Book - The brain of the tiger salamander 19

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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 XIX The Cerebral Hemispheres Subdivisions of the Hemisphere

LITTLE need be added here to the general descriptions given in ^ chapters iv and vii. For details the reader is referred to the paper of 1927. That description was based mainly on a survey of a small number of well-preserved specimens cut in the transverse plane. There are in our collection many more instructive Golgi sections cut in longitudinal planes which have not been critically studied, though preliminary surveys have been made. It is deemed unprofitable at this time to continue the study of these sections because their interpretation should be based on physiological experiments correlated with the anatomical analysis.


At the present time our knowledge of the details of the internal structure of the cerebral hemispheres of Necturus ('336) is more complete than of any other amphibian. This brain is not only larger than most of the others, but it is less compact and its great elongation is favorable for accurate localization of experimental studies by a wide variety of methods. This generalized brain provides a norm or basic pattern for the vertebrate phylum as a whole. The other urodeles and the anurans present a series of progressively more differentiated brains, and the sequence of stages of this process of specialization can readily be followed. That such a program of correlated histological and experimental work is practicable was demonstrated by Coghill in a restricted field of embryological research. With the refined experimental methods now at our disposal and with some reorientation in the fields of developmental mechanics, localized experimental extiipations, and electrical excitations, supplemented by oscillographic records, the steps in progressive phylogenetic differentiation of structure can be correlated with changes in overt behavior. For the completion of such a program frogs will probably prove to be more serviceable animals than the more sluggish salamanders (p. 98). These data will enable the comparative psychologists to identify and interpret prodromal stages of some of the basic patterns of human mentation.


Comparison of the amphibian cerebral hemisphere with the human shows a common plan of organization, and in the amphibian brains we find evidence of the beginning of differentiation of some mammalian structures at the earliest stages of their emergence from an undifferentiated matrix. The formative agencies which are operating to produce this local specialization are open to inspection and experimental investigation.


On the basis of position, internal structure, and connections the following mammalian organs have been identified in the brain of Amblystoma. First, the pallial field is distinguishable from the stem, and within this field primordia of hippocampal and piriform cortical areas are unmistakable. Some connections are suggestive of influences which may be precursors of neopallial differentiation, but these are vague and uncertain. Most of the mid-dorsal pallial area is probably represented in higher brains at the margins of archipallial and paleopallial cortex — such transitional cortex as the subicular and perirhinal areas.


In the subpallial part the lateral and medial walls of the hemisphere are organized essentially as in mammals. Laterally, the strioamygdaloid complex is well defined, though its subdivisions are not clearly separable. Of these, the amygdala is definitely organized, with connections very similar to those of mammals. In the corpus striatum the nucleus accumbens septi is present as in lower mammals, and associated with it is an area which probably corresponds with the head of the caudate nucleus. The remainder of the corpus striatum is an undifferentiated lentiform nucleus, within which large and small cells are mingled. The connections of these cells suggest that the dorsal part of this area becomes the putamen and the ventral part the globus pallidus (p. 96).


On the medial side of the hemisphere the structure and connections of the septum conform with the mammalian arrangement, and below this is an undifferentiated area which gives rise in some of the fishes and in mammals to the tuberculum olfactorium.


THE OLFACTORY SYSTEM

As outlined in chapter vii, the olfactory nerve and its connections have played the dominant role in the morphogenesis of the cerebral hemispheres of lower vertebrates. The brief summary of the structure at the end of chapter iv is here supplemented by further description of the distribution of the olfactory tracts.



THE CEREBRAL HEMISPHERES 207

Nervns terminalis. — These unmyelinated fibers enter the brain in small compact fascicles mingled with those of the olfactory nerve. Their peripheral and central courses can be accurately followed only in elective Golgi impregnations, which, fortunately, are frequently obtained. All fibers of the olfactory nerve end in the olfactory bulb, but none of the terminalis fibers do so. The latter enter the brain at the ventral border of the olfactory bulb and course backward in several small fascicles, which terminate in the septum, preoptic nucleus, and hypothalamus. In Necturus some of them reach the interpeduncular nucleus, and this may be true in Amblystoma also. This nerve is present in vertebrates generally, from fishes to man, but our knowledge is incomplete about its terminal connections and functions (McKibben, '11; for Necturus see my '336, p. 120, and '346, '34c; for the frog, '09). It is regarded here as a sensory nerve, but even this is a debatable question.


Olfactory bulb. — In the olfactory bulb, as in the retina, the peripheral receptors discharge into a field which receives few afferent fibers of other functional systems. In the other primary sensory centers there is a common pool of neuropil within which terminals of peripheral fibers of diverse sensory modality are mingled, and to these there are added terminals of other correlating fibers of central origin. The bulbar formation of Amblystoma receives an enormous number of fibers of the olfactory nerve and no others from the periphery. There are also terminals of fibers ascending from other parts of the brain:

(1) many of these are collaterals from the secondary olfactory tracts;

(2) some are axons of cells of the anterior olfactory nucleus; (3) some may be commissural fibers by way of the anterior commissure; (4) some may come from more remote parts of the hemisphere. By far the larger number of these ascending fibers belong to the first two classes, in which olfactory influence is clearly dominant. From this it follows that the impulses conducted by the secondary olfactory tracts are influenced relatively little by other functional systems. In this respect they differ from the lemniscus systems of the lower brain stem; and the fact that these almost purely olfactory tracts reach all parts of the cerebral hemisphere is probably the reason why this hemisphere remains at a low level of structural differentiation and physiological specificity.


Necturus and Amblystoma exhibit two well-defined stages in the histological differentiation of the olfactory bulb, but the mammalian type of structure has not been attained ('246, '31). Throughout the bulbar formation, except for the accessory bulb, the structure is nearly homogeneous, with little evidence of localization of function. The sense of smell lacks any provision for localizing in external space the source of odorous excitations. In the retina there is very complicated mechanism for analysis of the components of visual excitation (Polyak, '41). The analysis of olfactory sensibility for discrimination of odors is evidently a much simpler process. Judging by analogy with Polyak's description of the retina, there is little provision for this in the olfactory bulb. It is possible that the periglomerular cells may perform this function, but the structural organization of the bulbar formation gives clear evidence that the dominant activity here is not analysis but summation and intensification. The correlation of olfaction with other sensory systems is effected throughout the cerebral hemisphere, hypothalamus, and epithalamus, beginning in the nucleus olfactorius anterior.


Anterior olfactory nucleus. — This nucleus was first defined in Ambly stoma ('10, p. 497) as undifferentiated olfactory tissue of the second order, closely associated with the olfactory bulb and extending backward a longer or shorter distance between the bulbar formation and the more specialized parts of the hemisphere. It is of large extent in the amphibian brain, and to it considerable attention has been given ('246; '31; '27, p. 288; '336, p. 133; '34, p. 99). In higher brains it shrinks in size as progressively more of this generalized tissue is specialized. Its comparative anatomy was discussed in connection with a detailed description of it in the Virginia opossum ('24c?). In the amphibian brain it is a broad ring of gray bordering the bulbar formation on all sides. This cylinder is divided topographically into ventral, medial, dorsal, and lateral sectors, each of which has its own distinctive connections with other parts of the hemisphere. The ventral sector and the lower part of the medial contain the primordium of the tuberculum olfactorium. The arrangement of these sectors in Necturus is shown in figures 111 and 112, and their structure and connections have been described in detail ('336, p. 133). Transverse sections through this region of Ambly stoma are in the paper of 1927 (p. 288 and figures 2-5). The neurons and neuropil of this nucleus are illustrated in figures 105, 108, and 109 and in 1934, figures 1 and 2.


Typical neurons of the anterior nucleus have widely spread thorny dendrites, and axons which enter the olfactory tracts. Many other forms of cells are seen, some of which are transitional to those of the olfactory bulb. The axons of some of its cells are directed peripheral


THE CEREBRAL HEMISPHERES 3G9

Iv, to end with wide arborizations in the granular layer of the bulb. Most of the smaller cells have short, much branched axons, which participate in the formation of the dense axonic neuropil of this region.


Olfactory tracts. — Strictly defined, a tractus olfactorius includes axons of olfactory neurons of the second order only, that is, axons of mitral cells ; but practically all these fibers are mingled with those of higher order from the anterior olfactory nucleus and other parts of the hemisphere, so that the tracts so designated on the figures are all mixed })undles. These axons of mitral cells stream backward from all margins of the olfactory bulb. Only the shorter fibers to the anterior nucleus are drawn in figures 111 and 112. As shown in figure 6, these are accompanied by longer fibers, which join the tracts descending from the anterior nucleus. Olfactory tracts from the lateral and ventral borders of the bulb take direct courses backward in three series. The more dorsal fibers enter tr. olfactorius dorsolateralis for distribution to the dorsolateral olfactory nucleus, which is primordium piriforme. This is the largest of the olfactory tracts and is comparable with the lateral olfactory stria of mammals. Other lateral fibers pass to the corpus striatum and amygdala. Some of these fibers join tr. olfacto-peduncularis, most of the fibers of which arise in the anterior nucleus and primordial caudate nucleus. Fibers from the ventral border of the bulb enter tr. olfactorius ventralis and descend for an undetermined distance in the medial forebrain bundle.


As shown by figure 4, the lateral ventricle extends forward almost to the anterior end of the olfactory bulb. Many of the longer fibers from the bulb take tortuous courses to reach their terminal stations. They accumulate in the medial sector of the anterior olfactory nucleus and primordium hippocampi, where they form a very large compact sheet of fibers termed "fasciculus postolfactorius" (fig. 100, f.po.; '27, figs. 2, 3). These fibers run vertically around the tip of the lateral ventricle, some directed ventrally to enter tr. olfactorius ventralis and some dorsally to enter tr. olfactorius dorsolateralis (fig. 5).


Olfactory tracts of the third and higher orders, i.e., those separated by two or more synapses from the periphery, are generally designated by hyphenated compound words, as tr. olfacto-peduncularis; but, as mentioned above, many of these tracts are mixtures containing some axons of mitral cells. For further details of these connections see the summaries ('33&, p. 124; '27, p. 282).