblystoma it is notably larger, but still relatively unspecialized and probably concerned more with regulation of central excitatory state and the general physiological disposition and diathesis than with refined analysis. In mammals this tissue is elaborately specialized, with provision for precise localization of function correlated with the simultaneous differentiation of the neopallial cortex. But even in man the intrinsic functions of the thalamus are not supplanted by cortical development. The thalamo-cortical connections carry two-way traffic and all cortical activity may be profoundly affected by diencephalic influence.


The primary subdivisions of the adult diencephalon are more clearly evident in the gross preparation of the urodele brain than in most other vertebrates, as illustrated in figures IB and 2. I emphasized this in the paper of 1910, and the analysis there proposed has been widely accepted as applicable in all vertebrates. The epithalamus includes the habenular nuclei, the pars intercalaris, the epiphysis, and the membranous dorsal sac and paraphysis. The dorsal thalamus is an undifferentiated nucleus sensitivus, the ventral thalamus a field of motor co-ordination, and the hypothalamujs an olfactovisceral adjuster, including the pars nervosa of the hypophysis. These subdivisions and their more important connections have been described in previous publications (see the summary, '42, p. 204; also, '10, '21a, '27, '35a, '36, '39, '396, '41, '42; Necturus, '17, '336, '346, '34c, '41a; for other urodeles see the works cited on p. 11).


DEVELOPMENT

The larger part of the wall of the diencephalic sector of the early neural tube is evaginated to form the retina and the optic nerve. The details of this development, which begins at the anterior end of the medullary plate, have been recorded by Adelmann ('29, '36). I have described later stages of the development of these structures ('41). During these stages most of the dorsal median wall of this part of the neural tube remains membranous and is elaborately enlarged and folded to form the epiphysial vesicle, paraphysis, and diencephalic chorioid plexuses, the development of which has recently been described in several species of amphibians by Rudebeck ('45). The adult structure of these epithelial organs and the related meninges and blood vessels I described in 1935. The ventral medial wall of the adult (fig. 2) between the anterior commissure ridge and the tuberculum posterius is thin but nervous, except for the dorsal wall of the infundibulum. It has a massive thickening — the chiasma ridge — midway of its length. The arrangement of the chiasma ridge, the anterior commissure ridge, and the adjoining preoptic recess and precommissural recess is peculiar in the amphibian brain (fig. 2 and p. 291).


The massive side wall of the adult diencephalon is composed of four columns of gray substance, which project into the third ventricle as longitudinal ridges separated by deep sulci. As shown here in figures 1 and 2, these are epithalamus, dorsal thalamus, ventral thalamus, and hypothalamus. This adult configuration is achieved by a series of local pulses of proliferation and differentiation of cells, which shift in location and rate of growth from stage to stage, as indicated by changes in the sculpturing of the ventricular surface.


Rudebeck ('45) illustrates embryonic stages of Necturus, Triturus, and the anuran Pelobates which are younger than any of my models of Amblystoma. His pictures show that, in each of these species in stages preceding hatching of the eggs, the first ventricular markings to appear in the rostral part of the neural tube take the form of a series of transverse sulci. The most anterior of these, his "sulcus subpalliahs," separates the subpallial from the pallial parts of the hemisphere. Next follows von Kupffer's sulcus intraencephalicus anterior (p. 117), which separates the hemisphere and preoptic nucleus from the diencephalon. Posteriorly of this sulcus is a high transverse ridge, the di-telencephalic ridge, the adult derivatives of which apparently are the habenula and those structures ventrally of it which are listed in our next chapter as bed-nuclei of the di-telencephalic junctionnucleus of Bellonci, eminentia thalami, nucleus of tr. olfactohabenularis (p. 239), and part of the preoptic nucleus and hypothalamus. This ridge is bounded spinalward by his "sulcus diencephalicus ventralis," the dorsal part of which persists as the sulcus posthabenularis and the ventral part as the sulcus ventralis thalami plus the sulcus hypothalamicus. Posteriorly of the ditelencephalic ridge is an undeveloped area, giving rise to the adult pars intercalaris diencephali and the major parts of the dorsal thalamus, ventral thalamus, and hypothalamus.


After swimming is well established (about Harrison's stage 37),


DIENCEPHALON 233

differentiation is much further advanced, though there is no radical change in the topographic arrangement of the areas involved ('38, figs. 15-18). But, by the time the larvae begin to feed (Harrison's stage 46), the parts have shifted their positions toward the adult condition ('386, figs. 1, 2), which is fully attained in midlarval stages (38 mm, in length) of Amblystoma tigrinum ('39a, fig. 1).


These changes are brought about in two ways. There is, in the first place, a gradual straightening of the two great flexures of the neural tube, with a resulting rearrangement of the several areas in relation to the long axis of the brain and to one another. In the second place and accompanying these changes in the general shape of the forebrain, the regions of most rapid proliferation and difi'erentiation of cells shift their positions. There results a change in the pattern of the ventricular eminences and the intervening sulci. The original transverse ridges and sulci are broken up, and their parts are rearranged in the longitudinal series which we see in the adult brain.


The details of these changes have been described during recent years by a number of workers in Professor Holmgren's Zootomical Institute at Stockholm. Rudebeck ('45), in his admirable paper on the development of the forebrain of lungfishes, shows that the sequence of these changes is essentially the same in Dipnoi, Urodela, and Anura. In the youngest stages, proliferation of cells is most active along the lines of the primitive sulci. These active zones enlarge, and this results in obliteration of the primary sulci or a shift in their positions. The primary grooves, accordingly, do not mark the boundaries of the primordia of the different diencephalic centers, but, on the contrary, they are zones of more active growth from which these centers are developed. In some regions the original proliferation grooves disappear, in others they are gradually displaced and so become limiting sulci at the boundaries of the differentiated areas of the adult.


My interpretation of these changes as outlined above differs somewhat from that of Rudebek as summarized on page 63 of his monograph. Certainly, the details of these developmental processes vary from species to species, and examination of still more species of vertebrates along the lines of the critical studies made in the Stockholm Institute will doubtless clarify the questions which are still in controversy. One is impressed by the close resemblance of this development in lungfishes and amphibians, which suggests that these groups of animals are more intimately related phylogenetically than has hitherto been generally admitted.


EPITIIALAMUS

The epithalamus, like all other parts of the sensory zone, combines reception from the periphery with functions of correlation. It has two radically different parts that are separated by the pineal recess and the overlying pineal vesicle. The habenular nuclei comprise the anterior division. The posterior division is the pars intercalaris diencephali, which is relatively larger in urodeles than in higher animals. Because of this enlargement and the small size of the dorsal thalamus, the habenula lies farther forward than usual. The two habenulae are connected by the habenular or superior commissure and the pars intercalaris by the commissura tecti diencephali.


The pars intercalaris is sharply defined in front, but posteroventrally it merges with the posterior sector of the dorsal thalamus. There is a narrow subependymal layer of small nerve cells imbedded in dense periventricular neuropil, and externally of this lies a lentiform mass of crowded cells, mostly small, with some of larger size, which I term the "pretectal nucleus" (figs. ^B, 14, 15, 35, 36, nuc.pt.) without commitment about its homologies in other animals. It is probable that the pulvinar of mammals has been derived from the undifferentiated gray of the posterior part of this field.


As described in 1942 (pp. 205, 259, 279), the pretectal nucleus is permeated and covered with dense neuropil, which is continuous with that of all surrounding parts — tectum, habenula, and dorsal thalamus — with fibers passing in both directions. It receives terminals and collaterals from all chiasmatic bundles of the optic tracts (figs. 14, 22), tectum (figs. 12, 14, ir.t.pi.), posterior commissure, dorsal thalamus, habenula, and tr. strio-tectalis (figs. 14, 101, tr.st.pt.). Efferent fibers leave this nucleus for the tectum (figs. 11, 15, tr.pt.tec), thalamus (fig. 15), hypothalamus (figs. 15, 16, 25-36, tr.pt.hy.), and peduncle (tr. thalamo-peduncularis cruciatus and tr. thalamo-peduncularis dorsalis superficialis) . Some fibers accompany the fasciculus retroflexus to reach the area ventrolateralis pedunculi (fig. 23).


In my previous publications the tr. pretecto-hypothalamicus was not identified. Here its course is shown in figures 25-36. Many unmyelinated fibers, accompanied by a few with myelin sheaths, leave the pretectal nucleus (figs. 35, 36) and descend near the surface rostrally and internally of the marginal optic tract. Many of these fibers end in the thalamus (tr. pretecto-thalamicus, tr.pt. th.), and the remainder descend to decussate in the postoptic commissure and spread in the hypothalamus. These fibers and those here termed tr.



DIENCEPHALON 235

tecto-hypothalamicus anterior (tr.t.hy.a. of the figures) comprise the mixed system which in my previous papers was called "tr. tectothalamicus et hypothalamicus cruciatus anterior" (p. 296). The thickest myelinated fibers of this complex arise in the tectum and are clearly followed through the postoptic commissure into the hypothalamus.


The dominant afferent connections of the pretectal nucleus apparently are optic. The efferent connections to the hypothalamus and peduncle provide pathways for optic control of the intrinsic muscles of the eyeball, as in mammals, and also for possible conditioning or other influence upon the primary motor paths of the brain stem.


The pineal body appears in the premotile embryo as a low evagination from the roof of the diencephalon ('38, fig. 2). In early swimmers it is a hollow vesicle with a lumen, communicating with the third ventricle by a patent recessus pinealis ('38, figs. 3, 4, 18). Within three days after the early swimming stage this communication is closed ('38a, p. 18). The adult epiphysis is a small, flattened, simply lobulated epithelial vesicle, entirely detached from the brain except for a few fibers of the parietal nerve. No nervous elements other than these fibers have been found in it, and their connections are still unknown.


In Necturus there are more of these fibers, and their courses are more easily followed ('17, p. 236). These myelinated fibers are spread between the cells of the epithelial wall of the pineal vesicle, from which they pass into the brain in several small fascicles associated with the habenular commissure and posteriorly of it. They do not cross the mid-plane and they appear to have no functional connection with the epithalamus but to pass through it and join the f. retroflexus. They can be followed ventralward nearly to the cerebral peduncle, where they mingle with other myelinated fibers. The parietal nerve of Amblystoma has a smaller number of myelinated fibers than in Necturus (not more than 10), but their arrangement is similar ('42, p. 205). These certainly are not aberrant fibers of either the habenular commissure or the com. tecti, but their central connections and functions are uncertain. They resemble those associated with the IV nerve, which are distributed peripherally to the meninges and chorioid plexus (p. 181). It is not improbable that all these fibers have sensory functions, but for this there is no evidence. All chorioid plexuses are abundantly supplied with unmyelinated nerve fibers, probably of vasomotor function, but in the search for the sources of these fibers in Amblystoma I have been no more successful than I was with Necturus ('336, p. 15).


DOESAL THALAMUS

The dorsal thalamus is a well-defined part of the diencephalon and is intimately related by a web of connecting fibers with all contiguous parts and with many remote parts of the brain. It evidently is an important center of sensory correlation, but it is also the primary terminal station of a considerable fraction of the fibers of the optic nerve which come directly from the retina. The thalamic connection of optic fibers is not a late acquisition incidental to the differentiation of cerebral cortex, for it is present in all vertebrates, even the most primitive types. In corticated animals the larger part of the thalamus is clearly a cortical dependency, the neothalamus; but there is also a paleothalamus, which is a primordial part of the vertebrate brain, being present in all vertebrates, from the lowest to the highest. This paleothalamic component is not lost in mammals, as is shown by the surviving thalamic residue, which does not degenerate after total decortication.


The amphibian dorsal thalamus shows only vague outlines of an incipient subdivision into the local nuclei so characteristic of mammals. This is more evident in anurans than in urodeles, and in Amblystoma than in Necturus. In the urodeles, as in fishes, the undifferentiated dorsal thalamus may be regarded as a single "nucleus sensitivus," which acts, in the main, as a whole, without well-defined functional localization. Nevertheless, an early stage of local differentiation can be recognized. In Amblystoma there are three areas of gray, separated by shallow and variable ventricular sulci, namely :

1. Anteriorly the nucleus of Bellonci (fig. 2, nuc.B) produces a slight ventricular eminence wedged between the middle area, the ventral habenular nucleus, the eminentia thalami, and the ventral thalamus. Superficially of this gray is an area of very dense and complicated neuropil, which is intimately connected with that of all the surrounding areas. It receives terminals of the optic tract, and this justifies its inclusion within the sensory zone, though its other connections are those of correlating tissue of intermediate-zone type. The nucleus of Bellonci is primarily a bed-nucleus of the neighboring tracts — stria meduUaris, optic and postoptic systems, and others (chap, xviii).


2. The middle part of the dorsal thalamus is marked by a low ven


DIENCEPHALON 237

tricular eminence inclosed by the dorsal and middle thalamic sulci (fig. 2, s.d. and s.m.). It is the precursor of several of the sensory nuclei of the mammalian thalamus, though there is no visible local differentiation of its gray substance corresponding to these nuclei. This homology is clearly established by the location of these cells and their fibrous connections, which conform, as far as they are present, with the mammalian pattern (figs. 14, 15). This area receives terminals and collaterals of the optic tract, terminals of the spinal and bulbar lemniscus, strong tecto-thalamic tracts (precursors of the brachia of the superior and inferior colliculi), some fibers from the habenula, and probably also fibers from the hypothalamus by way of the postoptic commissure systems and from the cerebral hemisphere accompanying the tr. strio-pretectalis. These fibers all enter a common pool of neuropil, which is most dense in the intermediate alba but which also pervades the entire structure.


From this thalamic pool, efferent fibers go out to all surrounding parts of the brain, some in the diffuse neuropil and some as wellfasciculated tracts. An extensive and complicated system of efferents discharges into the underlying motor zone of the same and the opposite side, including direct and crossed tracts to the ventral thalamus, hypothalamus, cerebral peduncle, and isthmic and bulbar tegmentum. The more important of these are shown in figure 15. A large tract containing some thick myelinated fibers (tr.th.p.) joins similar fibers from the pretectal nucleus and tectum (fig. 18, tr.t.p.c), partially decussates in the commissure of the tuberculum posterius, and spreads through the alba of the peduncle on both sides, as illustrated in figures 30-36 {tr.th.p.c. and tr.t.p.c. 1.). Parallel with these deeper fibers there is a very large superficial uncrossed tract from the dorsal thalamus and pretectal nucleus to the peduncle (not shown on the figures) similar to that described in Necturus as tr. thalamo-peduncularis dorsalis superficiahs ('17, p. 264). Other similar fibers are dispersed in the neuropil, some of which go directly to the ventral part of the thalamus — tr. dorsoventralis thalami (shown but not labeled in figs. 15 and 16; see '17, p. 266). A small number of fibers go to the habenula (tr.th.hab.), and a larger number to the tectum in the brachia of the superior and inferior colliculus {br.coL). The large tr. thalamo-tegmentalis rectus includes deep and superficial fibers to the dorsal and isthmic tegmentum (figs. 32-34, 94, tr.th.teg.r.) ; for those which decussate in the postoptic commissure see chapter xxi and '42, p. 223.



?38 THE BRAIX OK THE TKJER SALAMANDER

The fibers wliich ascend to the heinispliere are in small, compact fascicles (tr. thalamo-froiitalis; see figs. 15, 19, oO-lU. 71. 7-2, 75. 95, 101, 1(H. 10:>, tr.fh.f.). which enter the lateral forebrain bundle. "Within this bundle the fibers pass to the strio-amygdaloid field in the ventrolateral wall of the hemisphere. Here they spread out and, so far as is now known, they end here, thus constituting a thalamostriatal system of projection fibers, which persists in mammals with the addition of the thalamo-cortical projection system. The thalamofrontal fibers arise as axons of cells of the middle part of the dorsal thalamus only, so that this "nucleus sensitivus thalami" may be regarded as the primordium of most of the mammalian thalamic nuclei which have cortical connections. This is not a new connection in the Amphibia, for it is pi-esent in various groups of fishes ('-2'-2a).


3. The posterior part of the dorsal thalamus is less differentiated and less clearly delimited. Its tissue is confluent with that of the pars intercalaris of the epithalamus. the eminence of the posterior commissure, the dorsal tegmentum, the peduncle, and the ventral thalamus. This suggests that it is functionally related with all these parts. All the connections of the middle part are more or less evident, except the thalamo-frontal tract. AYhat structures of higher brains have been derived from this area is not clear. Some of this tissue should probably be assigned to the ventral thalamus.


This posterior sector develops prtxxu^iously in company with the ventral thalamus vCoghill, "-2S, Paper VIII). Uncrossed impregnated fibers pass from it to the ventral thalamus and peduncle in the S-reaction stage — Harrison's stages 35, 36 (,'37, fig. 'i). In early swimmers these are more numerous, and similar fibers, which decussate in the post optic commissure, make their appearance (,'38, fig. 5: Coghill, '30, Paper IX, fig. -4). Differentiation of the anterior parts of the dorsal thalamus is relatively retarded.


The dense and intricate neuropil of the posterior part of the dorsal thalamus spreads to surrounding parts with no well-defineil boundaries, being intimately joined with that of the luiddle part and the ventral thalamus (fig. 73). This field (,figs. 14. 10, np.geu.) receives two systems of fibers which seem to have special phylogenetic significance, viz., abundant terminals of the optic tracts and the brachium of the inferior colliculus. The arrangement of these connections in Amblystoma is difi'erent from that seen in both Xecturus and the frog l,'-!'^, p. "278). Optic terminals are spread through the alba of the entire dorsal thalamus and most abundantly in this posterior neu


DIENCEPHALON >;{9

ropil. I have calleci tliis c'oiniiu)ii jxk)! of unditt'orentiated tissue of llie dorsal thalainiis and dorsal part of the ventral thalamus tlie "genieulate neuropil," on the assumption that out of it the medial and lateral geniculate bodies have emerged (]). '-I'-U). In the course of i)hylogeny the more sharp segregation of the optic terminals was accompanied by the differentiation of the lateral geniculate body, a process which is well advanced in the frog ('"25), and the differentiation of the cochlear apparatus and lateral lemniscus was accompanied by the emergence of the medial geniculate body from the same common pool. This hypothesis is held subject to revision, pending further study of the related species.


VENTRAL THALAMUS

On the ventricular surface (figs. 1, '2, 15>, 95) the ventral thalanuis is sharply delimited by the sulcus medius thalami (.s-.m.) above and the sulcus ventralis (s.r.) below, and it is separated into anterior and posterior parts by a depressed area containing in some si)ecimens a shallow sulcus. The two parts differ in embryological origin and connections, yet in the adult brain their most fundamental features are similar ('42, p. 207).


The posterior part of this field extends forward from the peduncle, and in early functional stages it has similar structure and connections, though it is clearly in diencephalic territory. In these stages the ])rimordium of the anterior i)art (area 7a of my analysis, ','??, p. 393) lies at the di-telencephalic junction ('38, p. 213 and fig. 18), and in prefunctional stages it is joined with area 7, which becomes the corpus striatum. This anterior part of the ventral thalamus may be genetically telencephalic, depending on how the arbitrary ditelencephalic boundary is defined.


In the adult ventral thalamus none of the nuclei of more highly differentiated brains are well defined, though some local differentiation is evident. Anteriorly, the eminentia thalami (figs. 2, 16, cni.fh.) belongs in a series of bed-nuclei related with important tracts of fibers at the di-telencephalic junction (chap, xviii). Both this area and the underlying nucleus of the olfacto-habenular tracts discharge fibers backward into the unspecialized gray of the anterior part of the ventral thalamus (fig. 17). Between the anterior part of the ventral thalamus and the dorsal (mamillary) part of the hypothalanuis there are fibers passing in both directions which seem to be precursors of the mammalian tr. mamillo-thalamicus ('396, p. 554 and figs. 22, 35).


The posterior part has a thicker gray layer, which produces a rather high ventricular eminence, the structure of which is similar to that of the peduncle. As illustrated by previously published figures ('396, p. 544 and figs. 23, 28), the large neurons of the ventral thalamus have widely spread dendrites and are evidently collectors of impulses from many sources (figs. 16, 17). Spinalward of the bednuclei at the anterior end, the two parts receive the strong tr. striothalamicus, which descends from the strio-amygdaloid field by way of the lateral forebrain bundle (f.lat.t.). Short fibers descend into both parts from the dorsal thalamus (tr. dorsoventralis thalami) and from the pretectal nucleus by tr. pretecto-thalamicus et hypothalamicus. There are much larger connections from the tectum, some uncrossed in the brachia of the superior and inferior colliculi (br.col.) and some decussating in the postoptic commissure (tr.t.th.h.c.p.).


Fibers stream backward from the ventral thalamus into the motor zone of all lower parts of the brain stem and into the hypothalamus, some uncrossed and some decussating in the postoptic commissure and in the ventral commissure of the isthmus. These have been seen especially clearly in larval brains ('39, fig. 2; '396, p. 546 and fig. 23). Most of these fibers end in the cerebral peduncle and tegmentum of the midbrain and isthmus ; many of them extend into the trigeminofacialis region; and relatively few which enter the f. longitudinalis medialis may go into the spinal cord.


The connections just described indicate that the ventral thalamus collects fibers from all parts of the cerebrum, and chiefly those concerned with somatic responses to exteroceptive stimuli. Its efferent fibers descend to those motor fields which supply the skeletal musculature. These fibers take a surprising variety of courses, the details of which need not be recounted here ('396, p. 544), but all the more important pathways are links in the chain of conductors which activate the skeletal muscles. For this reason this field is regarded as part of the motor zone, though it has no direct connections with the periphery.


This ventral thalamus corresponds in all essential respects with the subthalamus of mammalian neurology. In contrast with the mammals, it is here far larger than the dorsal thalamus, as is evident upon inspection of figure 2. This difference is probably due not to shrinkage of the ventral thalamus in mammals but to the great enlargement of the dorsal thalamus, i.e., to the addition of the neothalamus to the primordial paleothalamus as the latter is seen in Amblystoma.



DIENCEPHALON 241

HYPOTHALAMUS

The liypothalamus as here defined inchides the ventral part of the brain stem between the anterior commissure ridge and the tuberculum posterius. At its anterodorsal border the nucleus of the olfactohabenular tract might be included or assigned to the ventral thalamus. Its fibrous connections, like those of the preoptic nucleus, are mainly of hypothalamic type. Posteriorly of the chiasma ridge the deep sulcus hypothalamicus separates the dorsal from the ventral part of the hypothalamus, and each of these parts is further subdivided. A shallow and variable sulcus hypothalamicus dorsalis separates the dorsal hypothalamus again into dorsal and ventral lobes, both of which are confluent with the ventral thalamus and peduncle (fig. 2). These parts contain the primordium of the mamillary body, but this structure is not recognizably differentiated. The ventral hypothalamus is obscurely separable into posteroventral and anterodorsal parts. All these subdivisions are more clearly seen in Necturus than in Amblystoma ('35a, p. 253).


The sulcus preopticus separates the preoptic nucleus into anterior and posterior lobes. The lateral preoptic recess at the anteroventral border of the chiasma ridge is a remnant of the lumen of the hollow epithelial optic stalk of the embryo ('41), which persists in the endocranial part of the optic nerve of adult Necturus ('41a, p. 494).


The sequence of differentiation of the nervous connections of the hypothalamus has been summarized elsewhere, together with a description of these connections in midlarval stages ('396, p. 550). The adult structure and connections of the hypothalamus of Necturus have been fully described ('336, '346, '41a), and those of Amblystoma are essentially similar, though with considerable advance in differentiation throughout ('42, pp. 211 ff.). None of the hypothalamic nuclei described in mammals are here clearly segregated, though some of them are recognizable in more dispersed arrangement. The preoptic nucleus is an exception to this. It is very large, and its boundaries are well defined except anterodorsally, where it merges with the nucleus of the olfacto-habenular tract, and posterodorsally, where it merges with the ventral part of the hypothalamus.


AFFERENT CONNECTIONS

The nervus terminalis, as described by McKibben ('11), has endings distributed throughout the hypothalamus both before and behind the chiasma ridge. The only other peripheral fibers which reach the hypothalamus are a few which separate from the optic tracts and ramify in the vicinity of the chiasma (p. 221).


By far tlie larger part of the white substance of the hypothalamus is occupied by the great medial forebrain bundles (f.med.t. of the figures); and, since these bundles are composed chiefly of fibers descending from the olfactory field of the hemispheres, the implication is that olfactory functions are dominant here. The olfacto-peduncular tract arises chiefly from the head of the caudate nucleus and distributes some of its fibers to the dorsal part of the hypothalamus. Fibers descend in the dorsal fascicles of the medial forebrain bundle from the septum and primordium hippocampi and distribute chiefly to the preoptic nucleus and dorsal hypothalamus. Included here are the precommissural fornix and part of the stria terminalis system. The ventral fascicles have descending fibers from the olfactory bulb and from the ventral and medial sectors of the anterior olfactory nucleus, which spread throughout the preoptic nucleus and the ventral part of the hypothalamus. Strong collaterals of both dorsal and ventral fascicles enter the stria medullaris thalami. Many fibers descend from the preoptic nucleus in diffuse formation, to spread throughout the hypothalamus posteriorly of the chiasma ridge. One component of this preoptico-hypothalamic connection is the compact tr. preopticus, as described below.


The dorsal olfactory projection tract (ol.p.tr.) is a compact fascicle of unmyelinated fibers, which descend from the dorsal striatal nucleus and in larger number from the amygdala. This fascicle accompanies the lateral forebrain bundle, and above the chiasma it turns ventrad to connect with its nucleus at the posterior border of the chiasma ridge. This tract also contains ascending fibers. It was first described in Ambly stoma and the frog ('21a) and subsequently in Necturus ('336, p. 158) ; its course was fully illustrated in 1921 and is shown in many other figures (figs. 19, 25, 26, 27, 95, 96, 97, 101, 102, 103; '27, figs. 11-20, 24-32; '36, fig. 5).


Afferent fibers of the visceral-gustatory system enter the dorsal part of the hypothalamus (fig. 8) . The ventral part receives fibers from the pretectal nucleus by tr. pretecto-hypothalamicus (figs. 15, 16), and much larger numbers by the accompanying tr. thalamohypothalamicus, and also by tr. thalamo-hypothalamicus cruciatus (fig. 16). The entire tectum is connected with the hypothalamus by two large tracts — (1) the anterior division of tr. tecto-thalamicus et


DIENCEPHALON 243

hypothalamicus cruciatus (tr. tecto-hypothalamicus anterior) from the superior colliciilus and {%) the posterior division of this tract from both inferior and superior colhcuh (fig. l'^).


Through these manifold afferent connections nervous impulses are discharged into the hypothalamus from almost all parts of the brain, directly or indirectly. All sensory systems are represented here, but the olfactory connections evidently are preponderant. The significance of this convergence will appear after comparison with similar pools in the habenula and several other places (p. 252).


INTRINSIC HYPOTHALAMIC CONNECTIONS

All parts of the hypothalamus are interconnected by dense and intricately woven neuropil in both gray and white substance. Though all activities of the hypothalamus may thus be integrated, the structure is diversified, indicating the inception of specialization of the local nuclei as these are seen in higher animals. The most complicated and interesting of these local fields of neuropil surrounds and permeates the chiasma ridge, of which I have given a detailed description ('42, p. 214). Neuroblasts of this area are differentiated very early in embryogenesis ('37, '38), and some of their axons form tr. hypothalamo-peduncularis (figs. 18, 23) and tr. hypothalamo-tegmentalis (fig. 21), as elsewhere described ('42, p. 226). This area seems to be the primary focus into which converge all tracts of the ventral part of the hypothalamus for discharge through the two tracts just mentioned. Among these afferents are the intrinsic fibers of tr. preoptico-hypothalamicus from in front and of tr. infundibularis ascendens from the postero ventral lobe.


The last-mentioned ascending tract divides. One moiety is directed dorsally, providing a broad connection from the ventral part of the hypothalamus to the dorsal, and a larger moiety is directed forward into the medial forebrain bundle. Most of these ascending fibers apparently end in the postchiasmatic neuropil and preoptic nucleus, but some may go farther into the hemisphere ('336, p. 250; '346). This tract is quite independent of the ascending fibers of the olfactory projection tract previously mentioned. There are probably other fibers which ascend from the hypothalamus in the basal forebrain bundles, but our preparations have not revealed them.


From the anterior part of the ventral thalamus, fibers stream backward into the dorsal part of the hypothalamus. These thalamomamillary fibers are accompanied by fibers passing in the reverse direction — tr. mamillo-thalamicus — the combined tract being the probable precursor of the mammalian mamillo-thalamic bundle of Vicq d'Azyr.


EFFERENT CONNECTIONS

From the whole extent of the preoptic nucleus fibers pass dorsad to enter the stria medullaris thalami. Others probably ascend to the hemisphere in the basal bundles. The dorsal (mamillary) part of the hypothalamus is connected with the anterior part of the thalamus, as just mentioned. There is probably also a mamillo-cerebellar connection (p. 170). The other efferents from the mamillary region are the very extensive and complicated systems of mamillo-tegmental, peduncular, and interpeduncular fibers illustrated crudely in figures 18 and 21 and mentioned on page 278, where some references to literature are given (for fuller description see '396, p. 551).


The efferents from the ventral part of the hypothalamus include the ascending fibers and the hypothalamo-peduncular and tegmental systems already mentioned. Fibers may go out with some components of the postoptic commissure to the thalamus and tectum, but these have not been recognized.


The pars magnocellularis of the preoptic nucleus gives rise to the large hypophysial tract, which is one of the major features of the ventral hypothalamus. In most lower vertebrates, including some amphibians, these large cells are aggregated as a well-defined nucleus, homologous with the supraoptic nucleus of mammals. But in Amblystoma, as in Necturus, they are dispersed, being most numerous above and anteriorly of the chiasma ridge. These widely scattered large cells in the aggregate are here termed the nucleus magnocellularis. Their long dendrites are widely branched and may be activated by practically all nerve fibers of the preoptic and epichiasmatic areas. In this mixed collection of fibers there are two systems which seem to be specifically related with these large cells.


The first of these systems is the tr. preopticus (fig. 2C, 96, 97, tr.po.). These fibers are axons of cells at the postero ventral border of the anterior commissure ridge and the anterior part of the preoptic nucleus, which descend in the thin floor of the long preoptic recess, then recurve dorsally along the anterior face of the chiasma ridge, where they spread in the alba among dendrites of the cells of the nucleus magnocellularis. In the catfish, Ameiurus, the course is simi


DIENCEPHALON 245

lar, and here the connection of these fibers specifically with the compact nucleus magnocellularis is unmistakable ('41&). This, accordingly, is one source of activation of the hypophysial tract, the course of which in Ameiurus has been described by Palay ('45). His observations also indicate that in these fishes the nucleus magnocellularis itself is a source of endocrine secretion, as is clear also from the work of Scharrer and Scharrer ('40).


A second specific source of excitation of the nucleus magnocellularis is the small number of fibers which separate from the optic tracts near the chiasma and arborize in the area where these cells are most abundant (p. 221), thus providing visual control of endocrine secretion. This, however, is minimal in Amblystoma, and the very extensive hypophysial innervation must be concerned in the main with other functions.


The very numerous unmyelinated axons of the cells of the nucleus magnocellularis are diflficult to follow, for they descend in dispersed arrangement, penetrate the chiasma ridge, and then converge into the compact tr. hypophysius in the thin floor of the infundibulum. Their further course is clear. We have many brilliant Golgi impregnations of their distribution, some of which have been illustrated ('42, figs. 55-65). In the pars nervosa of the hypophysis they form a very dense neuropil, are less abundant in the pars intermedia, and a few of them penetrate the pars distalis. They terminate in tiny end-bulbs resting upon the epithelial cells. The pars nervosa is a rather thin sheet of convoluted epithelium which forms the posterodorsal wall of the wide infundibulum. These lobules take the form of irregular villi, in the axis of each of which is a capillary loop.


CONCLUSION

In most lower vertebrates the hypothalamus is relatively much larger than in the higher groups, and this seems to be correlated with the dominance of the olfactory system in the organization of the forebrain of the lower forms. All other functional systems have extensive representation here, and in this respect the hypothalamus resembles the habenular complex, but with the difference that in the latter the afferents converge into a very small compact area with one major efferent path, the f . retroflexus, while the much larger hypothalamus is diversified in structure and has a wider range of distribution of its efferent fibers. Though both these complexes are evidently concerned


24(5 THE BRAIN OP^ THE TIGER SALAMANDER

with the correlation of olfactory with a wide variety of other types of sensory experience, it is equally evident that the type of adjustment made is radically different. To this topic we shall return (p. 252).


The intricate connections described above are very different from those of the specialized fishes. How they are related with those of mammals remains uncertain, pending further study of intermediate species. The lack of differentiation of a localized mamillary body here is probably explained by the failure of the fibers of the primordial postcommissural fornix to reach the hypothalamus (p. %55) ; and this, in turn, is correlated with the primitive structure of the hippocampal formation. The analysis of the intricate system of postoptic commissures (chap, xxi) probably provides an instructive point of departure for further study of these connections, leading up to a solution of still controversial problems about the mammalian supraoptic commissures.


The nervous connections of the hypophysis in urodeles are very large, and they are so arranged as to be easily accessible for experimental study. It is hoped that advantage will soon be taken of this favorable material for investigation of some problems of endocrinology.