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

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

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

Chapter V Functional Analysis, Central and Peripheral

The brain of Amblystoma performs three general classes of functions, with corresponding local differentiation of structure. We recognize, accordingly, three zones on each side — a dorsal receptive or sensory zone; a ventral emissive or motor zone; and, between these and infiltrating them, an intermediate zone of correlation and integration.

The Longitudinal Zones

Figures 4 and 5 are sketches of longitudinal sections of the cerebrum of adult Amblystoma tigrinum to illustrate the areas included in the motor and sensory zones as here arbitrarily defined. The zones of the medulla oblongata as seen in transverse section are shown in figure 9. The sensory zone includes those parts of the brain which receive afferent fibers from the periphery, together with more or less closely related tissue of correlation. The motor zone includes those parts from which efferent fibers go out to the periphery, together with related apparatus of motor co-ordination. Both these zones contain some areas which, though not directly connected with the periphery, are nevertheless primarily concerned with specific types of sensory or motor adjustment. What is left over is assigned to the intermediate zone, and whether a particular area will be included here depends on one's estimate of its preponderant physiological character. The body of the cerebellum and the pallial part of the cerebral hemispheres are excluded from the zones as supra-segmental structures.

The lines drawn in this analysis are frankly arbitrary, chosen primarily for convenience of description; but, as will appear, this functional analysis contributes to an understanding of the meaning of the structure, and, moreover, it has morphological justification as well. These zones are not autonomous units when viewed either structurally or physiologically. Their interconnections are intimate and complicated. The more important of these connections are shown in a number of diagrams, some in this contribution and some in previous papers.

This analysis of the more obvious structural features of an amphibian brain in terms of physiological criteria is an artificial schematization of a complicated fabric, the several parts of which are so intimately connected that there is an over-all integration of their activities. The main highways of through traffic have been mapped, with signboards pointing the way at the crossroads. Selected examples of some of the lines of fore-and-aft through traffic are illustrated in the diagrams ; but this kind of analysis does not take us to our destination. It does not tell us how mixed traffic is actually sorted out and so reorganized as to give the body as a whole efficient adjustment to the momentarily changing exigencies of common experience. These problems are discussed in subsequent chapters, but, first, the schematic outline will be summarized here.

Each zone is structurally diversified, and many of these local differentiations are directly correlated on the sensory side with the modalities of sense represented in the end-organs with which they are connected and on the motor side in a similar way with synergic systems of muscles. Each sensory and motor system of nerves has a local primary central station in direct connection with the periphery, and each of these stations has widely spread connections within its own zone and with other zones, thus insuring efficient correlation of sensory data, co-ordination of motor responses, and integration of the action system as a whole. In this summary the sensory systems are given special attention because these are the most useful guides in the analysis of the structure of this brain.

The Sensory Zone

The sensory zone is defined as those parts of the brain that receive fibers from peripheral organs of sense. In some fields the number of such fibers is large, in others it is very small; and some parts of the brain, like the cerebral peduncle, have both sensory and motor peripheral connections. Within the sensory zone there is a complicated apparatus of correlation, and in lower vertebrates the receptive areas and surrounding tissues dominated by them are so large that most of the mass of the brain can be blocked into fields appropriately designated "nosebrain," "eyebrain," and so on ('31a, p. 129). This feature is due to the fact that in these animals the sense organs are well developed and highly specialized, but the motor apparatus is more simply organized, chiefly for co-ordinated mass movements. The sensory zone with its own apparatus of correlation, accordingly, bulks larger than the motor zone.

Though the peripheral sensory apparatus in Amblystoma differs from the human in many details, yet the general principles of its organization are similar; the structural organization of the central apparatus of adjustment, on the contrary, is so radically different that comparisons are difficult. Here the several functional systems of peripheral nerve fibers enter the brain in fascicles of the nerve roots, which are physiologically as specific as are those of mammals ; but at the first synapse this specificity may almost completely disappear, in so far as it has visible structural expression. The root fibers of all sensory systems (except, perhaps, the olfactory) terminate by wide arborizations in a few common fields of neuropil, in each of which several of these systems are inextricably mingled. This neuropil is a common synaptic pool for all entering systems. The several pools are interconnected with one another and with similar pools in other parts of the brain stem, and there is no supreme cortical regulator. The translation of sensory experience into adaptive behavior and the integration of this behavior are somehow accomplished within this interplay of the local activities of the brain stem.

The sensory zone is continuous from the dorsal gray colunm of the spinal cord to the olfactory field, comprising the dorsolateral part of the medulla oblongata, the auricle in the cerebellar region, the anterior medullary velum and a small amount of contiguous tissue, the tectum of the midbrain, pretectal nucleus, dorsal thalamus, olfactory bulb with the adjoining anterior olfactory nucleus, and optionally the septum and some other parts of the hemisphere, a portion of the hypothalamus, and the ventrolateral neuropil of the peduncle. The fields optionally included receive terminals of the nervus terminalis (p. 267); the hypothalamus has a small but significant connection with the optic nerve, the basal root of which also connects with the peduncle. In some vertebrates the epithalamus receives fibers from the parietal eye, but in Amblystoma these have not been seen, and in this animal the predominant functions of the "optional" areas are of intermediate-zone type. The body of the cerebellum and the pallial part of the cerebral hemisphere might be assigned to the sensory zone as here defined anatomically; yet, as previously mentioned, they are excluded from this zone because of their distinctive physiological characteristics and their remarkable specialization in higher animals.

In the sensory zone of the medulla oblongata there are two elongated synaptic pools of neuropil, into which terminals of the sensory root fibers converge (chap. xi). One of these receives terminals of all somatic sensory systems; the other lies more ventrally and internally in the visceral lobe and receives terminals of the visceral sensory and gustatory systems (figs. 9, 89). The secondary fibers which emerge from these pools are distributed locally to the motor zone of the medulla oblongata, downward to the spinal cord, and upward to higher levels. The last take different courses, some to the cerebellum, some to the tectum and thalamus, and some to the hypothalamus. Each of these pathways discharges into a higher synaptic pool of neuropil, where its terminals are in physiological relation with terminals of other related sensory systems. The relations to which reference has just been made are in terms of the types of response to be evoked. Thus the tectum becomes the dominant regulator of somatic adjustments to exteroceptive stimulation, the hypothalamus becomes the regulator of visceral responses to olfacto-visceral stimulation, and the cerebellum provides regulatory control of the action of the skeletal muscles. The dorsal thalamus is ancillary to the tectum and shows a very early stage in the evolution of the ascending sensory projection systems to the cerebral hemispheres.

These local differentiations, each with characteristic structure and connections, are receptive fields for the several systems of peripheral sensory fibers, though some of them receive few peripheral fibers and are concerned chiefly with sensory correlation.

The Motor Zone

The motor zone as here defined includes the peripheral motor neurons and those areas of the brain stem concerned with the organization of motor impulses in patterns of synergic action. It includes the following histologically different parts: (1) corpus striatum (paleostriatum); (2) anterior part of the ventral thalamus; (3) posterior part of the ventral thalamus; (4) nucleus of the tuberculum posterius ("peduncle" in the restricted sense); (3) isthmic tegmentum; (6) trigeminal tegmentum; (7) a poorly defined tegmental field extending farther posteriorly through the length of the medulla oblongata into continuity with the ventral gray column of the spinal cord.

The floor plate of the embryonic neural tube probably ends anteriorly at the fovea isthmi (fig. 2B, f.i.), and the adjacent basal plate, which is the primordial motor zone, extends forward of this to include the whole of the peduncle and probably more or less of the hypothalamus and ventral thalamus. This primordial zone contains not only nervous elements with peripheral connections, like the sensory zone, but also an elaborate apparatus of central co-ordination of the neuromotor systems.

Anteriorly of the peduncle the motor zone has no peripheral connections, but the apparatus of motor co-ordination extends forward through the thalamus into the lateral wall of the hemisphere. Since the present analysis is based primarily on physiological criteria, this anterior extension of the motor field is included in the motor zone. The anterior boundaries of this zone are, of course, arbitrarily drawn ; that they are artificial is emphasized by the fact that the large basal optic root terminates in the peduncle, which is in the motor zone as here defined. Efferent fibers have been described as leaving the brain in many places outside the motor zone, even as here broadly defined. Vasomotor and other visceral efferent fibers have been reported in various animals associated with the nervus terminalis and the olfactory and optic nerves and in other places for distribution to meninges and chorioid plexuses. We have nothing new to report about Amblystoma in this connection.

In the spinal cord and medulla oblongata the peripheral motor neurons are so mingled with the co-ordinating neurons of the tegmentum and reticular formation and they are so similar in form that it is often impossible to distinguish the peripheral neurons except in cases where their axons are seen to enter the nerve roots. The cells of the nuclei of the eye-muscle nerves are fairly clearly segregated, and in some reduced silver preparations they react specifically to the chemical treatment (fig, 104); but even here their dendrites are widely spread and intertwined with those of tegmental cells, so that both kinds of neurons would appear to be similarly activated by the neuropil within which they are imbedded. The cell bodies are locally segregated; but their dendrites, where most of the synaptic contacts are made, are not segregated.

In the medulla oblongata the motor tegmentum contains small and large cells in an endless variety of forms, but these elements are not segregated in accordance with either size or morphological type. It is true that the arrangement of their cell bodies may show some rather ill-defined local segregation, but their dendrites and axons are so intimately intertwined in the neuropil that nothing comparable with the localized nuclei of higher brains can be recognized. Farther forward in the isthmus and peduncle the tegmental tissue of coordination is much increased in amount and somewhat more differentiated. In some of my former papers (e.g., '30, p. 76) the term "nucleus motorius tegmenti" was used loosely (and inaccurately) to include a tegmental zone defined topographically. This seemed to be justified in the case of Necturus by the lack of localization of the large motor elements which characterize this region; but this justification is inadequate, both factually and morphologically — see the discussion by Ariens Kappers, Huber, and Crosby ('36, pp. 653, 666).

It is obvious that most of the tissue of the motor zone is concerned with co-ordination of the action of the peripheral elements, so that synergic groups of muscles are activated in appropriate sequence; but, with the technic available, it has not been possible to analyze this complex so as to reveal the mechanism employed. In the medulla oblongata this organization is chiefly for local control of bulbar and spinal reflexes, the intermediate zone participating. In the isthmus and peduncle the number of peripheral elements is relatively small and the co-ordinating apparatus larger, giving these areas control over all motor fields spinal ward of them. This intrinsic motor apparatus is supplemented by a segregated band of correlating tissue in the intermediate zone, the subtectal dorsal tegmentum. In mammals both these zones are further specialized into separate nuclei distributed in the tegmentum.

The primary patterns of somatic movements are predetermined by the course of central differentiation within the motor and intermediate zones in premotile stages of development. After connection with the peripheral musculature is established, each of these muscles seems to exert some sort of distinctive reciprocal influence upon that motor field of the central nervous system from which its innervation is derived. The nature of this influence is unknown, but its reality is well attested by experiments of Paul Weiss ('36, '41) and colleagues upon "myotypic response" and "modulation."

In later stages the primary motor patterns may be modified, or "inflected," by sensory experience and practice. Influence of use or some other functional factors seem to be essential for maintenance of motor efficiency, as graphically shown by Detwiler's observations ('45, p. 115; '46) on the behavior of decerebrate larvae of Amblystoma (to which further reference is made on p. 118). In young larvae of stage 37, swimming movements may be perfectly executed after transection immediately below the auditory vesicle under control of the lower medulla oblongata and spinal cord (Coghill, '26, Paper VI, p. Ill; '29, p. 15); but, subsequent to Harrison's stage 40, Detwiler finds that sustained motor activities, including swimming, fail rapidly if the influence of the midbrain is blocked in prefunctional stages, though feeding reactions are preserved after complete ablation of hemispheres and visual organs. The midbrain evidently supplies a factor essential for maintenance of motor efficiency.

The motor field of this brain is smaller and more simply organized than the sensory field because most of the activities are mass movements of total-pattern type. Within this larger frame of total behavior, the partial patterns of local reflexes are individuated with more or less capacity for autonomous action. The number of these local partial patterns is smaller than in higher animals, and all of them are far more closelj^ bound to the total patterns of which they are parts. The segments of each limb, for instance, may, upon appropriate stimulation, move independently; but in ordinary locomotion they move in a sequence related to the action of the entire limb, the other limbs, and the musculature of the trunk.

The peripheral motor nerves (omitting the general visceral components of preganglionic type not here considered) are in three groups: (1) the spinal nerves; (2) the eye-muscle nerves. III, IV, and VI pairs of cranial nerves, which are somatic motor; and (3) the special visceral motor nerves of the V, VII, IX, and X pairs, innervating the striated musculature of the visceral skeleton of the head. The primary movements of trunk and limbs are organized for locomotion in the motor zone of the spinal cord. This organization is under exteroceptive and proprioceptive control locally throughout the length of the cord and more especial!}^ at the bulbo-spinal junction; it is under additional proprioceptive control from the labyrinth and the cerebellum; there is further control from the cerebrum — optic, olfactory, and the related apparatus of higher correlation. The bulbar group of special visceral motor nerves is primarily concerned with movements of the head, notably those of respiration and feeding. The feeding reactions are under visual, olfactory, somesthetic, gustatory, and general visceral afferent control, and the pattern of performance seems to be organized in the large isthmic tegmentum. The very large and complicated interpeduncular nucleus is an isthmic structure which is physiologically of intermediate-zone type (chap. xiv). Details of the structure and connections of the various parts of the motor zone are in Part II, and the pecuhar features of its forward extension in the cerebral hemisphere are discussed in chapter vii.

The Intermediate Zone

The characteristics of this zone are imphcit in the preceding account of the sensory and motor zones. It is more elaborately developed and its boundaries are more clearly defined in parts of the cerebrum than in the rhombencephalon. These boundaries are necessarily arbitrary, for all parts of the brain are involved in correlation and integration of bodily activities ; but throughout the length of the spinal cord and brain there is a band of tissue between the sensory and motor zones primarily concerned with these adjustments. At lower levels I have termed this tissue the "reticular formation," and here it infiltrates the other zones with no clear boundaries (for details see chap. xi). At higher levels it increases in amount and is more clearly segregated. It would be appropriate to include in this zone most of the diencephalon and telencephalon except the specific optic and olfactory terminals ; but, for reasons mentioned above, a different subdivision has been adopted, primarily for convenience of description. The dorsal tegmentum, or subtectal area, is a typical representative of this zone in position and physiological connections. In the isthmic and bulbar tegmentum the characteristics of the intermediate and motor zones are inextricably mingled. The habenula, hypothalamus, and interpeduncular nucleus, as elsewhere described, clearly belong physiologically to the intermediate zone; and the whole cerebral hemisphere, except the olfactory bulb, might appropriately be so classified in all Ichthyopsida.

In the most primitive vertebrates the intermediate zone is scarcely recognizable as an anatomical entity. As the action system becomes more complicated in higher animals, this zone shows corresponding differentiation. This specialization is more directly dependent upon complication of the peripheral motor apparatus than upon sensory differentiation, for, so long as the action system is largely confined to mass movements, the patterning of these total activities is effected in the sensory and motor zones. In tetrapods and birds more complex central adjustors are required, and these are differentiated between the two primary zones and anteriorly of them. With the appearance of more autonomy of the local reflex systems, more efficient apparatus of integration is demanded. The final result is that in the human brain the apparatus of intermediate-zone type has increased so much that it comprises more than half the total weight of the brain, for both cerebral and cerebellar cortices are derivatives of this primordial matrix, as will appear in the ensuing discussions.

The Functional Systems

The preceding physiological analysis of the brain obviously rests upon the peripheral relations of its several parts. The two primary functions of the nervous system are, first, the maintenance of the integrity of the individual, with efficient co-operation of parts among themselves and with the total organization, and, second, the analysis of experience and the translation of the sensory data into appropriate behavior. The peripheral nerves are key factors in both these domains. Our knowledge of the functional analysis of the cranial nerves has been greatly increased during the last fifty years, largely by the work of the so-called American school of comparative neurologists, which I have recently reviewed ('43).

Before I discuss the components of these nerves, a few definitions are in order. In the attempt to envisage the nervous system from the operational standpoint, distinctions have been drawn between sensory correlation, motor co-ordination, and those central processes that provide integration, and some measure of spontaneity of action which might be grouped under the name "association" ('24c, p. 235; '31a, p. 35). This classification is necessarily artificial, for all these processes are interrelated. They interpenetrate, and they are not sharply localized in the structural fabric. Nevertheless, these several types of action are recognizable components of the unitary dynamic system, and there are local differentiations of the structural organization correlated with preponderance of one or another of them, more clearly so in higher vertebrates than in lower.

Sensory correlation, as the term is here employed, refers to interaction of afl^erent impulses within the sensory zone, that is, within the field reached by terminals oi peripheral sensory fibers. The interplay of these diverse afi'erent impulses takes two forms: (1) in fields of undifferentiated neuropil, the activation of which results in alterations of the central excitatory state or in mass movements of large numbers of synergic muscles; (2) in more restricted areas (nuclei), which activate the neuromotor apparatus of local reflexes. The members of both groups are interconnected by systems of internuclear fibers like the lemniscus systems, all within the sensory zone, so that all activities of this zone interact one with another. These internuncials are so arranged that functional systems of afferents, which normally co-operate to effect a particular type of motor response, are more intimately associated. Thus the tectum opticum receives most of the lemniscus fibers of all somesthetic systems and minimum numbers of olfactory and visceral systems. This basic pattern as seen in Ambly stoma is changed in mammals, where higher associational centers have taken over most of the functions of correlation.

Motor co-ordination is effected primarily in the motor zone, which is so organized as to activate synergic groups of muscles in appropriate sequence with inhibition of their antagonists. This grouping may be adapted for mass movements or for local reflexes. Internuclear tracts connect the various parts of the sensory zone directly with appropriate parts of the motor zone. More refined analysis and conditioning of motor responses are effected through the intermediate zone, and the tissues of the latter group are greatly enlarged and complicated in higher brains.

The activities of stimulus-response type which have just been considered are so interconnected with internuclear tracts and the interstitial neuropil as to facilitate the integration of all local activities in the interest of the requirements of the body as a whole. Every local part of the brain is a component of the apparatus of general integration, and some of these parts have this association as their dominant function. In Ambly stoma most of this suprasensory and supramotor tissue is dispersed as interstitial neuropil. In mammals, higher types of associational tissue have been differentiated locally, notably in the cerebral cortex and its dependencies, with corresponding enhancement of those synthetic functions which are manifested as conditioning, educability, and reasoning. Parallel with these changes there is an enormous increase in accumulated reserves of potential nervous energy, which come to expression as spontaneity, memory, and creative imagination.

A survey of the nerve fibers of Amblystoma as a whole in view of the principles just expressed shows that they may be classified in four groups: (1) the peripheral afferent systems and associated internuclear correlating tracts within the sensory zone (lemniscus systems, etc.) ; (2) the peripheral efferent systems and the related coordinating fibers of the motor zone; (3) the central internuclear systems intercalated between the preceding two and so interconnected as to yield appropriate responses to ordinary recurring stimuli; and (4) infiltrating these mechanisms of stimulus-response type, a different sort of adjusting apparatus which insures general integration of these systems, with provision for conditioning of reflexes and other forms of individually acquired behavior and for release of accumulated reserves of nervous potential as needed. These four groups intergrade but, in general, are recognizable.

The peripheral fibers are grouped in functional systems, each of which is defined as comprising all nerve fibers and related endorgans, which are so arranged as to respond to excitation in a common mode, either sensory or motor. These functional systems are convenient anatomical units also, for all fibers of each sensory system, regardless of variations in the peripheral distribution of their end-organs and regardless of the particular nerve trunks or roots through which they connect with the brain, are segregated internally and converge into local areas or zones. In higher vertebrates (but less so in lower) the secondary connections of these terminal stations tend to retain their physiological specificity. From this it follows that the peripheral systems of sensory analyzers are extended into the brain as far as related central pathways are separately localized — in the human brain it may be even up to the projection areas of the cerebral cortex. Accordingly, we include in the sensory zone as here defined not only the terminal nuclei of peripheral sensory nerves but also their related nervous connections, so far as these are with other parts of the sensory zone and not directly with the motor zone. The neuromotor apparatus can be similarly analyzed into functional systems, each of which is concerned with the synergic activation of some particular group of muscles.

This anatomical segregation of the functional systems is not carried to perfection, even in the human nervous system. The various modalities of cutaneous and deep sensibility, for instance, are not completely segregated and localized either peripherally or centrally. Yet this differentiation has gone so far that it provides our most useful guide in the analysis of the structure of the brain.

The activities of the body may be divided into two major groups; (1) those concerned with adjustment to environment, the somatic functions, and (2) those concerned with the maintenance and reproduction of the body itself, visceral functions. These, of course, are not independent of each other; nutrition, for instance, involves somatic activity in the search and capture of food and visceral activity in its digestion and assimilation. Nonetheless, these types of function are so different, especially in the responses evoked, that this strictly physiological criterion marks also the most fundamental structural analysis of the nervous system. Anatomically, the somatic systems of peripheral organs and nerves and central adjustors, including the proprioceptors, are, in general, rather sharply distinguished from the visceral. The systems are cross-connected by internuclear tracts, and some sensory systems, like the olfactory, may serve, on occasion, either somatic or visceral adjustments.

A phylogenetic survey of these systems reveals remarkable plasticity in their interrelationships. Thus, taste buds, which in most vertebrates are typical interoceptors, may in some fishes be spread over the external surface of the body, where they serve exteroceptive functions, with corresponding changes in the anatomical pattern of the central apparatus of adjustment (chap, x; '446). On the motor side the apparatus of feeding and respiration exhibits still more remarkable transformations. In fishes this musculature is connected with the visceral skeleton — jaws, hyoid, and gill arches — and the functions performed are obviously visceral, though the larger part of this musculature is striated. The related parts of the nervous system are classified as special visceral motor. With suppression of the gills in higher animals, some of these muscles undergo remarkable transformations. Those which are elaborated to form the mimetic facial musculature of mammals become physiologically somatic ('22, '43).

The classification of peripheral end-organs and their related nerves which has proved most useful grew out of the analysis of these nerves into their functional components, to which reference has just been made, by histological methods. Serial sections through the entire bodies of small vertebrates differentially stained for nerve fibers enable the investigator to reconstruct not only the courses of the nerves but also the arrangement of the functional systems represented in each of them and to follow these components to their peripheral and central terminals, a result that cannot be achieved by ever so skilful dissection. The first successful application of this method was Strong's analysis of the nerve components of the tadpole of the frog in 1895, a fundamental research which provided the generalized pattern which prevails, with endless modifications of details, throughout the vertebrate series, as has been abundantly demonstrated by numerous subsequent studies by many workers.

This was followed in 1899 by my Doctor's dissertation on the nerve components of the highly specialized teleost, Menidia. From these and subsequent studies the peripheral nervous system of the head was analyzed into functional systems as follows:

  1. Somatic sensory fibers of two groups. — (1) Exteroceptive systems, including (a) the specialized olfactory (in part), optic, auditory, and lateral-line nerves with differentiated end-organs, and (6) the nerves of general cutaneous and deep sensibility with simple free endings, these entering chiefly in the V nerve root with some in the VII, IX, and X roots. (2) Proprioceptive fibers from specialized endorgans of the internal ear and (probabljO lateral-line organs and also fibers from muscles, tendons, and other deep tissues. Here belongs also the peculiar mesencephalic root of the V nerve. See chapter x for further comments on the proprioceptive system.
  2. Visceral sensory fibers of two types. — (1) With specialized endorgans, viz., the olfactory organ (in part) and the taste buds, the latter entering by the VII, IX, and X nerve roots. (2) Fibers of general visceral sensibility with free endings, entering in the same roots as the preceding and mingled with them.
  3. Somatic motor fibers. — Somatic motor fibers which supply striated muscles derived from the embryonic somites, those in Amblystoma being limited to the nerves of the extrinsic muscles of the eyeball in the III, IV, and VI nerves.
  4. Visceral efferent fibers of two types. — (1) Special visceral motor fibers of cranial nerves supplying striated muscles, not of somitic origin, related with the visceral skeleton, jaws, hyoid, branchial arches, and their drivatives (in the V, VII, IX, and X roots). (2) Preganglionic fibers of the general visceral (autonomic) system, terminating in sympathetic and parasympathetic ganglia, where they activate postganglionic fibers distributed to unstriated and cardiac muscles and glands (in the III, VII, IX, and X roots). The last system is not further considered in this work. For application of this analysis to the human nervous system see my Introduction to Neurology ('31a, chaps, v and ix).

This analysis has yielded our most useful clues for resolution of the complexity of both peripheral nerves and brain. Descriptions of the peripheral end-organs and the courses of the nerves do not lie within the scope of this work. Some of these details which are significant for understanding their central connections are included in chapter x.

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

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


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

Cite this page: Hill, M.A. (2019, September 18) Embryology Book - The brain of the tiger salamander 5. Retrieved from

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