Book - The brain of the tiger salamander 13

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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" (textbooks, papers, people, recommendations) 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, interpretations and recommendations 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 XIII Isthmus

THE strategic position of the isthmus as a transitional sector of the brain stem between rhombic brain and cerebrum has been commented upon (pp. 45, 118). In early neural-tube stages of the developing human brain it is a rather large sector, but in the adult it is hardly recognizable as a distinct entity because its tissues are dispersed among other structures of relatively recent phylogenetic origin. In contrast, the isthmus of adult Amblystoma retains its embryonic separateness and, indeed, accentuates it. Here its distinctive features are most clearly seen, and these will be described as fully as available material permits. Its dorsal part is contracted and includes only the anterior medullary velum and some adjoining structures. To the intermediate zone is assigned the superior visceral-gustatory nucleus, and for convenience of description the remainder is included in the motor zone, though its functions are, in large part, of intermediate type. The interpeduncular nucleus is an isthmic structure, to which a separate chapter is devoted.


The configuration of this sector of the brain exhibits an interesting series of changes in the course of development, due to the rapid and radical dislocations caused by the cerebral flexures and to other inequalities of intrinsic growth. Successive modifications of the isthmic sulcus provide a useful indicator of the course of these changes.

From the coil stage (Harrison's stage 35) to the adult, the isthmic tegmentum is intimately joined with the trigeminal tegmentum, with no constant external or ventricular boundary visible. "Isthmus rhombencephali," therefore, is an appropriate name. But its anterior boundary is sharply marked by the fovea isthmi, the deep ventricular sulcus isthmi, and the external fissura isthmi. Even in early stages these grooves are not exactly parallel, and as development advances they deviate from this relationship more and more widely because of unequal growth of the deep and superficial structures.

Reference was made on page 177 to von Kupffer's sulcus intraencephalicus posterior as the precursor of the sulcus isthmi. In coil ('37, fig. 1), S-reaction ('37, figs. 7, 8), and early swimming stages ('37, fig. 2) the external fissura isthmi lies a little farther spinalward than the internal sulcus. In slightly older early swimmers ('38, figs. 15-18) both the isthmic fissure and the isthmic sulcus are deep, and the sulcus is displaced farther rostrally of the fissure. This means that the gray of the isthmus is proliferating rapidly and pushing the isthmic sulcus farther forward. The sulcus is now a very deep cleft and lies approximately transversely to the long axis of the rhombencephalon. From this stage the enlargement of the isthmic tegmentum continues on the ventricular side, and in the early feeding stage (Harrison's stage 46) the isthmic sulcus extends from the cerebellum almost horizontally forward before dipping downward to the fovea isthmi ('386, figs. 1, 2). The sulcus and the fissure are still deep, and the sulcus has come to lie far rostrad of the fissure. This is not evident in transverse sections but is shown clearly in horizontal sections of this and subsequent stages ('396, fig. 16). In later larval stages, internal differentiation, particularly the elaboration of the neuropil and thickening of the auricle, result in smoother contours both internally and externally, and the isthmic sulcus and fissure are shallower in the adult.

At the close of the early swimming stage (about Harrison's stage 38) the cerebellum and auricle are so little developed that the plica rhombo-mesencephalica is marked externally by a wide depression ('38, fig. 17). Between this and the early feeding stage (Harrison's stage 46) the sharp cerebral flexures are straightened, resulting in a backward thrust of the plica rhombo-mesencephalica in the dorsal wall, while the ventral wall of the isthmus remains fixed (compare '38, fig. 18, with '386, fig. 1). Though there has not yet been any very great enlargement of the isthmic tegmentum, the isthmic sulcus in early feeders lies more nearly horizontally than vertically. In midlarval stages the deep sulcus isthmi (shown but riot named in fig. 2 of '14a) is inclined more than 45° from the vertical, and between midlarval and adult stages there is enormous enlargement of the gray of the isthmic tegmentum, which thrusts the sulcus isthmi forward and upward toward the horizontal position.

The external fissura isthmi does not follow the sulcus isthmi in these shifts of position, but, as mentioned above, it lies in the adult a considerable distance spinalward of the sulcus. That this involves a mechanical stretching of the wall between the fissure and the sulcus in later stages is evident from the arrangement of the ependymal elements in the lips of the isthmic sulcus. This traction begins early. It was noticed in early swimmers ('38, p. 220) that the ependymal elements of the sulcus isthmi are thickened and sharply bent outward and backward. This was illustrated also in early feeding stages ('386, figs. 11, 12) and in Golgi impregnations of midlarval stages ('396, p. 518, and figs. 31, 93). At all stages, including the adult, these thick elements form a dense band of parallel fibers bordering the sulcus on both sides and extending outward and backward to an attachment at the pial surface in the floor of the isthmic fissure. In the adult the sulcus isthmi usually does not extend ventrally so far as the fovea isthmi, but its locus here is indicated by a band of modified ependyma. Two mechanical factors, accordingly, can be recognized in the shifting relations of the isthmic fissure and sulcus : first, the backward thrust of the roof during the straightening of the cerebral flexures between early swimming and early feeding stages and, second, unequal growth of superficial and deep structures, especially in later stages. The rapid enlargement of the gray of the isthmic tegmentum in late larval stages produces an upward and forward thrust ventrally in the direction opposite to the dorsal backward thrust of early stages.

The developmental history and adult position of the sulcus isthmi of Amblystoma are radically different from those of Hynobius as described by Sumi ('26), who illustrates models showing the ventricular sculpturing from closure of the neural tube to the adult. At none of these stages is von Kupffer's sulcus intraencephalicus posterior visible at the plica rhombo-mesencephalica. The sulcus isthmi appears rather late at the fovea isthmi, and it does not at any stage reach the dorsal surface.


The sensory field of the isthmus is contracted. Its scanty gray substance includes a compact cluster of cells of the mesencephalic V nucleus within and adjacent to the anterior medullary velum. These cells are smaller and more densely crowded than are those of this nucleus in the tectum, and they may have dift'erent functional significance (p. 140). Their connections are unknown. Associated with them are other cells at the posterior border of the inferior colliculus which appear to have peripheral connections. From this region a few myelinated, and many unmyelinated, fibers go out to the meninges and chorioid plexus of the fourth ventricle and some with the IV nerve.

These fibers appear to be related with cells bordering the velum, but no clear evidence of their connections has been found ('36, p. 343; '42, pp. 255, 291). Within and adjoining the anterior medullary velum are fibers of passage of several kinds, including the decussation of the IV nerves, mesencephalic root of the V nerve, and tractus tecto-cerebellaris.


The intermediate zone as defined here is represented by the small gray area of the superior secondary visceral-gustatory nucleus and associated neuropil. This gray lies dorsally of the isthmic tegmentum and incompletely separable from it. In figures 2 and 13 it is shown wedged between the isthmic tegmentum below and the nucleus cerebelli and dorsal tegmentum above. The cells of the superior visceral nucleus are of medium size, scattered and clumped in an open neuropil, with a few outlying cells in the alba. Their dendrites are directed ventrolaterally through the alba into the posterior isthmic neuropil (described below), where they engage terminals of the ascending visceral tract ('42, pp. 244, 253, and fig. 43) and of fibers from many other sources.

There is an obscure vestige here of the nucleus isthmi, which is well differentiated in the frog ('42, pp. 244, 253). In anurans and reptiles this nucleus is in intimate relations with the lateral lemniscus, and it is probably part of the auditory reflex apparatus, with, perhaps, vestibular connections also. In Ambly stoma the auditory system is poorly developed, and the crawling habit maintains the balance of the body without nervous control; the nucleus isthmi, accordingly, is undeveloped.


This field is so intimately related anatomically and physiologically with the tegmentum of the trigemino-facialis region that these will be described together. The tegmentum isthmi is bounded anteriorly by the isthmic sulcus and fovea isthmi, separating it from the cerebral peduncle and dorsal tegmentum. Posteriorly it is bounded by the nucleus cerebelli and the trigeminal tegmentum. Its gray substance includes the nucleus of the IV cranial nerve (as described in chap, x) ; a compact central nucleus of the isthmus (fig. 29, ; and, surrounding this, a pars magnocellularis which is continuous spinal ward with the large-celled component of the trigeminal tegmentum (fig. 30).


The tegmentum of the trigeminal field forms a low eminence in the floor of the fourth ventricle internally of the V roots, the erainentia trigemini (figs. 2C, 90), which is evident in Necturus also ('30, figs. 11, 12, em.V). The gray under this eminence comprises a small-celled dorsal and medial part and a large-celled ventral part (fig. 27) . The small-celled part is continuous medially with a band of similar cells, which extends spinalward under the ependyma of the paramedian sulcus (fig. 28), and anterodorsally it is continuous with the ventral border of the central nucleus of the isthmic tegmentum and more intimately with the deep periventricular gray of the isthmus (fig. 29). The large-celled part is continuous anterodorsally with the largecelled component of the isthmic tegmentum (figs. 28, 29, 30). Imbedded among these large tegmental cells are the motor V nuclei. There are two motor V roots (fig. 27), but the two motor V nuclei seen in Necturus ('30, p. 13) are here merged. Figure 40 shows three of these cells in Golgi impregnation. The heavily myelinated fibers of the mesencephalic V root are more dorsal, passing upward toward the tectum at the outer border of the gray (figs. 13, 29-32).

The configuration at the boundary between trigeminal and isthmic tegmentum is variable, and the position of this boundary is debatable. A posterodorsal extension of the isthmic tegmentum is continuous with a low ventricular eminence, the superior visceral nucleus. The latter is separated by a shallow and variable sulcus from the nucleus cerebelli posteriorly and a tegmental area ventrally. Comparison of the two key drawings, figures 2B and 2C, reveals differences due partly to individual variations in relative sizes of parts and partly to lack of any well-defined boundary between the large-celled components of the isthmic and trigeminal tegmentum. In figure 2B the dorsal part of the area marked contains only large cells (fig. 31), and these may be assigned to either the isthmic or the trigeminal tegmentum. More ventrally (figs. 29, 30), both large and small cells occupy a similar ambiguous position.

The large cells of the trigeminal tegmentum take various forms, one of which is shown in figure 62 (others are illustrated in '14a, figs. 21-26). Like those of the nucleus motorius tegmenti elsewhere, they are concerned with motor co-ordination in patterns as yet imperfectly known. Some of these cells in the auricle have dendrites directed forward into the posterior isthmic neuropil, where they engage terminals of the tegmental fascicles (fig. 46). Other similar neurons direct their dendrites laterally among terminals of the ascending sensory V root and related tracts ('396, figs. 47-49, 53, m, 68). The axons of some of these elements are directed forward, as shown in figure 43. These short fibers pass from the trigeminal tegmentum to arborize within the gray of the isthmic tegmentum and are doubtless part of the apparatus of intrinsic co-ordination of patterns of action within the motor zone.

Some of the small cells at the anterior end of the trigeminal tegmentum are shown in figures 66 and 91. These resemble those of the adjacent interpeduncular nucleus. Their dendrites extend laterally through the entire thickness of the alba of the ventral part of the auricle, where they are in contact with all kinds of fibers passing this region. The axons of some of them descend to the interpeduncular neuropil (figs. 63, 80, 84; compare similar cells of the isthmic tegmentum, figs. 60, 84).


Horizontal sections are most instructive in the analysis of this region (figs. 29-33). The nervous elements are arranged in five groups: (1) A narrow and inconstant layer of subependymal cells. (2) These are separated by a thick sheet of neuropil from the lens-shaped area of small and medium cells, here termed the "central nucleus of the isthmus." (3) Externally and spinalward of this area is the pars magnocellularis, containing scattered nerve cells of medium or large size. The extensive neuropil within which these cells are imbedded is broadly continuous with that of the surrounding alba. This largecelled area envelops the lentiform area of small cells laterally, ventrally, and caudally and is continuous spinalward with a similar field of the trigeminal tegmentum. In figure 2B the dotted line separating the pars magnocellularis ( is arbitrarily drawn to indicate a posterior sector which may be assigned to either isthmic or trigeminal tegmentum. (4) Dorsally of all the preceding, the gray of the isthmic sector extends upward into contact with that of the dorsal tegmentum and ventral cerebellar nucleus. This area is the superior secondary visceral-gustatory nucleus (fig. 2B, iiuc.vis.s.). (5) A probable vestige of the nucleus isthmi.

The first component of the preceding list is the ventral part of the zone of deep subependymal gray, which everywhere surrounds the aqueduct. In the isthmus it is thin and contains few nerve cells, but more posteriorly in the trigeminal tegmentum it expands to a wide zone of subependymal small cells (figs. 29, 30). This and the associated neuropil comprise one of the most primitive and conservative features of this part of the brain and also of the diencephalon and the mesencephalon. In mammals it contributes fibers to the dorsal longitudinal fasciculus of Schiitz, as described in the opossum by Thompson ('4'-2).

The second comi)onent, the central nucleus, is clearly delimited in its middle part. Anteriorly, it merges with the small cells of the posterior gray of the peduncle (figs. 29, 30, 31) and more dorsally with the ventral border of the dorsal tegmentum (fig. 32). Posteriorly, it is continuous with the small-celled component of the trigeminal tegmentum (fig. 91), though the boundary between these is clearly marked in many preparations. At its ventral border the gray is continuous with that of the interpeduncular nucleus, with difference in form and arrangement of the cells, though many transitional forms are seen ('42, fig. 43). The boundaries here mentioned are in some preparations quite indeterminate; in some reduced silver preparations they are evident; and in Golgi preparations each of the regions mentioned has characteristic structure and connections. All these areas are intimately connected by a web of interstitial neuropil, the structure of which is characteristically different in each of them. All this gray is connected with the interpeduncular nucleus by fibers passing in both directions, and in chapter xiv, devoted to that nucleus, it is suggested that the central nucleus of the isthmus contains, among other constituents, the primordium of the dorsal tegmental nucleus of mammals.

The large-celled component 3 surrounds the central nucleus on all sides, and many of these larger neurons lie deeper, mingled with those of the small-celled component. These cells have massive dendrites, which spread widely in the overlying alba. Among these are cells of the nucleus of the nervus trochlearis (fig. 70). Figure 61 shows three neurons from this region which probably belong to the IV nucleus. They are surrounded by other similar elements of the tegmentum. Most of the axons fi-om the large tegmental neurons descend, crossed or uncrossed, in the tegmento-bulbar tracts, as described in the larva ('396, p. 590). Some axons from the isthmic tegmentum, as also from the trigeminal tegmentum, are directed forward. So far as observed, these take short courses, though some may extend farther (figs. 43, 46). This may be a precursor of the connection from the substantia nigra to the corpus striatum (Kodama, '29) which has recently been confirmed by several students of nianinialian neurology. Component 4, I lie superior visceral nucleus, has been described above (p. 18'-2).


In the white substance of the isthmus the most conspicuous features are the numbered tegmental fascicles described in chapter xx. Externally of these fascicles are the massive teclo-bulbar and tectospinal tracts and, suj)erficially close to the pial surface, a layer of line fibers, most of which are unmyelinated. The latter includes tr. thalamo-tegmentalis rectus from dorsal and ventral thalamus (fig. 94; '36, }). .'UO) and, more dorsally, similar fibers from the dorsal thalamus, which decussate in the postoptic commissure — tr. thalamo-tegmentalis dorsalis cruciatus (p. '-29!); '39, pp. 95, 116).

Mingled with these fibers and more abundantly at deeper levels are fibers of tr. tecto-peduncularis and tecto-tegmentalis (figs. 18, 22, 24; '42, p. 267 and figs. 14, 30; Necturus, '17, figs. 9-14, 32, 33, tr.t.ped.p.). Short fibers from the entire tectum and dorsal tegmentum to the isthmic tegmentum are dispersed in both white and gray substance, and a large number of the longer fibers are assembled in dorsal tegmental fascicles of group (7) accompanying tr. tecto-bulbaris rectus (p. 283).

Figure 21 shows only the more prominent att'erent connections; there are many others. In the aggregate these include fibers from the motor zone above and below the isthmus, collaterals of the lemniscus sj^stems in the isthmic neuropil, terminals of the ascending visceralgustatory tract, cerebello-tegmental fibers, short fibers from the overlying dorsal tegmentum and tectum, longer fibers from the tectum which decussate in the postoptic and ventral commissures, direct fibers from the dorsal and ventral thalamus, strong tracts from the dorsal and ventral thalamus which decussate in the postoptic commissure, fibers from both cerebral hemispheres by way of the lateral forebrain bundle and tr. olfacto-peduncularis, and strong tracts from the hypothalamus, crossed and uncrossed. To this list one might add the fasciculus retroflexus, passing from the habenula to the interpeduncular nucleus and, through the latter, acting upon the isthmic tegmentum.

Most of the efferent fibers descend in the f. tegmentalis profundus (p. 286) and the ventral tegmental fascicles. Some are dispersed in the neuropil accompanying tr. interpedunculo-bulbaris dorsalis, and these are regarded as jn-ecursors of the bulbar part of the f. longitudinalis dorsalis of Schlitz (p. 208). The relativelj^ short fibers which descend from the hir^e cells of theisthmic<ind trigeminal tegmentum to the motor zone of the medulla oblongata are especially clearly seen in very young larvae from early swimming to early feeding stages and are interpreted as provision for activation of the mandibular and hyoid musculature involved in feeding. Longer fibers descend into the spinal cord; but these, so far as is sliown in our material, are not numerous.


Both the gray and the white substance of the isthmus are permeated with dense neuropil, the axonic component of which is composed mainly of terminals of afferent fibers and collaterals of fibers of passage. The superficial neuropil is composed chiefiy of branched terminals of the unmyelinated thalamo-tegmental and tecto-tegmental tracts and dendritic terminals from the underlying gray. The intermediate neuropil is in the form of sheets or plaques insinuated between the tegmental fascicles and composed largely of terminals and collaterals of fascicular fibers. The deep neuropil of the alba and the grisea is a c(mfused entanglement of fibers and dendrites, the analysis of which is quite impossible except where clarified l^y elective Golgi impregnations. These preparations reveal a num})er of more or less well-defined tracts imbedded within this interstitial matrix. In the gray substance the cells are arranged in lamellae separated by sheets of neuropil, which is continuous with that of all neighboring parts. A large proportion of these axons trend dorsoventrally from tectum and dorsal tegmentum to isthmic tegmentum and interpeduncular neuropil and posteroventrally, with or without decussation in the ventral commissure. The latter axons accompany thicker fibers of the tr. tegmento-bulbaris arising from large cells of the motor tegmentum.


The name "posterior isthmic neuropil" is given to a wide zone of neuropil in the alba at the boundary between the isthmus and the bulbar and cerebellar structures. It invades the auricle below and the nucleus posterior tecti above. It is permeated by dendrites from all surrounding parts, including the trigeminal tegmentum (fig. 46; 'S9b, figs. 47-49, 62, 6P>), auricle ('396, fig. 53), cerebellum (fig. 47), superior visceral nucleus ('42, fig. 43), and dorsal and isthmic tegmentum. It receives terminals of peripheral sensory fibers of the V, VH, VIII, and lateral-line nerves, of a bulbo-isthmic tract (figs. 38, 39), of the secondary visceral-gustatory tract; terminals and collaterals of the lemniscus systems; vast numbers of terminals of descending fibers of the tegmental fascicles; and collaterals of tr. tecto-bulbaris rectus. This evidently is a strategic region in the organization of both ascending and descending systems of conduction. Within this undifferentiated field the primordia of a considerable number of specific nuclei of other animals can be recognized, including specialized structures peculiar to fishes and a number of mammalian nuclei. Among these components (with their probable derivatives in more specialized species) the following can be identified:

1. The chief sensory nucleus of the trigeminus. Some ascending sensory V root fibers terminate here, though most of them more posteriorly, and some pass through to reach the commissura cerebelli and the corpus cerebelli of the same and of the opposite side. Secondary fibers go from this area upward into the cerebellum and forward with or without decussation as the primordial trigeminal lemniscus.

2. Similarly, some of the ascending root fibers of the VIII and lateral-line nerves terminate more laterally in the posterior isthmic neuropil (superior vestibular nucleus), and some of the VIII fibers reach the lateral (auricular) part of the cerebellum of the same and of the opposite side, the latter decussating in the com. vestibulo-lateralis cerebelli.

3. The longitudinal bulbar correlation tracts a and b reach the ventral part of the same neuropil. This field is incorporated within the cerebellum of mammals.

4. Spinal lemniscus fibers, as they recurve dorsal ward along the anterior face of the auricle to reach the tectum, are spread through the middle part of this neuropil, with collaterals and branched terminals within it (figs. 42, 43, 44).

5. Associated with the spinal and bulbar lemniscus and laterally of them in the isthmus are fibers of the lateral bulbo-tectal tract (fig. 11, tr.b.t.L), which is the probable primordium of the lateral lemniscus. These ascending fibers are mingled with descending fibers of tr. tecto-bulbaris posterior (fig. 12, tr.t.b.p.; '396, p. 606 and fig. 96). The position of the mixed fascicle in horizontal sections is shown in figures 31-34, here marked tr.t.b.p. Terminals and collaterals of the ascending lateral bulbo-tectal fibers spread throughout the posterior isthmic neuropil (figs. 33, 34), and these endings are comparable with those of the lateral lemniscus in the nucleus isthmi of the frog (Larsell, '24). This field also contains precursors of some of the nuclei of the lateral lemniscus of mammals, specifically the dorsal nucleus of the lateral lemniscus (Ariens Kappers, '29, p. 304; Clark, '33).

6. The nucleus isthmi, which is large in the frog (Larsell, '24), is probably represented in Amblystoma by a few outlying cells in the dorsal alba of the isthmus, which are more numerous in the larva ('42, pp. 244, 254). If present here, it is an insignificant vestigial structure.

7. The neuropil associated with the superior visceral-gustatory nucleus (fig. 33, nucris.n.) is an important field of sensory correlation. It is penetrated by dendrites of its nucleus ('42, fig. 43) and by terminals of the ascending secondary visceral tract ('25, fig. 19) and also by fibers from several other sources, including the lemnisci and collaterals of tr. tecto-bulbaris rectus (fig. 37; '42, fig. 67). This complex is very large in fishes (the Uebergangsganglion of Mayser) and is present, though small, in the frog (Larsell, '24).

8. The neighboring large cells of the motor tegmentum are undoubted precursors of several mammalian nuclei of this region, including the substantia nigra and the locus coeruleus.


The afferent fibers which terminate in the isthmus come, directly or indirectly, from practically all parts of the brain, and they carry nervous impulses activated from all the major functional systems. The efferent discharge is also widely spread, but the chief distribution seems to be to upper levels of the medulla oblongata which are concerned with movements of the musculature of the head.

It is obvious that the isthmus contains a motor adjustor of prime importance. Its distinctive features can perhaps best be appreciated by comparing it with some of the other important centers of adjustment. The tectum of the midbrain and the dorsal thalamus, which in this animal is ancillary to it, comprise the dominant apparatus of correlation, on the sensory side, of all exteroceptive and proprioceptive systems. In the habenula the olfactory organ as an exteroceptor is tied in with the other exteroceptive systems. In the hypothalamus the olfactory system is similarly related with the visceral and gustatory systems. All fields of sensory correlation discharge into the corpus striatum, ventral thalamus, and peduncle, where they are integrated and co-ordinated primarily in the interest of mass movements of the skeletal musculature, such as those involved in locomotion. Efferent fibers from all the centers just enumerated and from others converge into a common pool in the isthmic tegmentum. Since apparatus adequate for executing the primary mass movements of the trunk and limbs is found elsewhere — primarily in the midbrain.

acting through the ventral tegmental fascicles upon the lower bulbar and spinal apparatus — and, since the isthmic tegmentum matures later in development, with notable enlargement preceding metamorphosis, it seems safe to infer that the isthmic apparatus is the chief regulator of the musculature concerned with feeding. The courses of the efferent fibers from the isthmus support this supposition. In Ambly stoma the feeding activities are thoroughly integrated movements, where posture of the body, conjugate movements of the eyes, and action of jaws, pharynx, and esophagus are organized as a "total pattern," as Coghill ('36) has demonstrated. For further consideration of the olfactory and hypothalamic components of feeding reactions see pages 210 and 252 and 19346, page 384.

It is evident that control of feeding reactions is not the only function of the isthmic sector. For instance, it has been shown by Aronson and Noble ('45) that the removal of the entire brain of the male frog in front of this region (including the tectum, cerebellum, and anterior part of the tegmentum) did not interfere with spawning movements but that "lesions in the tegmentum at the level of the motor nucleus of the trochlear nerve markedly disturbed or completely abolished these spawning responses."

In addition to this apparatus of activation of behavior, there are inhibitory functions here also, implying a participation in motor control of wide import. The intricate connections of the isthmic tegmentum described in the next chapter are part of this inhibitory apparatus.

The unitary character of most of the activities of Amblystoma, involving the synergic action of large masses of musculature in invariable orderly sequence, with relatively less capacity for the local autonomous action of the individual members than in higher animals, explains the simplicity and relative homogeneity of the nervous apparatus involved — also why the isthmic tegmentum is so large in Amblystoma. The integrated functional complexes which here are controlled from this major pool are individually differentiated in man, with corresponding specialization and segregation of the apparatus of the several component parts; and, parallel with this differentiation and increase of local autonomy, the central control apparatus has been transferred from the stem to the cerebral cortex. Accordingly, the isthmic tegmentum, which bulks so large in the brain of Amblystoma (and in the early human embryo), is dismembered in the adult man, and its parts are submerged within surrounding, structures.

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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


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

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