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

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Chapter III Histological Structure

General Histology

In amphibian brains the histological texture is generalized, exhibiting some embryonic features; and it is at so primitive a level of organization as to make comparison with mammals difficult. Most of the nerve cells are small, with scanty and relatively undifferentiated cytoplasm. There are some notable exceptions, such as the two giant Mauthner's cells of the medulla oblongata and related elements of the nucleus motorius tegmenti. With the exceptions just noted, Nissl bodies are absent or small and dispersed.

Almost all bodies of the neurons are crowded close to the ventricle in a dense central gray layer, with thick dendrites directed outward to arborize in the overlying white substance (figs. 9, 99). The axon usually arises from the base of the dendritic arborization, rarely from its tip, and sometimes from the cell body; it may be short and much branched or very long, with or without collateral branches. The ramifications of the short axons and of collaterals and terminals of the longer fibers interweave with dendritic arborizations to form a more or less dense neuropil, which permeates the entire substance of the brain and is a synaptic field. Some of the nerve fibers are myelinated, more in the peripheral nerves, spinal cord, and medulla oblongata than in higher levels of the brain. Both myelinated and unmyelinated fibers may be assembled in definite tracts, or they may be so dispersed in the neuropil as to make analysis difficult. The arrangement of recognizable tracts conforms with that of higher brains, so that homologies with human tracts are in most cases clear. These tracts and the gray areas with which they are connected provide the most useful landmarks in the analysis of this enigmatic tissue.

In the gray substance there are few sharply defined nuclei like those of mammals, but the precursors of many of these can be recognized as local specializations of the elements or by the connections of the related nerve fibers. In most cases the cells of these primordial nuclei have long dendrites, which arborize widely into surrounding fields (figs. 9, 24, 61, 66), so that the functional specificity of the nucleus is, at best, incomplete. This arrangement facilitates mass movements of "total-pattern" type, but local differentiations serving "partial patterns" of action (Coghill) are incipient. Localized reflex arcs are recognizable, though in most cases these are pathways of preferential discharge within a more dispersed system of conductors (chap. vi).

Tissue differentiation is more advanced in the white substance than in the gray. The most important and diversely specialized synaptic fields are in the alba, and this local specialization is correlated with differences in the physiological properties of the nervous elements represented. This means, as I see it, that functional factors must be taken into account in both ontogenetic and phylogenetic differentiation and that in the long view the problems of morphogenesis are essentially physiological, that is, they resolve into questions of adaptation of organism to environment (chap. viii). This is the reason why in this work the histological analysis is made in terms of physiological criteria, even though these criteria are, in the main, based on indirect evidence, namely, the linkage of structures in functional systems of conductors.

The nonnervous components of this tissue comprise the blood vessels, ependyma, and a small number of cells of uncertain relationships which are regarded as undifferentiated free glial cells or transitional elements ('34, p. 94; '336, p. 17). The ependymal elements everywhere span the entire thickness of the brain wall with much free arborization. They assume various forms in different regions, and their arrangement suggests that they are not merely passive supporting structures, though if they have other specific functions these are still to be discovered. For illustrations see figures 63, 64, 70, 79, and 81.

More detailed descriptions of the histological structure of urodele brains may be found in earlier papers ('14a, p. 381; '17, pp. 232, 279 ff.; '335, pp. 16, 268; '33c; '33cf; '34; '34a,- '346; '42, p. 195; '44a). In the olfactory bulbs of Necturus ('31) and Amblystoma ('246) we find an interesting series of transitional cells between apparently primitive nonpolarized elements and typical neurons, as described on page 54.

The Neuropil

In the generalized brains here under consideration the neuropil is so abundant and so widely spread that it evidently plays a major role in all central adjustments, thus meriting detailed description.

Only the coarser features of this tissue are open to inspection with presently available histological technique. In my experience its texture is best revealed by Golgi preparations, and very many of them, for the erratic incidence of these impregnations may select in different specimens now one, now another, of the component tissues — blood vessels, ependyma, dendrites, or axons. In each area of neuropil these components are independent variables, and in most of these areas axons from many sources are so intricately interwoven that the tissue can be resolved only where fortunate elective impregnations pick out one or another of the several systems of fibers in different specimens. It is difficult to picture the neuropil either photographically or with the pen, and the crude drawings in this book and in the literature give inadequate representations of the intricacy and delicacy of its texture.

A survey of the neuropil of adult Amblystoma as a whole has led me to subdivide it for descriptive purposes and somewhat arbitrarily into four layers ('42, p. 202). From within outward, these are as follows:

The periventricular neuropil pervades the central gray so that every cell body is enmeshed within a fabric of interwoven slender axons (figs. 106, 107). This persists in some parts of the mammalian brain as subependymal and periventricular systems of fibers.
The deep neuropil of the alba at the boundary between gray and white substance knits the periventricular and intermediate neuropil together, and it also contains many long fibers coursing parallel with the surface of the gray. The latter are chiefly efferent fibers directed toward lower motor fields (fig. 93, layer 5; '42, figs. 18-21, 24, 29-45, 47).
The intermediate neuropil in the middle depth of the alba contains the largest and most complicated fields of this tissue. It is very unevenly developed, in some places scarcely recognizable and in others of wide extent and thickness. Its characteristics are especially well seen in the corpus striatum (figs. 98, 99, 108, 109), thalamus ('396, fig. 81; '42, figs. 71, 81), and tectum opticum (figs. 93, layer 2, 101; '42, figs. 26, 30, 32, 79-83). Many of the long tracts lie within this layer and have been differentiated from it. Most of the specific nuclei of higher animals, including the outer gray layers of the tectum, have been formed by migration of neuroblasts from the central gray outward into this layer. Here we find much of the apparatus of local reflexes and their organization into the larger, innate patterns of behavior.
The superficial neuropil is a subpial sheet of dendritic and axonal terminals, in some places absent, in others very elaborately organized. Here are some of the most highly specialized mechanisms of correlation in the amphibian brain, from which specific nuclei of higher brains have been developed. Notable examples are seen in the interpeduncular neuropil (chap, xiv) and the ventrolateral neuropil of the cerebral peduncle described in the next section. This neuropil seems to be a more sensitive medium for strictly individual adjustments (conditioning) than the deeper neuropil, but of this there is no experimental evidence. This hypothesis is supported by the fact that in higher animals cerebral cortex develops within this layer and apparently by neurobiotactic influence emanating from it.

In the first synapses observed in embryogenesis numerous axonic terminals converge to activate a single final common path (Coghill, '29, p. 13), This is the first step in the elaboration of neuropil. As differentiation advances, neurons are segregated to serve the several modalities of sense and the several systems of synergic muscles, and these systems are interconnected by central correlating elements. In no case are these connections made by an isolated «hain of neurons in one-to-one contact between receptor and effector. The central terminals of afferent fibers from different sense organs are widely spread and intermingled. Dendrites of the correlating cells branch widely in this common receptive field, and the axons of some of them again branch widely in a motor field, thus activating neurons of the several motor systems. This arrangement is perfectly adapted to evoke mass movement of the entire musculature from any kind of sensory stimulation, and this is, indeed, the only activity observed in early embryonic stages.

It is the rare exception rather than the rule for a peripheral sensory fiber to effect functional connection directly with a peripheral motor neuron. One or more correlating elements are interpolated; and, as differentiation advances, the number of these correlating neurons is enormously increased in both sensory and motor zones and also in the intervening intermediate zone. The axons of these intrinsic elements ramify widely in all directions, and, as a rule, they or collaterals from them interweave to form the closely knit fabric which pervades both gray and white substance everywhere. This is the axonic neuropil, within the meshes of which dendrites of neurons ramify widely. These axons are unmyelinated, and every contact with a dendrite or a cell body is a synaptic junction. Eveiy axon contacts many dendrites, and every dendrite has contact with many axons, and these may come from near or from very remote parts.

All neuropil is a synaptic field, and, since in these amphibian brains it is an almost continuous fabric spread throughout the bram, its action is fundamentally integrative. But it is more than this. It is germinative tissue, the matrix from which much specialized structure of higher brains has been differentiated. It is activated locally or diffusely by every nervous impulse that passes through the substance of the brain, and these impulses may irradiate for longer or shorter distances in directions determined by the trend of the nerve fibers of which it is composed. This web of conductors is relatively undifferentiated, but it is by no means homogeneous or equipotential, for each area of neuropil has characteristic structure and its own pattern of peripheral and central connections. Many of these fibers, which take long courses, may connect particular areas of gray substance either in dispersed arrangement or assembled as recognizable tracts. In fact, we find all gradations between an almost homogeneous web of neuropil and long well-fasciculated tracts of unmyelinated or myelinated fibers. Many of these long fibers have collateral connections throughout their length; others are well-insulated conductors between origin and termination.

The web of neuropil shows remarkable diversity in different regions. In some places it is greatly reduced, as, for instance, in the ventral funiculi, where the alba is densely filled with myelinated fibers; in other places there are local concentrations of dendrites and finest axons so dense that in ordinary haematoxylin or carmine preparations they appear as darker fields, the Punksuhstanz of the early histologists and the glomeruli of the olfactory bulb and interpeduncular nucleus (chap. xiv). When some of these fields are analyzed, it is found that their fibrous connections conform with those of specific "nuclei" of higher brains. In Ambly stoma such a field is not a "nucleus," for it contains no cell bodies; but examination of the corresponding area in anurans and reptiles may show all stages in the differentiation of a true nucleus by migration of cell bodies from the central gray outward into the alba ('27, p. 232). Such a series of phylogenetic changes can be readily followed in the corpus striatum, the geniculate bodies, the interpeduncular nucleus, and many other places and particularly in the tectum opticum and the pallial fields of the cerebral hemispheres.

In phylogeny the long, well-organized tracts seem to have been formed by a concentration of the fibers of the neuropil. The diffuse neuropil is probably the primordial form, going back to the earliest evolutionary stages of nervous differentiation (coelenterates) . Local reflex arcs and specific associational tracts have been gradually differentiated within it, that is, integration precedes local specialization, the total pattern antedates the partial patterns. In ontogeny, especially of higher animals, this history may not be recapitulated, and tracts serving local reflexes may appear very early; but in Amblystoma, even in the adult stage, there are few tracts which are compactly fasciculated and free from functional connection with the surrounding neuropil. Most of the long, well-fasciculated tracts have some myelinated fibers that seem to have functional connections only at their ends; but these are accompanied by others, which are without myelin and are provided with numberless collaterals tied into the enveloping neuropil. There is, accordingly, a seepage of nervous influence along the entire length of these tracts.

From the primordial diffuse neuropil, differentiation advanced in two divergent directions. One of these, as just pointed out, led to the elaboration of the stable architectural framework of nuclei and tracts, the description of which comprises the larger part of current neuroanatomy. This IS the heritable structure, which determines the basic patterns of those components of behavior which are common to all members of the species. The second derivative of the primordial neuropil is the apparatus of individually modifiable behavior — conditioning, learning, and ultimately the highly specialized associational tissues of the cerebral cortex. In both phylogeny and ontogeny, differentiation of the first type precedes that of the second. Primitive animals and younger developmental stages exhibit more stable and predictable patterns of behavior; and the more labile patterns are acquired later. In Amblystoma both these types of differentiation are at low levels, but they are sufficiently advanced to be clearly recognizable. Tissue differentiation is further advanced in the white substance than in the underlying gray.

During the course of this progressive specialization of tissue, the primordial integrative function of the neuropil is preserved and elaborated. The mechanism employed is seen in its most generalized form in the deep periventricular neuropil, layer 1 of the preceding analysis. This is a close-meshed web of finest axons, within which all cell bodies are imbedded. It is everywhere present, providing continuous activation (or potential activation) of every neuron, summation, reinforcement, or inhibition of whatever activities may be going on in the more superficial layers and affecting the general excitatory state of the whole central nervous system. It receives fibers from all sensory and motor fields and seems to be the basic apparatus of integration. In man this type of tissue survives in the periventricular gray of the diencephalon, and, as suggested by Wallenberg ('31), it probably plays an important part in determining the disposition and temperament of the individual ('34rt, pp. 241, 245). This unspecialized tissue may serve the most general totalizing function. From it there have been derived the complicated mechanisms for synthesizing the separate experiences and organizing them in adaptive patterns — a process of differentiation which culminates in the human cerebral cortex. The relations of neuropil to reflex arcs are discussed in chapter vi.

The relative abundance of myelinated fibers is a rough indicator of the relation between stable and labile types of performance. Thus the myelinated white substance is relatively greater in the spinal cord than in the brain, and the ratio of myelinated to unmyelinated tissue diminishes as we pass forward in the brain, as is well illustrated by a published series of Weigert sections drawn from a single specimen (no. IIC) from the olfactory bulb to the spinal cord ('10, figs. 8-21; '25, figs. 2-9; '44&, figs. 1-6). This is because the myelinated fibers, most of which are long conductors of through traffic, tend to be compactly arranged, with relatively scanty collateral connections with the neuropil.

In the gray substance there is a similar increment m relative amount of neuropil as we pass forward from spinal cord to hemispheres, the higher levels being specialized for correlation, association, conditioning, and integration and the lower levels for stabilized total activities and reflexes. In the phylogenetic series this principle takes a form which can be quantitatively expressed as Economo's coefiicient of the ratio between total gray substance and the mass of the nerve cells contained within it. The lower the animal species in the scale, the greater is the mass of the cells compared with the gray substance. This law may be expressed in the converse form: Higher animals have a larger proportion of neuropil in the gray substance, thus giving them capacity for conditioning and other individually acquired patterns of behavior (Economo, '26, '29). No mathematical precision should be claimed for these laws, but they do express the general trend of our experience. For additional data and critical comment see von Bonin on page 64 of the monograph by Bucy and others ('44).

The properties of amphibian neuropil have been discussed in several places, in addition to the summary already cited ('42, p. 199). In one of these papers ('33c?) the geniculate neuropil of Necturus was described, and later ('42, p. 280) the quite different arrangement of Anibly stoma (compare the corresponding structure of the frog, '25), together with a full account of the neuropil of the optic tectum and its connections (here, again, comparison with the more differentiated structure of the frog is instructive). In connection with a general survey of the neuropil of Necturus, its peculiar relations in the pallial part of the hemisphere were discussed in four papers ('336, p. 176; '336?, '34, '34a). Amblystoma is similar ('27), though the details have not been fully explored. In the pallial field the four layers of neuropil tend to merge into a single apparatus of association. This is corticogenetic tissue within which the earliest phases of incipient differentiation of laminated cortex can be recognized in the hippocampal area, as described in chapter vii.

Some samples of the appearance of elective Golgi impregnations of amphibian neuropil are shown in the accompanying illustrations, and many others are in the literature. The related morphological and physiological problems can best be presented in the form of illustrative examples, one of which is the striatal neuropil (chap, vii), another the interpeduncular neuropil described in chapter xiv, and still another in the superficial ventrolateral neuropil of the peduncle, the area ventrolateralis pedunculi, which will next be described.

THE VENTROLATERAL PEDUNCULAR NEUROPIL

The "peduncle" in the restricted sense as here defined, together with the adjoining tegmentum, is the chief central motor pool of the skeletal muscles. Efferent fibers from it are among the first to appear in the upper brain stem in embryogenesis, descending in the primary motor path, which is the precursor of the fasciculus longitudinalis medialis, and activating the musculature of the trunk. A second efferent path, which appears very early, goes out directly to the periphery through the oculomotor nerve. The first sensory influence to act upon this pool comes from the optic tectum through the posterior commissure in the S-reaction stage. In subsequent stages it is entered by fibers from many other sources, including the basal optic tract (fig. 14) and the olfacto-peduncular and strio-peduncular tracts (fig. 6). In the adult animal, motor impulses issuing from this pool are probably concerned primarily with synergic activation of large masses of muscle, notably those concerned with locomotion and conjugate movements of the eyes. Control of the movements directly involved in seizing and swallowing food is believed to be chiefly in the isthmic tegmentum, and this apparatus matures later n ontogeny.

The chief synaptic connections of the great motor pool of the peduncle are in the intermediate, deep, and periventricular layers of neuropil. External to these is a ventrolateral band of dense neuropil extending from the root of the III nerve forward along the entire length of the peduncle. In former papers this has been termed "area lateralis tegmenti" and "nucleus ectomamillaris," but both these names now seem to me inappropriate ('4'-2, p. 233). This band receives at the anterior end all the terminals of the large basal optic tract (figs. U, 94) and, at the posterior end, terminals of the secondary and tertiary ascending ^'isceral sensory and gustatory tracts (figs. 8, 23). The terminals of these two systems of fibers are intimately interlaced, and among them are dendritic arborizations of tlie underlying cells of the peduncle into which optic, olfactory, and visceral systems of nervous impulses converge and through which the combined effect is transmitted in the outgoing motor pathways. These seem to be the primary components of this neuropil, and to them are added axonic terminals from a wide variety of sources, the most notal)le of which are sketched in figure '23 (compare Necturus, '34, p. 103 and fig. 4).

The peduncular dendrites, which arborize within this neuropil, include some from the nucleus of the oculomotor nerve (fig. 24; '42, p. 275), so that here fibers of the basal optic tract coming directly from the retina may synapse with peripheral motor neurons — one of the rare examples of a two-neuron arc witli only one synapse between the peripheral receptor and the effector, though even here there are at least two additional synapses in this arc within the retina. Other peduncular neurons may transmit retinal excitations downward through the ventral tegmental fascicles to all lower motor fields.

At its anterior end this neuropil is connected by fibers conducting in both directions with the dorsal (mamillary) part of tlie hypothalamus, as in Necturus ('346, fig. 3), and also with its ventral (infundibular) part (fig. 23; "42, pp. 226, 227). This is an extension of the visceral-gustatory tract into the hypothalamus, and it may be that some of the visceral fibers pass without interruption through the peduncular neuropil to the hypothalamus, though this has not been satisfactorily demonstrated. In some fishes such a direct connection is evident and large; in mammals the course of the ascending visceral-gustatory path is still uncertain.

Other connections of this neuropil, as shown in figure 23, include terminals of the olfacto-peduncular tract from the anterior olfactory nucleus, probably the nervus terminalis (observed in Necturus) fibers from the tectum, pretectal nucleus, dorsal and ventral thalamus, and terminals of the fasciculus retroflexus (p. 202 and fig. 20). These terminals of fibers from surprisingly diverse sources are all closely interwoven with one anotlier and with the terminal dendrites of peduncular neurons, a unique arrangement occurring, so far as known, only in Amphibia. I have indulged in the following speculations upon its possible physiological significance.

In the first place, it is clear that this curious tissue is either an undifferentiated primordium of a number of structures which are separately differentiated in more specialized brains, or else it is a retrograde fusion of several such structures. The former supposition seems more probable, for the phylogenetic history of one of its components is easily read ('25, pp. 443-49). In Necturus there are no cell bodies directly associated with this neuropil; in Amblystoma a few cells have migrated out of the gray layer to its border (fig. 24) ; and in the frog the optic component of the neuropil is separate and surrounded by a spherical shell of cell bodies, which is a true basal optic nucleus (Gaupp, '99, p. 54, fig. 19). This nucleus attains large size and complexity in some reptiles, as described by Slianklin ('3.'3) and others. In the mammals it retains its individuality as the chief terminus of basal optic fibers, though other fibers of this system are rather widely spreatl in the surrounding tegmentum (Gillilan, '41). The history of the visceral and other components of the amphibian ventrolateral neuropil has not yet been written.

In my recent description ('42) of the optic system of Amblystoma, special attention was given to the central distribution of thick and thin fibers from the retina, and this was followed ('42, p. 295) by some speculations about its physiological significance. In development the thick fibers appear first, and the number of thin fibers is enormously increased in later stages, particularly at the time of metamorphosis. Thick fibers conduct more rapidly than thin fibers, and this time factor may play an important part in the central analysis of mixed retinal excitation.

Both thick and thin fibers end in the thalamus, pretectal nucleus, and tectum; and in each of these fields their terminals are mingled, not segregated. The optic fibers to the basal peduncular neuropil are moderately thick; upon retinal stimulation this field, accordingly, is the first to be activated, and its excitation of the underlying peduncular neurons may precede any influence upon these cells from the longer paths by way of the tectum and thalamus. Figure 22 shows in continuous lines the major optic tracts and the motor paths from the peduncle, and in broken lines some internuncial connections. The thick fibers which descend from the tectum and thalamus are myelinated; some of them are crossed in the posterior commissure and the commissure of the tuberculum posterius, and some are uncrossed. They connect primarily with the descending pathway in the ventral tegmental fascicles (f.r.t.). The thinner correlating fibers take other pathways, and they may make synaptic connection with peduncular neurons in any one or all of the four layers of the neuropil. The efl'erent path may be to low er motor centers through the ventral tegmental fascicles or to the muscles of the eyeball through the III nerve.

Figures 24 and 93 are diagrammatic transverse sections at the level of the III nuclei and the middle of the tectum, illustrating some of the tecto-peduncular connections. These fibers are all of medium or thin caliber and, for the most part, unmyelinated. The dendrites of the peduncular neurons ramify widely throughout the entire thickness of the brain wall, and few, if any, of them are in physiological relation with any single one of the afferent systems of fibers. Each of them may be activated by any or all of these systems. The only significant localized specialized tissue here seems to be in the neuropil, where the texture is different in the four layers and where all peduncular neurons spread their dendrites in all these layers. Since the movements which are activated from this motor pool are not disorderly convulsions, it is evident that the discriminative and well co-ordinated responses which follow its excitation are not ordered primarily by the arrangement in space of the motor elements. There are differences in the structural arrangements of the synaptic junctions, though these are not so pronounced as in most other animals, and there may be chemical and other factors yet to be determined. As I have elsewhere pointed out, a time factor can be recognized by physiological experiment, and its structural indicator can be identified histologically because large fibers have a faster rate of conduction than small fibers and the interpolation of synaptic junctions in a nervous pathway retards transmission.

In the structural setup before us it may be inferred that the first result of a retinal excitation is the activation of the entire tectum, pretectal nucleus, and dorsal thalamus through the tliick myelinated fibers of the optic nerve and tract and also of the ventrolateral neuropil of the peduncle through the basal optic tract. This is presumably a generalized nonspecific effect, and it will come to motor expression, first, through the basal tract, for this is the shortest path. The resting state is changed to a state of excitation in both the peduncular gray and the peripheral musculature with which it is connected. This is immediately followed by volleys from the tectum, pretectal nucleus, and thalamus tlirough the myelinated tectoand thalamo-peduncular tracts; and this may contribute a spatial factor determined by the position of the exciting object in the visual field and the sector of the tectum upon which this local stimulus is projected. The first overt movement, accordingly, is an orientation of the body and the eyeballs with reference to the source of stimulus. After an appreciable time the smaller fibers from the retina deliver their volleys, and the small fibers of the correlating tracts are activated. These deliver to the peduncle, not unmixed or purely visual impulses but discharges, fired or inhibited, as the case may be, by the existing excitatory state of the correlating apparatus: and this, in its turn, is determined by numberless nonvisual features of the total situation, present and past. If, for instance, one of the visual or nonvisual components is fatigued, this will affect the pattern of tectal discharge.

In salamanders the delay between the preliminary orientation of the body and the consummation of the reaction may be, and commonly is, very long — a period of tension, which in a man we would call "attention" and which Coghill called the "regarding" reaction (p. 78; Coghill, '33, Paper XI, p. 334; '36). During this period an inconceivably complicated resolution of forces is in process within the central apparatus of adjustment. Inhibition plays an important role here, and it may be that the chief function of the ventrolateral superficial neuropil of the peduncle is control of the inhibitory phase of these activities comparable with tliat suggested in chapter xiv for the specific interpeduncular neuropil. Of the details of these adjustments our knowledge is scanty, but some hints may be gathered from urther examination of the structure involved; and here, to simplify the problem, we shall confine tJie discussion to the superficial peduncular neuropil.

This neuropil is interpolated "in series" in the visual-motor path of the basal optic tract, and this short circuit is connected "in parallel" with the longer visualmotor circuits by way of the tectum and thalamus. The latter are very much larger and more complicated, and they evidently comprise the major part of the apparatus by which behavior is regulated by visual experience. The basal optic system seems to be ancillary to the major system and to be related with it in two quite different ways, first, as a nonspecific primary activator, as already explained, and, second (and subsequently), as a device for modifying or "inflecting" (to borrow Arnold Gesell's expression) the standard patterns of behavior by intercurrent influence of present activity in other fields, including, perhaps, conditioning and the individuation of hitherto unaccustomed types of response. This second feature involves further consideration of the nonvisual components. These have already been listed, and some of them are shown in figure 23.

The largest components of this peculiar neuropil are the optic and tlie visceralgustatory systems, and their fibers end exclusively here. This is why these are regarded as the primary components; all other systems of entering fibers are spread more or less widely also in the deeper layers of peduncular neuropil. In the visceralgustatory system this neuropil is inserted in series in the ascending pathway toward the hypothalamus, and there is probably a parallel system of conduction with no synapse in the peduncle (fig. 8). As seen in tliis figure, peduncular neurons activated from this neuropil send axons around the tuberculum posterius into the hypothalamus; and there is a return path from both dorsal (mamillary) and ventral (infundibular) parts of the hypothalamus to this neuropil and contiguous parts of the peduncle (figs. 18, 'TIS). All visceral-gustatory influences which reach the hypothalamus are, accordingly, previously modified or "inflected" in the superior visceral nucleus or in the peduncle. The excitatory state of the hypothalamus, in turn, affects all activity in the peduncle.

This sort of circular activity is everywhere present in these urodele brains, working through the diffuse periventricular and deep neuropil and in many places also through specially differentiated tracts (p. 76). This is the primordial apparatus of integration, and in its more differentiated form it is part of the apparatus by which individuated partial patterns of local reflexes are kept in appropriate relationship with the larger total patterns of behavior.

In a recent survey ('44o) of tlie optic and visceral nervous circuits here under consideration, several different patterns of linkage of the component units were listed. These include direct activation of eye muscles through the basal optic tract, similar activation of the skeletal musculature, indirect activation of either or both of these sj^stems of muscles by visual stimuli through the tectum and thalamus, direct activation of either or both systems of muscles through the visceral-gustatory path, indirect visceral -gustatory action by way of the superior visceral nucleus, with or without intercurrent influence of the tectum upon this nucleus by way of the tecto-bulbar tract, and the variable effects of concurrent discharge of many other systems of fibers into each of the centers of adjustment involved. Since eacli of these patterns of linkage is complex and the structural units themselves are not simple, it is evident that in actual performance the number of ways in which the known units may be combined and recombined is practically unlimited. The simplest possible activity of stimulus-response type involves a central resolution of forces in an equilibrated dynamic system of inconceivable complexity. Oversimplification of the problem will not hasten its solution.

The preceding description illustrates the relations of a specific field of specialized neuropil to other parts of the brain with known functions. These visible connections exhibit a structural arrangement which can be interpreted as putative pathways of transmission of the several components of a complex action system, some concerned with stable reflex patterns and some with more labile individually acquired components. The relations of these two classes of components of the action system to each other and of local units to the integrated whole present the major problems of neurology.

Book - The brain of the tiger salamander 3

Chapter III Histological Structure

General Histology

The Neuropil