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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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.
