Book - The brain of the tiger salamander 4

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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 IV Regional Analysis

Since the brain of Amblystoma presents a generalized structure which is probably close to the ancestral type from which all more highly specialized vertebrate brains have been derived, the salient features of internal organization are here summarized in schematic outline. Tlie accompanying diagrammatic figures 1-24 give the necessary topographic orientation, and the details may be filled in by reference to the corresponding sections of Part 11. What is here described may be regarded as the basic organization of the brains of all vertebrates above fishes, that is, the point of departure from which various specialized derivatives have been differentiated. Amblystoma possesses the equipment of sensory and motor organs typical for vertebrates at a rather low level of specialization and in evenly balanced relations. All the usual systems are present, and none shows unusual size or aberrant features. The great lateral-line system of sense organs so characteristic of fishes is preserved, though somewhat reduced after metamorphosis. On the motor side the organs of locomotion and respiration have advanced from the fishlike to the quadrupedal form, but in very simple patterns. In early phylogeny the specialization of the motor systems seems to lag behind that of the sensory systems because the aquatic environment of primitive forms is more homogeneous than that of terrestrial animals, and, accordingly, fewer and simpler patterns of behavior are needed.

Our search in this inquiry is for origins of human structures and for an outline of the history of their evolution. From this standpoint it is evident that in the central nervous systems of all vertebrates there is a fundamental and primary difference between the cerebrum above and the rhombencephalon (and spinal cord) below a transverse plane at the posterior border of the midbrain (for further discussion of this see chaps, viii and xiii).

The spinal cord and rhombic brain contain the central adjustors of the basic vital functions — respiration, nutrition, circulation, reproduction, locomotion, among others. This apparatus is elaborately organized in the most primitive living vertebrates, as also no doubt it must have been in their extinct ancestors. The cerebrum, on the other hand, except for the olfactory component, is a hiter acquisition. This is suggested by what is seen in Amphioxus and by the retai-ded development of the cerebrum in all vertebrate embryos, as illustrated especially clearly in the early fetal development of the opossum.

At an early (and unknown) period of vertebrate ancestry a pair of eyes was differentiated. These and the olfactory organs are the leading distance receptors, and as such they gave to the vertebrate ancestors more information about their surroundings and hence greater safety in moving about freely. The nose and eyes, with the associated oculomotor apparatus, early assumed the dominant role in the recognition of food, mates, and enemies, and their cerebral adjustors were enlarged accordingly. The contact receptors are adequate for sedentary, crawling, or burrowing ancestors, and here the response to stimulation follows immediately. But, as Sherrington long ago pointed out, in a free-swimming animal there is a time lag between reception of the stimulus from a distant object and the consummation of the response. The pregnant interval between the anticipatory and consummatory phases of the reaction gives the clue to an understanding of the entire history of forebrain evolution. During this interval there is a central resolution of forces, which eventuates in appropriate behavior; and, with increasing complication of patterns of behavior, this central apparatus of adjustment assumes more and more structural complexity and physiological dominance over the entire bodily economy (chap. vii).

The details of these internal connections are not relevant here. It suffices to present two summaries, one in this chapter in topographic arrangement by regions from spinal cord to olfactory bulbs as conventionally described and one in the next chapter on a different plan, i.e., an arrangement in longitudinal zones which are functionally defined. For the present purpose it is convenient to recognize seventeen subdivisions of the central nervous system, each of which is characterized by special physiological activities, though these activities are not localized here exclusively. This subdivision might be carried further into detail indefinitely. Numbers 2-6 in the following paragraphs are in the rhombic brain; the others are in the cerebrum.

The Subdivisions, Spinal Cord to Pallium

1. The Spinal Cord

The spinal cord is not described in this report except for some features closely related to the brain, to which reference is made in the next paragraph. The cord segments are organized for the regulation of local reflexes of the limbs and the integration of these reflexes with one another and with the action of the trunk musculature, as in ordinary locomotion.

2. The Bulbo-Spinal Junction

The sector of the bulbo-spinal junction includes the upper segments of the spinal cord and the lower part of the medulla oblongata. It is the first center of correlation to become functional in embryonic development (Coghill, '14, Paper I). Its dorsal part around the calamus scriptorius receives fibers from the entire sensory zone of the bulb and cord, so that this gray of the funicular and commissural nuclei is a general clearing-house for all exteroceptive, proprioceptive, and visceral functions of the body except vision and olfaction. Here these functions are integrated in the interest of control of posture, locomotion, visceral activity, and other basic components of mass-movement type. Some of these connections are shown diagrammatically in figures 3, 7, 8, 87; for details and discussion see chapter ix and a recent paper ('446).

Efferent fibers from the dorsal nuclei are directed spinalward and forward. Most of the latter connect with motor nuclei of the medulla oblongata; some go farther forward to the cerebellum, tectum, and thalamus; and there is a strong, ascending visceral-gustatory tract to the isthmus and peduncle. The motor zone of this sector is occupied chiefly by fibers of passage. The moderately developed gray substance includes motor neurons for muscles of the neck region, for the tongue muscles, and for special visceral motor elements of the accessorius component of the vagus.

3. Medulla Oblongata

The medulla oblongata, or bulb, includes all the stem between the isthmus and the calamus scriptorius except the cerebellum, there being no pons. Its dorsal field receives all sensory fibers from the head except the optic and olfactory, fibers from lateral-line organs widely distributed over the body, and general visceral sensory fibers chiefly by way of the vagus. The general visceral sensory and gustatory root fibers are segregated from the others in the fasciculus solitarius; and this group has its own system of secondary fibers, which converge into the visceral motor nuclei of the medulla oblongata and spinal cord. There is also a strong ascending secondary visceral tract {tr.v.a.) to the isthmus, peduncle, and hypothalamus, through which all cerebral activities may be influenced by visceral and gustatory functions (fig, 8 and chaps, x, xi).

The other afferent fibers of the V to X cranial nerves, upon entrance into the brain, are fascicuhitetl according to the functional systems represented, as outlined in the next chapter and shown in figures 7, 9, 88, 89, 90. The general cutaneous, lateral-line, and vestibular fibers are arranged in a series of fascicles bordering the external surface; the visceral sensory and gustatory fibers are assembled in a single deeper bundle, the fasciculus solitarius. The marginal fascicles of root fibers are arranged from ventral to dorsal in the following order : general somatic sensory (chiefly cutaneous), vestibular, and, dorsally of these, five or six fascicles of fibers of the lateral-line roots of the VII and X nerves. The details of this arrangement are variable within the species and from species to species of urodeles. The fascicles of vestibular and lateral-line roots, together with the underlying gray and the intervening neuropil, comprise the area acusticolateralis. The dorsal part of this, which receives only roots of the lateral-line nerves, is lobulated on the ventricular side.

Most of these root fibers divide into ascending and descending branches, and each fascicle spans the entire length of the medulla oblongata. Some of the general cutaneous and vestibular fibers extend far down into the spinal cord and upward into the cerebellum (figs. 3, 7). Terminals and collaterals of all these fibers end in a common pool of neuropil, from which secondary fibers go out to effect local connections in the medulla oblongata, to enter the cerebellum, to descend to the spinal cord, and to ascend in the lemniscus systems to the tectum and thalamus. The physiological specificity of the root fibers i.s largely, though not entirely, obliterated at the first synapse in the neuropil of the sensory field, in sharp contrast with the mammalian arrangement (chap. xi).

From this arrangement of the sensory systems of fibers and their central secondary connections it is clear that the bulbar structure is so organized as to facilitate mass movements of total-pattern type, which may be activated by any one of the exteroceptive or proprioceptive systems or by any conibination of these. There is some provision here for local reflexes of the muscles of the head, but the structure indicates that most of these are patterned from higher centers. The central receptive field of the visceral-gustatory system is well segregated from that of the general cutaneous and acousticolateral systems; and this is the structural expression of the fundamental distinction between the visceral and the somatic sensory physiological systems, to which further reference is made on pages G7 and 83.

Otherwise, there is httle histological evidence of precise localization of function in the medulla oblongata. The visceral and somatic sensory fields are cross-connected within the sensory zone, and they converge into a common sensory field at the bulbo-spinal junction. Proprioceptive adjustments are made throughout the spinal cord, medulla oblongata, cerebellum, and tectum; and each of these regions evidently plays a different role in the adjustments. Arcuate fibers connect all parts of the sensory zone with the motor zone of the same and the opposite side, and many of these divide into longdescending and ascending branches, thus activating extensive areas of the intermediate and motor zones.

The motor field of the medulla oblongata and the intimately related reticular formation contain the complicated apparatus by which the nuclei of the motor nerves are so interconnected as to act in groups, each of which may execute a series of co-ordinated actions in patterns determined by these connections. The tissues of the motor tegmentum, which effect this analysis of motor performance, are so intricately interwoven that it has not been possible to recognize the components of the several functional systems, and further analysis of this field is desirable.

4. Cerebellum

The cerebellum is small and very simply organized, but the chief structural features of the mammalian cerebellum are present except the pontile system, which is totally lacking. The urodele cerebellum consists of three major parts: (1) the median body, activated from the spinal cord, trigeminal nerve, and tectum (figs. 1, 3, 10); (2) the lateral auricles, which are enlargements of the anterior ends of the sensory zones of the medulla oblongata (figs. 7, 91); and (3), ventrally of the body of the cerebellum, a nucleus cerebelli, which is the primordium of the deep cerebellar nuclei of mammals (figs. 10, 32, 91).

This analysis of cerebellar structure is based on the comprehensive studies of Larsell ('20-'47) and Dow ('42), whose descriptions of Amblystoma, published in 1920 and 1932, I have confirmed in all respects. It should be noted that my definition of the amphibian auricle includes more than Larsell's, for he includes in this structure only the vestibular and lateral-line components. I find that these terminals and the related field of neuropil are so intimately related with the terminals of the trigeminus, the visceral-gustatory system, and lemniscus fibers that their segregation is not practical anatomically. The auricle, accordingly, is here regarded as the common primordium of several structures which in higher animals are diversely specialized for different functions. The most notable of these are the superior or pontile nucleus of the trigeminus and the floccular part of the flocculonodular lobe of the cerebellum. The primordia of these structures are clearly evident, and the history of their further differentiation in higher animals has been written.

Efferent fibers of tractus cerebello-tegmentalis leave all parts of the cerebellar complex for the underlying gray; and one fascicle of these — the brachium conjunctivum — passes forward to a ventral decussation and distributes its fibers to the isthmic tegmentum of both sides (figs. 10, 71). No primordium of the nucleus ruber or of the inferior olive has been recognized.

This primitive cerebellum exhibits the typical vertebrate pattern in very instructive form, with localization of the vestibular system laterally and the other systems medially. It is an appendage added to the basic sensori-motor systems; it supplements them, not as an aid in determining the pattern of performance, but to insure efficient execution. In species in which it is greatly enlarged, it contains enormous reserves of potential nervous energy, which is released during motor activity to reinforce and stabilize the operation of the effectors. For additional details see chapters x and xii.

5. Isthmus

The isthmus is unusually large in urodeles and is clearly circumscribed from surrounding parts. Dorsally it is small, containing in and near the superior medullary velum a special segment of the mesencephalic V nucleus and probably other peripheral connections through the nerves of the chorioid plexuses and meninges. Below this there is the superior visceral-gustatory nucleus (figs. 2B, 8, 23, 34). The nucleus isthmi, which is large in the frog, is here undifferentiated. The ventral part of the isthmus is very large, containing the nucleus of the IV nerve and a mass of tegmental cells. This isthmic tegmentum is interpolated between the primary sensori-motor systems of the medulla oblongata and midbrain, and it serves as an intermediary between them. There is a large central nucleus of small cells which receives fibers from a wide variety of correlation centers of intermediate-zone type. These enter by all the dorsal tegmental fascicles and by several other paths (figs. 16, 17, 21). This nucleus is enveloped by a group of larger cells, which is continuous posteriorly with similar tegmental cells of the region of the trigeminus (figs. 29, 30, 91). The complex as a whole is believed to have two chief functions: (1) Here are organized the patterns of the local reflexes of the musculature of the head, particularly those concerned with feeding. (2) The smallcelled central nucleus is a special differentiation of the periventricular gray, which serves, in addition to the specific functions just mentioned, a more general, nonspecific, totalizing function; that is, it is a part of that integrating apparatus which appears in mammals as the dorsal tegmental nucleus and the related fasciculus longitudinalis dorsalis of Schiitz (p. 208). The details of structure are given in chapter xiii.

The isthmic tegmentum occupies a strategic position between the primitive bulbo-spinal mechanisms and the higher cerebral adjustors; it plays a major role both in the patterning of local reflexes and in the integration of all bodily activities. This mass of tissue, which in urodeles is at a low level of differentiation, in higher animals is split up and distributed so that in mammals the identity of the isthmic tegmentum as an anatomical entity is lost in the adult brain, though the isthmic sector is plainly marked in the early embryonic stages.

6. Interpeduncular Nucleus

The interpeduncular nucleus also is unusually large in urodeles. It is not interpeduncular but interisthmic, extending from the fovea isthmi back to the level of the V nerve roots. The histological texture is extraordinary. A well-defined, trough-shaped column of cells borders the ventral angle of the ventricle, with dendrites extending downward through the ventral commissure, to arborize with tufted endings in a ventromedian band of neuropil (figs. 65, 66, 82, 83, 91). The axonic components of this interpeduncular neuropil come from various sources: (1) terminals of the fasciculus retroflexus, which take the form of a flattened spiral (fig. 50); (2) terminals of tr. tegmento-interpeduncularis from small cells of the overlying tegmentum with tufted endings, which join with the dendritic tufts of the interpeduncular nucleus to form small glomeruli (figs. 60-66, 84) ; (3) collaterals of thick fibers of tr. tegmento-bulbaris from the large cells of the tegmentum with similar tufted endings in the glomeruli (fig. 68); (4) collaterals of tr. interpedunculo-bulbaris, which also enter glomeruli (figs. 83, 84) ; (5) terminals of tr. mamillo-interpeduncularis with dispersed free endings (figs. 60, 61); (6) similar terminals of tr. olfacto-peduncularis (fig. 59); (7) less numerous terminals from several other sources. The slender, unmyelinated axons of the interpeduncular cells branch freely in the interpeduncular neuropil and continue from the nucleus in two strands (figs. 83, 84). The ventral interpedunculo-bulbar tract descends beyond the nucleus for an undetermined distance in the lip of the ventral fissure. The dorsal tract descends dorsally of the fasciculus longitudinalis medialis and turns laterally to end in wide arborizations in the tegmentum as far back as the IX nerve roots. Associated with these dorsal fibers are interpedunculo-tegmental fibers, which end in the neuropil of the isthmic tegmentum. The dorsal and interpedunculotegmental fibers are regarded as comparable with the isthmic and bulbar parts of the mammalian f. longitudinalis dorsalis of Schiitz.

The physiological problems suggested by this peculiar structure are puzzling. In the light of such scanty experimental evidence as we possess, I have ventured to suggest that the interpeduncular complex provides both activating and inhibitory components of reflex and general integrative activities, the actual patterns of which are elsewhere determined.

Topographically, this nucleus lies in the motor zone, but its functions clearly are of intermediate-zone type. It is present in all vertebrates at the anteroventral border of the isthmus, that is, at the boundary between cerebrum and rhombencephalon. Most of its afferent fibers come from the cerebrum, and evidently it serves chiefly as an intermediary adjustor between the sensory and intermediate zones of higher levels and the motor zone of the rhombic brain (for details see chap. xiv).

At this point in our analysis we. cross the boundary between rhombencephalon and cerebrum. The radical differences in structure and physiological properties of these two chief parts of the brain are masked and in large measure overruled, especially in higher animals, by ascending and descending connectives, of which the interpeduncular system is a typical illustration.

7. Tectum And Pretectal Nucleus

The tectum and the pretectal nucleus, as sectors of the sensory field, together with the dorsal thalamus, form a physiological unit within which all exteroceptive sensory systems are integrated in the interest of cerebral control of all lower sensori-motor systems involved in the operation of the skeletal musculature. This unit is intimately related with the cerebral peduncle and ventral thalamus. In the most primitive vertebrates and in early embryonic stages of all vertebrates, these structures might appropriately be united as a middle subdivision of the brain, which serves as the dominant center of cerebral control of all somatic activities. But in the adult animal the parts of this natural subdivision have so many distinctive connections and physiological properties that it seems preferable to treat them separately.

In vertebrates below the mammals the tectum opticum is the chief central end-station of the optic nerve; and, since the eyes are the chief distance receptors in most of these animals, fibers of correlation of all other sensory systems concerned with external adjustment naturally converge to this station. The tectum, accordingly, becomes the dominant adjustor of all exteroceptive systems. The tectum mesencephali of Aml^lystoma has a larger optic part — the superior colliculus — and a small, poorly differentiated nucleus posterior — the primordial inferior colliculus. The latter is interpolated between the tectum opticum and the cerebellum, and its connections suggest that its most primitive functions are proprioceptive. It receives a small primordial lateral lemniscus and evidently also serves such generalized auditory functions as this animal possesses (chap. xv). The development of the optic nerve and adult tectal structure and connections have been described in detail ('41, '42). Chapter xvi is devoted to the visual system ; for the arrangement of the mesencephalic nucleus and root of the V nerve see page 140 and figure 13.

Optic and lemniscus tracts and smaller numbers of fibers from various other sources all terminate in a broad sheet of intermediate neuropil, which is spread through the entire tectum and is nearly homogeneous in texture (figs. 11, 93). The tectum is not definitely laminated, though separation of the layers, which are conspicuous in the frog, is incipient. Fibers diverge from it in all directions (figs. 12, 18, 21-24, 93). It is inferred from this structure that movements activated directly from the tectum are of total-pattern type. Such local visual reflexes as this animal possesses are probably patterned elsewhere — in the thalamus and dorsal and isthmic tegmentum. Conditioning of reflexes is probably effected in these areas and perhaps also in the ventrolateral peduncular neuropil (p. 38). Experiments upon Triturus and anurans (Stone and Zauer, '40; Sperry, '43, '44, '45, '456) demonstrate anatomical projection of retinal loci upon the tectum opticum. This is true also in Amblystoma (Stone, '44; Stone and Ellison, '45), though the nervous apparatus employed has not been described.

The pretectal nucleus (figs. 2B, 35, 36, receives fibers directly from the retina and from the tectum, habenula, and cerebral hemisphere. Its efferent fibers go to the tectum, thalamus, hypothalamus, and cerebral peduncle (figs. 11, 12, 14, 15, 16, 22, 23). Its functions are unknown, but, by analogy with mammals, this may be part of the apparatus for regulation of the intrinsic musculature of the eyeball. Doubtless other functions are represented also. This area is the probable precursor of the mammalian pulvinar and neighboring structures.

The thalamus receives many fibers from the retina, and it is broadly connected with the tectum by uncrossed fibers passing in both directions in the brachia of the superior and inferior colliculi (figs. 11, 12). There are also systems of tecto-thalamic and hypothalamic and thalamo-hypothalamic tracts which decussate in the postoptic commissure ; some of these crossed fibers take longer courses to reach the peduncle and isthmic tegmentum (figs. 12, 15). This intimate thalamo-tectal relationship is radically changed in higher animals, where the thalamo-cortical connections are highly elaborated.

8. Dorsal Thalamus

I have separated the dorsal thalamus into three sectors: (1) anteriorly, the small nucleus of Bellonci of uncertain relationships ; (2) a well-defined middle part, an undifferentiated nucleus sensitivus, which is the primordium of most of the sensory nuclei of the mammalian thalamus; and (3) a vaguely delimited posterior sector, which apparently contains the undifferentiated primordium of both lateral and medial geniculate bodies (chap. xvii).

The middle and posterior sectors receive numerous terminals and collaterals of the optic tracts, terminals of the general bulbar and spinal lemnisci, and, through the brachia of the superior and inferior colliculi, these sectors are broadly connected with the tectum by fibers running in both directions. There is a similar, but much smaller, connection with the habenula.

From the middle sector a small, well-defined tr. thalamo-frontalis goes forward to the hemisphere (figs. 15, 71, 72, 95, 101,; this is the common primordium of all the thalamo-cortical projection systems of mammals, though here few, if any, of its fibers reach the pallial area. Other efferent fibers go to the ventral thalamus, hypothalamus, peduncle, and tegmentum. These thalamic reflex connections antedate in phylogeny the thalamo-cortical connections, and they persist in mammals as an intrinsic paleothalamic apparatus, an important part of which is the periventricular thalamic contribution to the f. longitudinalis dorsalis of Schiitz. The largest pathways of efferent discharge from the dorsal thalamus go backward to the peduncle and tegmentum by both crossed and uncrossed tracts (figs. 15, 18, 21, 23). The peduncular connection puts all the primary systems of total-pattern type under some measure of thalamic control. The connections with the dorsal and isthmic tegmentum probably co-operate in the patterning of local reflexes, particularly supplying the visual component of the feeding reactions.

9. Peduncle

The "peduncle" described here is not the equivalent of the human cerebral peduncle (p. 21). The intimate relations of this field with the overlying tecto-thalamic field have been commented upon in the preceding paragraphs. This ventral field is a well-defined column of cells, differentiated at the anterior end of the basal plate of the embryonic neural tube. It is the head of the primary motor column (of Coghill), which in all vertebrates, from early embryonic stages to the adult, contains the nucleus of the oculomotor nerve and a much larger mass of nervous tissue, which activates the primitive mass movements of locomotion. It maintains cerebral control of the lower bulbo-spinal segments of the latter systems, and some other motor functions also are represented here. Into it fibers converge from all other parts of the cerebrum (figs. 12, 14, 15, 17, 18, 20-24), and from it efferent fibers go out in four groups: (1) Ventromedial tracts go to the medulla oblongata and spinal cord. The longest of these fibers are in the f. longitudinalis medialis (fig. 6). (2) The oculomotor nerve supplies intrinsic and extrinsic muscles of the eyeball (figs. 22, 24). Associated with these peripheral fibers are central connections with the nuclei of the IV and VI nerves, so arranged as to execute conjugate movements of the eyes. The details of the apparatus employed are unknown. (3) Visceral sensory and gustatory fibers enter the peduncle (fig. 8), and with these are related efferent fibers to the hypothalamus and to lower levels of the motor zone, The pathways taken by the latter in the amphibian brain have not been clarified. (4) From both ventral thalamus and peduncle, fibers diverge to various surrounding parts, notably to the hypothalamus and isthmic tegmentum. These probably provide for co-ordination of various local reflex activities with the basic peduncular functions.

At the ventrolateral border of the peduncle there is an area of superficial neuropil, which is the terminus of the basal optic tract, large secondary and tertiary visceral-gustatory tracts, some fibers of the f. retroflexus, and fibers from several other sources (figs. 22, 23, 24). This is an undifferentiated primordium of the basal optic nucleus and some other structures of the mammalian brain (pp. 35, 221). It is related with the olfacto-visceral functions of the hypothalamus and probably also with conditioning of the fundamental peduncular activities.

10. Ventral Thalamus

There are anterior and posterior sectors of the ventral thalamus which differ in embryological origin (p. 239) and in certain connections of intermediate-zone type. Both sectors are here included in the motor zone because their chief efferent connections resemble those of the "peduncle," of which the posterior part is physiologically an anterior extension. The ventral thalamus is the primordium of the motor field of the mammalian subthalamus. The anterior sector contains a nucleus specifically related to the stria medullaris and amygdala and, above this, the eminentia thalami, which is a bednucleus of tracts related to the primordium hippocampi (chap, xviii; figs. 16, 17, 19, 20, 96).

The ventral thalamus and peduncle of urodeles form a single massive column, which is anatomically well defined. The specialized structures derived from it in mammals are dispersed among large masses of tissue of more recent phylogenetic origin; but in the human brain this region still retains cerebral control of the primordial coordinated movements of the musculature of the eyeballs and of the trunk and limbs.

11. The Retina and its Connections

In early embryonic stages the retina is part of the brain, and, as development advances, it absorbs much of the diencephalic sector of the early neural tube. This precociously accelerated development results from the dominance of vision in exteroceptive adjustment from the time that the larva begins to feed. For further details of this development and of the organization of the visual-motor apparatus see chapter xvi.

12. Habenular

As described in chapter xviii, this specialized part of the epithalamus receives fibers from almost all parts of the telencephalon and diencephalon and from the tectum (fig. 20). The habenular commissure connects the two habenulae, and it also contains two commissures of pallial parts of the hemispheres — commissura pallii posterior and com. superior telencephali. The chief efferent path from the habenula is the f. retroflexus (chap, xviii), which terminates in the cerebral peduncle and interpeduncular nucleus. In the brains of lower vertebrates the habenular complex is one of the most widely spread and physiologically important members of the central correlating apparatus. Its primary function seems to be to integrate the activities of all parts of the brain that are under olfactory influence with the exteroceptive functions of the tectum and thalamus in the interest of higher cerebral control of the feeding reactions of the skeletal muscles.

13. Hypothalamus

In the large preoptic nucleus and hypothalamus, olfactory connections dominate the picture, as they do in the habenular system; but here the nonolfactory functions represented are interoceptive instead of exteroceptive. All parts of the cerebral hemisphere are connected with the hypothalamus by fibers passing in both directions in the medial forebrain bundles (p. 273), stria terminalis (p. '^55), and fornix (p. 254) systems. The visceral-gustatory afferent paths are shown in figure 8. Large tracts from the thalamus and tectum also end here, so that all kinds of sensory experience of which the animal is capable are represented in the hypothalamus. This experience is here organized in terms of visceral responses. The efferent tracts go to the peduncle, tegmentum, interpeduncular nucleus, and descending fibers in the deep neuropil which are precursors of the f. longitudinalis dorsalis of Schiitz. There is a large tract to the hypophysis for nervous control of endocrine activity. There is also evidence that some neuro-endocrine functions are performed in the hypothalamus itself (Scharrer and Scharrer, '40). The structure of the hypothalamus has been described ('21a, '27. ^5a, '36, '42, and in the embryological papers, '37-'41). It is similar to that of Necturus, of which more detailed descriptions have been published ('336, '346). For the composition of the postoptic commissure see chapter xxi.

14. Strio-Amygdaloid Complex

The primordial corpus striatum occupies the thickened ventrolateral wall of the cerebral hemisphere and, like all the rest of the hemisphere, is under olfactory influence. This is stronger at its anterior and posterior ends. Anteriorly, it is divided by a striatal sulcus into dorsal and ventral parts (fig. 99), and posteriorly it is much enlarged as the amygdala (figs. 1, 96, 97), which has the typical mammalian connections (fig. 19).

The ventral sector of the anterior olfactory nucleus is interpolated between the olfactory bulb and the corpus striatum, as in Necturus (figs. 6, 86). This is the primordium of the tuberculum olfactorium ('27, p. 290), which is enormously enlarged in the lungfishes (Rudebeck, '45). Posteriorly of this nucleus is a poorly defined field, which embraces the ventral angle of the lateral ventricle and is regarded as the probable precursor of the head of the caudate nucleus (fig. 99) It is intimately connected with the rest of the striatum and the septum. The chief efferent path is the olfacto-peduncular tract to the dorsal hypothalamus, ventral border of the peduncle, and interpeduncular nucleus.

The middle sector of this complex is the undifferentiated primordium of the mammalian lentiform nucleus, as shown by its fibrous connections. It is characterized by very dense, sharply circumscribed neuropil in the white substance (p. 96; figs. 98, 99, 108, 109) and has the typical striatal connections with the overlying pallium and the thalamus, including a small sensory projection tract from the dorsal thalamus (fig. 15, The chief efferent path is the lateral forebrain bundle (f. lateralis telencephali, /.Za^./.), which contains strio-thalamic, strio-peduncular, and strio-tegmental fibers comparable with the corresponding components of the mammalian extrapyramidal system. There is also a strio-tectal and strio-pretectal connection (figs. 11, 14, 101). The separation of the lentiform nucleus into globus pallidus and putamen is incipient (p. 97).

15. Septum

The septum complex occupies the ventromedial wall of the hemisphere between the anterior olfactory nucleus and the lamina terminalis and hippocampal area (figs. 4, 98, 99) . Its position and connections are similar to those of mammals. It is directly connected with the olfactory organ by the nervus terminalis, and it receives fibers from the olfactory bulb, anterior olfactory nucleus, pallium, and hypothalamus. The chief efferent paths are by the medial forebrain bundle (f . medialis telencephali, and to the overlying pallium by the f. olfactorius septi ('27, p. 291). There is also a broad connection across the ventral aspect of the hemisphere with the amygdala and the piriform area, the diagonal band of Broca (figs. 96, 97, 98, d.b.), and a connection with the habenula by tr. olfactohabenularis anterior and tr. septo-habenularis (chap, xviii).

16. Olfactory Bulb and Anterior Olfactory Nucleus

The olfactory bulb is very large, embracing the anterior end of the lateral ventricle and extending back in the lateral wall for about half the length of the hemisphere (figs. 1, 3, 4, 85, 100, 105, 110; '246, '27). All peripheral olfactory fibers end in the glomeruli of the bulb. Fibers of the second order pass in large numbers to the anterior olfactory nucleus, and they enter longer olfactory tracts with wide distribution (fig. 6), The olfactory tracts are mixtures of fibers from the bulb and the anterior nucleus, as in Necturus ('336, figs. 6 16). They reach all parts of the cerebral hemisphere, the habenula, and the hypothalamus. Some of these decussate in the ventral part of the anterior commissure and some in the habenular commissure (com. superior telencephali).

The histological texture of the olfactory bulb is more differentiated than that of Necturus ('31), but more generalized than that of higher vertebrates. I have contrasted this with the mammalian pattern and added a theoretic interpretation of probable differences in physiologic properties of the tissue ('246). In brief, this tissue is interpreted as illustrating several transitional stages in the differentiation of polarized nervous elements from an unpolarized or incompletely polarized matrix. In Necturus ('31) the transitional character of this tissue is still more clearly evident. The granule cells, in particular, give no structural evidence of physiological polarity, i.e., of differentiation of dendrites from axon, though the connections of these cells in Amblystoma suggest that they have a transient and reversible polarity. In connection with this description ('246, pp. 385-95) there are some speculations regarding possible phylogenetic stages in the differentiation of permanently polarized neurons from an unspecialized nonsynaptic nerve net or neuropil.

In Amblystoma there is a moderately developed accessory olfactory bulb, but no other evidence of local specialization in the primary olfactory center. (There are hints of this in some mammals, e.g., the mink, Jeserich, '45, and references there cited). In 1921, I described the peripheral and central connections of the accessory bulb of Amblystoma and compared these with the more specialized structures of the frog. The anatomical connections there described are, I believe, correct, but the theoretic interpretation of the relationships in vertebrates generally between the vomeronasal organ, accessory bulb, and amygdala is less secure and awaits confirmation or correction.

The anterior olfactory nucleus is a zone of relatively undifferentiated cells interpolated between the bulbar formation and the more specialized areas posteriorly of it (figs. 6, 86B and C, 105, 109; '27, p. 288). In higher animals much of this tissue seems to be specialized and added to the adjoining fields ('24(/). A very large proportion of the fibers of the olfactory tracts, arising from both the bulbar formation and the anterior nucleus, are assembled in a dense superficial sheet of fibers in the medial sector of the anterior olfactory nucleus, which I have named the "fasciculus postolfactorius" (figs. 5, 100, 105, 110, /./JO.). Here these fibers take a vertical course and then are distributed to all the olfactory tracts. In chapters vii and xix there is further discussion of the significance of the olfactory system in the morphogenesis of the hemisphere.

17. Pallial

The pallial part of the hemisphere can be distinguished from the stem part, though there is no laminated cortex. There are three sectors (figs. 96-99) — the dorsomedial primordium hippocampi {p.hip.), the dorsolateral primordium piriforme {p.pir., or nucleus olfactorius dorsolateralis, nuc.oLd.L), and between these a dorsal sector of uncertain relationships (p.p.d.). The gray, as elsewhere in these brains, is confined to a thick periventricular layer except in the hippocampal sector, where the cell bodies are dispersed through the entire thickness of the wall and are imbedded in dense neuropil. This is evidently a first step toward differentiation of superficial cortex. The homologies of the hippocampal and piriform sectors with those of mammals are clear, as shown by substantially similar nervous connections. Further discussion will be found in chapter vii and the references there given.

The Commissures

Throughout the length of the central nervous system all parts of the two sides are broadly connected by systems of commissural and decussating fibers. These are in two series, dorsally and vent rally of the ventricles. Their composition is summarized in chapter xxi, with references to more detailed descriptions. In the aggregate they make provision for the co-ordinated action of the motor organs on right and left sides of the body.


The preceding outline of a regional analysis is framed in very general terms. The evidence upon which it is based is assembled in Part II of this work and references there given. This evidence, though far from complete, is regarded as adequate for the anatomical arrangements described. The physiological inferences drawn from these arrangements and the general theory expressed in the following chapters rest on a less secure basis. The correctness of these conclusions can be tested experimentally, and the hope that this will be done has motivated the labor expended upon this program of histological study.

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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.

Cite this page: Hill, M.A. (2021, June 15) Embryology Book - The brain of the tiger salamander 4. Retrieved from

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