Book - The brain of the tiger salamander 20

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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 XX The Systems of Fibers

THE principles of classification and nomenclature of nerve fibers here employed are explained on page 9. A systematic account of them is very difficult because of their dispersed arrangement, their many deviations from the familiar mammalian pattern, and a cumbersome and confused nomenclature; yet gratifying success has attended efforts to resolve the amphibian tissue and discover mammalian homologies or their primordia. It is not our purpose in this chapter to give a comprehensive list of the tracts of Amblystoma that have been identified and named. References are given to the literature in which incomplete lists have been published and to pages of this book where particular systems of fibers are described. To these there are added descriptions of a few other systems of fibers of special importance and complexity.

Lists of tracts of Necturus have been published for the medulla oblongata ('14rt, '30), midbrain and thalamus ('17), and forebrain ('336). At the end of the latter paper is an alphabetical list of abbreviations which includes many tracts, with references to previous descriptions and synonyms. For Amblystoma a similar list of abbreviations ('36, pp. 309-12) includes many tracts of the cerebral peduncle, with page references to the text. Other lists of tracts of Amblystoma have been published ('27, '36, '39, '396, '42), and Bindewald ('14) gave in tabular form a useful summary of previous descriptions of the parts of the amphibian forebrain and related fiber tracts, with homologies. The tracts of several species of urodeles have been described in the literature cited on page 11. The following systems of fibers of Amblystoma have been more or less completely described: peripheral nerves (Coghill, '02), olfactory tracts ('27, p. 282; Bindewald, '14; compare Necturus, '336, p. 124, '346), optic tracts and tectal connections ('25, '41, '42; compare Necturus, '41a), postoptic (supra-optic) commissures ('42, p. 219; Necturus, '41a, p. 513), the visceral-gustatory system ('44a, '446), cerebellum (Larsell, '20, '32), tegmental fascicles ('36).

In this work under appropriate headings there are lists of tracts



related with the several parts of the brain, accompanied by diagrams, and some classified lists, including the lemniscus and visceralgustatory systems (chap, xi); connections of the interpeduncular nucleus (chap, xiv) ; some tectal connections (chap, xvi) ; stria medullaris thalami, fasciculus retroflexus, stria terminalis, and fornix (chap, xviii); some olfactory connections (chap, xix); and in the next following sections the basal forebrain bundles, tegmental fascicles, f. tegmentalis profundus, and the commissural systems.


In a survey of the forebrains of fishes, attention was called to the important part played in morphogenesis by these great systems of longitudinal conduction ('22a, p. 175), and here this theme has received further consideration in chapter vii. These bundles of Necturus were described in comparison with those of other amphibians ('336, p. 166), and their arrangement in Amblystoma has been illustrated ('27, p. 285; '36, p. 335 and figs. 5, 6; '396, p. 533 and fig. 1). In sections of these brains the basal bundles are the most obvious landmarks, extending from the hemispheres backward through the brain into the medulla oblongata. Some of their fibers, in other species, have been described as extending into the ventral funiculi of the spinal cord, but our material yields no evidence that in Amblystoma any fibers which descend from the hemispheres in these bundles go back without interruption farther than the level of the roots of the VIII nerve. These bundles contain ascending and descending fibers, many of the latter decussating in the anterior commissure. In addition to these main lines of through descending traffic, many other kinds of fibers enter these bundles, and these connections will now be summarized.

The fibers of these bundles are arranged in three groups of fascicles as indicated in figures 19, 20, 21, 101; but these groups are not sharply separated, for there is much interchange of fibers among them. Their descending fibers are roughly comparable with the subcortical components of the mammalian extra-pyramidal systems, and the more dorsal ascending fibers of the thalamo-frontal tract (figs. 19, 101, correspond with the thalamo-striatal system. The dorsal group of fascicles, the lateral forebrain bundle (i. lateralis telencepha\\,, connects with the lateral wall of the hemisphere and has dorsal and ventral components with different connections. The ventral group, the medial forebrain bundle (f. medialis telencephali,, connects with the medial and ventral walls and also has dorsal and ventral components related, respectively, with the dorsal and ventral parts of the hypothalamus. The third group of fascicles comprises the tractus olfacto-peduncularis (tr.ol.ped.), which lies between the lateral and the medial bundles, connecting the anterior olfactory nucleus and the head of the caudate with the hypothalamus, peduncle, and interpeduncular nucleus.

Lateral forebrain bundle. — Most of these fibers are connected with the primordial corpus striatum and amygdala, and some of them with the piriform area. The descending fibers correspond with the human ansa lenticularis, and most of them are well myelinated. They appear earlier in ontogeny than do the ascending fibers. The dorsal and ventral fascicles of this bundle interchange fibers and in Necturus are not clearly separable ('336, p. 170). They are connected mainly, though not exclusively, with the dorsal and ventral nuclei, respectively, of the corpus striatum, as described in chapter vii. When first studied, this relationship could not be demonstrated ('27, p. 286), but subsequent examination of sagittal sections ('36, p. 335) convinces me that this incomplete separation marks the beginning of differentiation of the globus pallidus. In 1927 the ventral fascicles were named tr. strio-peduncularis ('27, p. 287) and the dorsal fascicles tr. strio-tegmentalis, but these terms are inappropriate because both of them terminate in the peduncle and also in the tegmentum, though in different areas and evidently with different physiological import. The dorsal fascicles enter group (9) of the tegmental fascicles and the ventral fascicles enter group (10), as described below.

The descending fibers of the ventral fascicles { and (10) of the figures) arise from the large cells distributed throughout the striatal gray. Their terminals are widely spread in the ventral parts of the peduncle and isthmic tegmentum. They d6 not extend so far spinal ward as do some fibers of the dorsal fascicles. In the analysis of the tegmental fascicles ('36) these fibers comprise group (10). They make their chief synaptic connections with the large cells of the peduncle and tegmentum, the axons of which enter the ventral tegmental fascicles of groups (4), (5), and (6), descending to the medulla oblongata, and some of which continue in the f . longitudinalis medialis into the spinal cord. This connection is regarded as provision for cerebral control of mass movements of the trunk, limbs, and eyeballs.

The descending fibers of the dorsal fascicles ( and tegmen


tal fascicles of group (9)) extend farther spinalward with different distribution. They give numberless collaterals to the thalamus, the dorsal part of the peduncle, and the dorsal and isthmic tegmentum; and posteriorly they turn laterally into the trigeminal tegmentum dorsally of its motor zone (figs. 29, 30). The chief synaptic connections of these fibers are not with the primary motor column but with areas of intermediate-zone type. They appear later in embryonic development than do the ventral fascicles, and their functions are believed to be cerebral control and conditioning of bulbar reflexes.

Another important descending component of these dorsal fascicles is the recently described tr. strio-tectalis (figs. 11, 101,; '42, p. 262) and the associated tr. strio-pretectalis (figs. 14, 101, These fibers probably arise from the corpus striatum, though this has not been demonstrated. They end by wide arborizations in the optic tectum, pretectal nucleus, and geniculate neuropil of the thalamus.

Some fibers descend from the dorsal striatal nucleus in company with more from the amygdala in that component of the stria terminalis complex known as the dorsal olfactory projection tract (, as described on page 242.

The most noteworthy ascending system of fibers of the lateral bundle is tr. thalamo-frontalis { These slender unmyelinated axons arise from cells of the middle sector of the dorsal thalamus and descend in several small compact strands to the dorsal fascicles of the lateral forebrain bundle, within which they turn forward. As in Necturus ('336, p. 170), they probably all end in the striatal neuropil, though their terminals in Ambly stoma have not been described. These are precursors of the ascending thalamic radiations of mammals. They arise from the undifferentiated nucleus sensitivus of the thalamus, and no evidence has been seen of any separation among them of projection tracts related with different functional systems. The course of these fibers, so far as known, has been fully illustrated (figs. 3, 15, 30-34, 75, 95, 101, 102, 103; '396, figs. 1, 7, 8, 13-17).

Medial forebrain bundle. — Most of the fibers of this large system are unmyelinated, passing in both directions between the olfactory bulb and the medioventral parts of the olfactory area of the hemisphere and the preoptic nucleus and hypothalamus. Many of these fibers decussate in the ventral part of the anterior commissure ridge. Analysis of this complex is possible only with the aid of elective Golgi impregnations, and there are few of these in our collection of Amblystoma sections. The Necturus material has been more instruc


live, and the following summary is based on data from both species (Amblystoma, figs. 6, ^25, 71, 75; '^27, p. 285; '396, p. 534 and figs. 1, 79; Necturus, '336, pp. 173, 261 and fig. 14; '346). The medial bundle is incompletely separable into dorsal and ventral fascicles connected, respectively, with the dorsal and ventral parts of the hypothalamus. Both fascicles have extensive connections with the preoptic nucleus, and both contain descending and ascending fibers. Posteriorly of the anterior commissure ridge, thick collaterals of fibers of both groups of fascicles ascend to the habenula in tr. olfacto-habenularis of the stria medullaris (p. 257; '396, fig. 79).

The dorsal fascicles contain descending fibers arising in the septum (tr. septo-hypothalamicus) and in the primordium hippocampi (precommissural fornix, p. 254), also some components of the stria terminalis system (p. 256). These are accompanied by some ascending fibers the connections of which are not clear.

The ventral fascicles contain secondary olfactory fibers from the bulb and some of higher order from the ventral and medial sectors of the anterior olfactory nucleus. These fibers are distributed to the preoptic nucleus and ventral part of the hypothalamus and are accompanied by many preoptico-hypothalamic fibers. Slender filaments of the nervus terminalis are spread among these fascicles for their entire length. The large hypophysial tract arises from neurons which are widely scattered throughout the hypothalamic field reached by both the ventral and the dorsal fascicles of the ventral bundle. In the floor of the preoptic recess there is a median fascicle of unmyelinated fibers, among which are a few with myelin sheaths. This is tr. preopticus (p. 244).

Very slender unmyelinated fibers arise from small cells at the extreme posterior end of the ventral hypothalamus and ascend for undetermined distances in the ventral fascicles. There are doubtless other ascending fibers, but their courses have not been recorded.

Olfacto-peduncular tract. — This is a well-defined round bundle, lying between the lateral and the medial bundles and less myelinated than the former (figs. 18, 21, 25-30, 53, 54, 59, 72, 95-99, 101; '27, p. 286; '36, p. 336; '396, p. 534, figs. 1, 79, 80). Its fibers arise from the head of the caudate nucleus and neighboring parts and distribute to the dorsal part of the hypothalamus, ventral part of the peduncle, and interpeduncular nucleus. The hypothalamic connection allies this tract with the dorsal fascicles of the medial forebrain bundle, the peduncular connection with the ventral fascicles of the


lateral forebrain bundle. The connection with the interpeduncular nucleus is extensive and physiologically distinctive. This tract is the most direct connection between the olfactory area and the primary motor field of the peduncle and the interpeduncular nucleus. It is an internuncial of intermediate-zone type, with the olfactory influence predominant.


The strio-thalamic and strio-peduncular components of the lateral forebrain bundle activate large neurons of the ventral thalamus and peduncle, and the axons of these cells transmit impulses downward to the tegmentum from the isthmus to the spinal cord. These descending fibers form the most conspicuous components of a series of ventral tegmental fascicles, some of which at lower levels are assembled in the f. longitudinalis medialis (figs. 6, 18, 91). Ventrally and dorsally of these are many other tegmental fascicles, composed chiefly of descending fibers; and still farther dorsally are the ascending lemniscus systems (fig. 14). The analysis of the composition of these longitudinal fascicles has been very diflficult, yet this knowledge is essential for an understanding of the brain stem.

The topographical analysis of the brain stem of 1935 was followed in 1936 by a reconnaissance survey of the related fibers in the peduncle and tegmentum. The more obvious and constant bundles of longitudinal fibers were enumerated as ten fascicles or groups of fascicles ('36, p. 303), and the specific tracts represented in these and some other bundles, so far as then known, were Hsted ('36, pp. 33446). These fascicles are divided into a ventral and a dorsal series, the former including fascicles (1) to (6) and part of (10); the latter, (7) to (10). Figures 91, 92, 94, and 102-4 are here reproduced from the paper of 1936; compare figures 30-33 and 101.

This classification was arbitrary for descriptive purposes only and included only the most clearly defined and constant fascicles bordering the gray substance. The symbol for each group is an Arabic number inclosed in parenthesis. Externally of these ten groups, in the intermediate alba, there are other less well-defined fascicles with a larger proportion of unmyelinated fibers. These are imbedded in much neuropil and are more variable than the deeper fascicles. Superficially in the subpial neuropil there is another series of fascicles, chiefly of unmyelinated fibers, some of which form recognizable tracts. Dorsally of all these fascicles are the lemniscus systems; these and the ascending secondary visceral-gustatory tract are described elsewhere in this work. Further study has yielded additional details about the composition of the ten groups of tegmental fascicles. These and some other fascicles and tracts of this region are here analyzed as far as their composition is now known. Most of these fibers are descending. There are ascending fibers also, but our material has yielded little information about them.

During the preliminary study it was anticipated that each group of tegmental fascicles would prove to be composed chiefly or wholly of fibers of a single tract or related group of tracts, as are the lemniscus systems. This proves not to be the case, for most of these bundles are mixtures of fibers of diverse sorts from unexpectedly widely separated sources. The reasons for their fasciculation in the pattern observed are not clear. Some specific tracts, like the mamillointerpeduncular and strio-tegmental, are fairly clearly segregated (in groups (2) and (9) in the cases mentioned), but most of the bundles are heterogeneous mixtures. The pattern of fasciculation seems to be determined more by the ultimate destination of the fibers than by their nuclei of origin. In Weigert and especially in reduced silver preparations these fascicles, particularly those bordering the gray, are clearly defined for long distances; but Golgi sections show that there is much anastomosis among them and that there are numberless unmyelinated fibers which are not fasciculated but spread diffusely in the alba.

As the lateral forebrain bundles recurve dorsally at the anterodorsal border of the peduncle (figs. 6, 16, 101, 102; '36, figs. 5, 6), sagittal Weigert sections show that the compact bundles of myelinated fibers disintegrate, with diffuse spread of the fibers in the alba of the posterior part of the thalamus, dorsal tegmentum, and peduncle. Some of these fibers are reassembled farther spinal ward in fascicles (9) and (10). Golgi sections show a similar dispersal of the thinner unmyelinated fibers. Reduced silver preparations, however, reveal many slender fascicles of unmyelinated fibers which traverse this region without loss of their individuality. It is evident that these bundles, like most of the other tegmental fascicles, are mixtures of fibers of diverse distribution and physiological significance. In transverse Golgi sections of adult brains, in which the myelinated fibers of the deeper fascicles are slightly darkened and the unmyelinated fibers are electively impregnated, it is seen that the proportion of unmyelinated fibers is greater in the dorsal fascicles than in the ven


tral and that almost all fibers bordering the gray in the ventral fascicles are myelinated.

Ventromedian fascicles (1). — These are limited to the midbrain and isthmus, extending spinalward from the commissure of the tuberculum posterius. Most of their fibers decussate obliquely in the ventral commissure. They comprise 5 to 12 well-defined bundles of rather thick myelinated fibers arranged close to the gray under the ventral angle of the ventricle, with some admixture of unmyelinated fibers. These fibers are derived from various sources, and they are variously distributed. The bundles as definite anatomical entities are assembled only in the space bounded approximately by the levels of the nuclei of the III and IV cranial nerves. This distance is greater in urodeles than in most other vertebrates, and the arrangement of these fascicles found in Amblystoma has not been described in any other species. In Necturus reduction of the optic system involves corresponding shrinkage of these median fascicles ('36, p. 348), though the general plan is similar. Three components of these fascicles have been distinguished.

a) Tractus tecto-bulbaris et spinahs cruciatus, pars anterior (fig. 12, tr.t.h.c.l.). — This is by far the largest component of the group. Its fibers, chiefly myelinated, descend from the tectum and turn spinalward in the mid-plane, here decussating in component 4 of the commissure of the tuberculum posterius, and posteriorly of this very obhquely in the ventral commissure ('36, pp. 303, 330, figs. 2, 7). At the level of the nucleus of the IV nerve they turn laterally and descend in tr. tecto-bulbaris et spinalis within the ventromedial alba of the medulla oblongata. The course of this tract, as seen in horizontal sections, is shown in figures 27-36 (for complete description see '36, p. 340, and '42, p. 268). The posterior division of this tract (figs. 27-36, tr.t.h.c.2.) does not enter the tegmental fascicles but decussates transversely, at the level of the nucleus of the IV nerve.

b) Tractus pedunculo-bulbaris ventralis cruciatus ('36, figs. 2, 7, f.m.t.{l)hr, '39&, p. 582 and figs. 23, 24).— These fibers arise from the ventral thalamus and peduncle and enter ventral fascicles (1). Here they decussate obliquely, mingled with those of the crossed tectobulbar tract; and after crossing they separate from the latter to descend in the ventrolateral fasciculi of the medulla oblongata, and some of them in the f. longitudinalis medialis, as described in the references cited.

c) Tractus pedunculo-tegmentalis cruciatus ('36, figs. 2, 7, fjn.t.{l)c.). — These fibers enter the ventral fascicles from the peduncle and perhaps also from the ventral thalamus. At the posterior end of these fascicles they do not turn laterally with the others but continue posteriorly and dorsally as one of the components of the f. tegmentalis profundus. They arborize in the deeper layers of neuropil of the isthmic tegmentum.

V entromedian fascicles {2) . — These fascicles of thin unmyelinated fibers comprise tr. mamillo-interpeduncularis, lying laterally and ventrally of those of group (1). These are shown in figure 19 and separating from tr.mam.teg. in figure 21. They assemble in the periventricular neuropil of the ventral lobe of the dorsal part of the hypothalamus. Figure 27 {tr.mam.inp.{2)) shows them converging into a ventricular protuberance of this lobe, which, immediately dorsally of this level, extends across the mid-plane to join the corresponding structure of the opposite side at the attenuated ventral border of the commissure of the tuberculum posterius (compare the median section, fig. 2C). Here some of these fibers decussate as component 1 of this commissure, as shown in figure 28. The crossed and uncrossed fibers continue dorsalward and then spinalward, recurving around the cerebral flexure at the extreme ventral surface (figs. 29, 30), to end in open arborizations within the interpeduncular neuropil. Their entire course in the larva is shown in a published diagram ('396, fig. 22; other details are also illustrated in that paper — figs. 6-9, 35, 41, 42, 57-61). These fibers comprise the whole of group (2) of the tegmental fascicles, described in 1936. They do not form a compact bundle but are rather loosely spread, and their courses can be followed only in favorable Golgi sections. Unlike the other connections of the dorsal part of the hypothalamus, these fibers are aggregated in the deep periventricular neuropil, and evidently their physiological properties are radically different, from those of the mamillo-peduncular and mamillo-tegmental tracts.

V entral fascicles {3). — The chief component is tr. mamillo-tegmentalis (fig. 21; '36, figs. 3, 8; '42, fig. 3). These thin myelinated and unmyelinated fibers pass, probably in both directions, between the dorsal hypothalamus and the tegmentum in close association with similar fibers related with the peduncle and thalamus. The mammalian equivalents of the complex are found in the mamillo-thalamic and mamillo-peduncular tracts and the mamillary peduncle.


Aniblystoma has no differentiated corpus inamillare; its primordiuni is in the dorsal part of the hypothahiniiis, from which efferent fibers go out dorsalward to the peduncle, forward to the ventral thalamus, and backward to the tegmentum. They are dispersed in the alba of the hypothalamus, and as they leave it those for the thalamus and tegmentum accumulate rostrally of those for the peduncle. Some of them decussate in components 1 and 2 of the commissure of the tuberculum posterius ('36, fig. 2). Afferent fibers to the hypothalamus are known to be present in the mamillo-peduncular and mamillo-thalamic tracts, and this may be true also of the mamillo-tegmental.

Mamillo-peduncular fibers have wide distribution in the alba of the nucleus of the tuberculum posterius, including the neuropil of the area ventrolateralis pedunculi (figs. 6, 18, 27-30, tr.mam.ped.; '36, p. 338, figs. 3, 8; '39/;, p. 338, figs. 6-12, 22, 35, 42, 89; '42, fig. 39; Necturus, '336, p. 246; '346, p. 422). The accompanying pedunculomamillary fibers transmit visceral sensory, gustatory, and optic impulses received by the area ventrolateraHs pedunculi to the hypothalamus. The mamillo-thalamic tract and the accompanying thalamo-mamillary tract put the mamillary region into reciprocal relations with the anterior part of the thalamus, as has been fully illustrated ('396, figs. 22, 35; '42, fig. 39; Necturus, '336, p. 247; '346, p. 423, figs. 2, 8, 9).

The fibers of tr. mamillo-tegmentalis arise in company with those of the two preceding systems and form a loose fascicle rostrally of those of tr. mamillo-peduncularis (figs. 27-31; '36, p. 338, figs. 3, 8-21; '396, p. 552, fig. 43; '42, fig. 3). Most of these fibers enter ventral tegmental fascicles of group (3), as shown in figures 92 and 94, some of them first decussating in the two ventral components of the commissure of the tuberculum posterius. These fascicles lie ventrolaterally of those of group (4), which contains thicker and more heavily myelinated fibers from^ the ventral hypothalamus. They terminate in the alba of the isthmic tegmentum, chiefly through the f. tegmentalis profundus. Mingled with these hypothalamic fibers are some thicker well-myelinated fibers of tr. pedunculo-tegmentalis. Some of the latter, and probably some of the hypothalamic fibers also, descend for long distances in the f . longitudinalis medialis.

Ventral fascicles (4). — These are mixed bundles derived chiefly from the postoptic commissure. The largest component is tr. hypothalamo-peduncularis et tegmentalis (figs. 21, 23), which contains both descending and ascending fibers. In the first description these were designated simply as "fibers from the postoptic commissure" ('36, p. 304, figs. 3, 8, 19, po.{Jt-)); but now their connections are better known ('42, p. 226 and fig. 3), thanks to the fact that they mature very early in ontogeny and so can be seen in young stages, despite their dispersed arrangement. A few elective Golgi impregnations confirm these findings in the adult.

Fibers related with the entire ventral part of the hypothalamus converge into the commissura tuberis in the caudal part of the postoptic commissure complex. After crossing here in diffuse arrangement, they recurve around the tuberculum posterius, where most of them spread and end in the peduncle (tr. hypothalamo-peduncularis) . Others descend in ventral tegmental fascicles of group (4) as tr. hypothalamo-tegmentalis. Only the longer fibers of the latter tract are entered on the drawings of the horizontal sections (figs. 25-32, marked tr.hy.teg.{Jf.), or simply (4)). Figure 32 cuts these fibers (^) at the most dorsal level of their arched course through the peduncle; compare their projection on the sagittal plane ('36, fig. 3, po.{4-)). Other similar fibers extend dorsally from both ventral and dorsal parts of the hypothalamus to enter the midbrain without decussation in the chiasma ridge ('42, figs. 22, 23, 39). Some of these decussate in component 2 of the commissure of the tuberculum posterius, but most of them are uncrossed.

This system of fibers is evidently the main descending pathway from the ventral hypothalamus and neuropil of the chiasma ridge to the motor field of the peduncle and tegmentum. The associated fibers connected with the dorsal part of the hypothalamus probably shoiild be classed with the mamillo-peduncular system, though they are not included in the bundles so designated. This more dorsal system, and perhaps the ventral system also, contain some fibers which are afferent to the hypothalamus, though most of them evidently are efferent. Most of the longer fibers of this system which reach the tegmentum descend in bundles of group (4), and here they are joined by thalamo-tegmental and pedunculo-tegmental fibers ('42, p. 225 and fig. 4). The longest fibers enter the f. longitudinalis medialis (figs. 27, 28), and it is uncertain whether these are of hypothalamic or of peduncular origin.

Most of the hypothalamic fibers of groups (3) and (4) pass into f. tegmentalis profundus, and most of these end in the neuropil


related with the central nucleus of the isthmic tegmentum. These are doubtless precursors of the hypothalamic component of the mammalian f. longitudinalis dorsalis of Schiitz.

Tegmental fascicles of groups (5) to (10) contain fibers of diverse origin, and there is much anastomosis of their finer fibers. The coarser fibers, however, are well fasciculated in an arrangement which seems to be determined primarily by the terminal distribution of the descending systems. Their analysis has been clarified by the previously published embryological studies; see particularly the general survey ('396) and for the decussating systems the description of the postoptic commissure of the adult ('42). The data upon which this summary of the composition of the several fascicles is based are to be found mainly in the two papers just cited.

Ventral fascicles (5). — This is a group of large fascicles mainly composed of fibers (chiefly myelinated) descending from the ventral thalamus and peduncle. Most of them are uncrossed, but some decussate in company with those of tr. tecto-bulbaris cruciatus in the ventromedial fascicles of group (1) ('396, p. 546 and fig. ^S,f.v.t.{5)). They arise from all parts of the ventral thalamus and peduncle, and most of them terminate in the alba of the isthmic and trigeminal tegmentum, where they are in synaptic contact with the large cells of this region. The thicker and more heavily myelinated fibers are closely fasciculated laterally of the ventral angle of the ventricle, and many of these descend for undetermined distances in the f . longitudinalis medialis. The origin of these fibers from the ventral thalamus is shown in figures 31 and 32. At those levels similar thick fibers enter fascicles (5) from large cells of the peduncle, but they are not drawn in these figures. Figures 6 and 18 show fibers entering the f. longitudinalis medialis from large cells of the dorsal and ventral parts of the peduncle. The former (shown but not named in fig. 32) are in synaptic connection with terminals of the posterior commissure and doubtless correspond with the mammalian nucleus of Darkschewitsch (p. 217). The more ventral large cells of the peduncle (fig. 31) correspond with the interstitial nucleus of Cajal. In Amblystoma, unlike Necturus and most other vertebrates, the f. longitudinalis medialis is definitely organized only spinalward from the isthmus ('36, p. 334).

The fascicles of group (5) also receive some fibers from other systems, as will appear below. For their courses as seen in horizontal sections see figures 27-32; in transverse sections, figures 91-94 and '36, figures 9-16; in sagittal sections, '36, figures 3, 18-21. The thalamoand pedunculo-tegmental fibers of this group are similar in many respects to the crossed and uncrossed fibers of tr. thalamo-tegmentalis ventralis of groups (4), (6), and (8). Those of group (5) arise chiefly from the peduncle, the others from the thalamus. In the aggregate these thick descending paths comprise the chief final common paths from the cerebrum to the peripheral neuromotor apparatus of the primary activities of the skeletal musculature, notably those of locomotion and feeding. In early larval development these long fibers from the peduncle and ventral thalamus are among the first to appear in the cerebrum. Their adult distribution in the several tegmental fascicles seems to be determined primarily not by the arrangement in space of the groups of cells from which they arise but by the lower motor fields into which they discharge their nervous impulses. Those in groups (4), (5), and (6) descend more medially and ventrally, the longest in the f. longitudinalis medialis. These longer fibers evidently activate the trunk and limbs. Collaterals of these fibers and accompanying shorter fibers end throughout the isthmic and bulbar tegmentum ('396, figs. 84, 93), thus insuring coordination of head movements with those of the trunk and limbs. These more ventromedial fibers, accordingly, comprise the final common paths of fundamental mass movements, total patterns of behavior.

The more dorsal fibers of group (8), accompanied by the striotegmental fibers of group (9), descend along the outer border of the isthmic gray (figs. 29, 30) to end far laterally in the white substance of the isthmic and trigeminal tegmentum. Here the fundamental reflexes of the musculature of the head, concerned primarily with feeding, are organized. These more dorsal fibers, crossed and uncrossed, have innumerable terminals and collaterals in the peduncle, dorsal tegmentum, and lower tegmental fields ('396, figs. 79-82).

Ventral fascicles (6). — These fascicles, like those of the fourth group, were originally designated as fibers from the postoptic commissure ('36, figs. 4, 8, 18, po.{6)). They are now known to be composed chiefly of two systems of thick myelinated and unmyelinated fibers which decussate in the postoptic commissure, derived, respectively, from the tectum and the ventral thalamus — tr. tecto-thalamicus et hypothalamicus cruciatus posterior ('42, p. 221 and fig. 5) and tr, thalamo-tegmentalis ventralis cruciatus ('42, p. 225 and fig. 4). The coarsest fibers of these fascicles, probably including some of both


the systems just mentioned, traverse the whole length of the peduncle and isthmic tegmentum, and some of them enter the f . longitudinalis medialis. Figure 3^2 cuts these fascicles a few sections ventrally of their most dorsal course as they arch across the peduncle; compare the sagittal section (fig. 103).

The thickest fibers of tegmental fascicles (4), (6), and (8) decussate in the dorsal and posterodorsal part of the postoptic commissure, those of group (8) being the most dorsal fibers of this complex at their crossing in the mid-plane. Most of the latter enter group (8), but some enter group (6). These are fibers of tr. tecto-thalamicus et hypothalamicus cruciatus posterior of my former descriptions, but these longer fibers are properly called tr. tecto-peduncularis et tegmentalis cruciatus. Posteriorly and ventrally of these at the decussation are similar coarse fibers of tr. thalamo-tegmen talis ventralis cruciatus, which cross in more dispersed arrangement posteriorly and ventrally of the preceding system. These enter the three groups of fascicles, (4), (6), and (8). In the horizontal sections here illustrated the courses of these fibers are in some places not very clearly seen, for they are not separately fasciculated. Their courses are indicated on the drawings as determined (where doubt arises) by comparison with sections in other planes and with the early larvae, where their courses are clear. It should be noted that this crossed system of ventral thalamo-tegmental fibers is accompanied by similar uncrossed fibers, most of which enter fascicles of group (8). These were first described in the larva ('39&, p. 546, figs. 13-16, 81) and in the adult are seen here in figures 30-33 as fibers joining bundle (8) from the thalamus. There is also a broad uncrossed connection from both ventral and dorsal parts of the thalamus to the tegmentum by tr. thalamotegmentalis rectus (figs. 30-34, These fibers pass from the thalamic gray to the pial surface of the tegmentum and end here in the superficial neuropil and obviously have physiological properties different from the deeper fibers of groups (4), (6), and (8), which make synaptic contacts with the large cells of the tegmentum in the intermediate and deep neuropil.

Dor ml fascicles (7). — Two systems of fibers have been identified in these fascicles: {a) one from the dorsal thalamus and dorsal tegmentum and (6) one from the tectum.

a) Tractus tegmento-isthmialis (fig. 21). — This tract was first described as the chief component of fascicles of group (7) under the name tr. tegmento-bulbaris ('36, p. 334, figs. 4, 6, f.d.t.{7); '39&, p. 584). That name is inappropriate for two reasons, first, because few of its fibers reach the bulb and, second, because there is a large tegmento-bulbar tract more ventrally (figs. 27, 28, 29, tr.tecj.h.). The tract here under consideration passes from the posterior part of the dorsal thalamus, eminence of the posterior commissure, and dorsal tegmentum into fascicles of group (7). Its fibers enter f. tegmentalis profundus and end in relation with the small cells of the central nucleus of the isthmic tegmentum. Its entire course is shown in figures 29-33, here marked (7). The isthmic tegmentum is regarded as the pool within which the bulbar reflexes concerned with feeding are organized, and this tract probably plays a critical role in l)ringing to this center the appropriate aft'erents from the intermediate zone. This tract carries only part of the efl^erent fibers from the dorsal tegmentum. Many of these fibers pass directly ventrally to enter the peduncle and lower tegmental levels uncrossed or with decussation in the ventral commissure. Others ascend or descend in various other tegmental fascicles.

b) Tractus tecto-bulbaris rectus. — It has recently been found that these fibers leave the tectum by various courses and that many of those from the anterior part of the tectum enter fascicles of group (7), from which they separate in the isthmus to enter the medulla oblongata in company with other tectal fibers (p. 225). These are the fibers which in 1936 were followed from fascicles (7) into the bulb.

Dorsal fascicles (8). — This conspicuous group contains thick and thin fibers, many of which are well myelinated. Those which come from the postoptic commissure ('36, pp. 304, 338, figs. 4, 8, 17, 18, ■po.{8)) comprise two quite separate systems, («) from the tectum and {})) from the ventral thalanms. The latter are joined by many uncrossed fibers of the same system.

a) Tractus tecto-thalamicus et hypothahinueus cruciatus i)osterior. — The coarsest and most heavily myelinated fibers of the complex so named in earlier papers, after decussation in the postoptic commissure, enter fascicles of group (8) and distribute to the dorsal, isthmic, and trigeminal tegmentum ('42, p. 221). Other shorter fibers enter fascicles (6) to reach the isthmus, and others end in tlie peduncle and hypothalamus. These longer and coarser fibers to the tegmentum were variously interpreted and named in my earlier papers, but good elective impregnations have now clarified their connections as tr. tecto-tegmentalis cruciatus.

b) Trjictus thalamo-tegmentalis ventralis.— As previously men


tioned, this large and complicated system of crossed and uncrossed fibers was overlooked until early larval stages revealed its essential features ('396, p. 546). These thick fibers arise in all parts of the ventral thalamus. Some of them decussate in the postoptic commissure and enter fascicles (4), (6), and (8). Others descend uncro sed in all these groups and also in group (5). Golgi sections show that these tracts have numberless terminals and collaterals throughout their lengths. The thicker fibers of group (8) course more dorsally and terminate more laterally in the isthmic and trigeminal tegmentum.

The composition of group (8) as it leaves the postoptic commissure is shown in figures 25 and 26 and its further course in figures 27-33. Figures 30-33 show uncrossed fibers from the ventral thalamus joining the bundles of crossed fibers (compare '39&, fig. 2, and see the concluding comment about fascicles of group (9)).

Dorsal fascicles (.9). — These are composed chiefly of tr. strio-tegmentalis (fig. 101; '27, p. 287; '36, pp. 304, 335; '396, fig. 79), which is the dorsal component of the lateral forebrain bundle, passing from the dorsal nucleus of the primordial corpus striatum to the trigeminal tegmentum. Some of these fibers decussate in the anterior commissure, and a small number descend as far as the level of the VII nerve roots, with terminals and collaterals along the way.

These fascicles are easily followed in both horizontal and transverse sections (figs. ^S-SS, and (9)), and especially clearly in sagittal sections (figs. 16, 19, 21, 72, 102; '36, fig. 5; '39, figs. 3, 7-12; '396, figs. 1, 79). There is interchange of fibers between these fascicles and those of groups (7a) and (8), and the thicker fibers of both groups turn laterally in the trigeminal tegmentum close to the gray and spray out dorsally of the V and VII nuclei, where they apparently activate both large and small tegmental cells. As the fascicles of group (9) reach the posterior end of the ventral thalamus, they turn sharply dorsad, parallel witlv the limiting sulcus, s, around the anterodorsal border of the nucleus of the tuberculum posterius. Here those of (9) lie ventrolaterally of (8) (figs. 16, 91-94; '36, fig. 5), with much anastomosis between them. The finer fibers of both groups spread widely in the neuropil of the peduncle and dorsal and isthmic tegmentum, and some descend in fascicles of group (7). The primary function of groups (7), (8), and (9) appears to be the control of the feeding musculature of the jaws and hyoid from the corpus striatum, thalamus, and tectum.

Fascicles {10). — These comprise the greater part of the ventral fascicles of the lateral forebrain bundles (figs. 26-32, 101, 102, and (10)). In 1927 (p. 287) this was termed tr. strio-peduncularis, but it now appears that these fibers arborize not only in the peduncle but also throughout the length of the isthmic tegmentum, though they do not extend as far spinalward as those of group (9). This tr. strio-peduncularis et tegmentalis arises from the ventral nucleus of the corpus striatum, decussates partially in the anterior commissure, traverses the ventral thalamus, and in the peduncle breaks up into a number of small fascicles which are widely distributed among the ventral and dorsal tegmental fascicles. Its fibers are less myelinated than those of group (9), and, in general, they distribute in the alba more ventrally and laterally (figs. 92, 102, 103; '36, pp. 304, 336, fig. 6; '396, fig. 79; '42, figs. 2, 17); compare the preceding description of the lateral forebrain bundles.

Most of the numbered groups of fascicles which have just been listed lie in the deeper layers of the alba close to the gray. Superficially of these and in part mingled with them are many dispersed fibers, loosely fasciculated in variable arrangements. These include many of the shorter tracts mentioned in the descriptions of the several regions, such as thalamo-tegmental, pedunculo-tegmental, tectotegmental, and tegmento-bulbar tracts. The well-defined tr. olfactopeduncularis also belongs in this series. The thalamo-tegmental fibers are very numerous, and some of them are well fasciculated. From both dorsal and ventral thalamus uncrossed fibers stream backward to the isthmus, some deeply and some superficially — tr. thalamotegmentalis rectus (figs. 15, 31-34, These are in addition to the thick fibers from the ventral thalamus, which enter tegmental fascicles (6) and (8), some uncrossed and some decussating in the postoptic commissure (fig. 17, and c. '3.96, p. 546). Analysis of the fibers from the dorsal thalamus which decussate in this commissure is very difficult. The earlier accounts are confused and inaccurate, but the essential features are now clear, as described in the next chapter.


This name has been given to a loosely arranged sheet of fibers which borders the gray in the tegmental region of the midbrain and the isthmus. Most of these fibers are unmyelinated and are part of an


extensive tegmento-peduncular and tegmento-bulbar system of internuncials, passing obliquely from the tegmental to the motor field; but fibers of various other systems are mingled with these. They comprise important components of the periventricular, deep and intermediate tegmental neuropil, with a dorsoventral trend. At the outer border of the gray they are especially numerous, and here some of the components are fasciculated as anatomically separate tracts for part of their courses. Other more dispersed components also are named as tracts in cases in which their terminal connections are revealed by elective impregnations. Many of these fibers decussate in the ventral commissure in the tuberculum posterius and spinalward of it through the medulla oblongata. In the midbrain and upper rhombencephalon there are numberless short crossed and uncrossed fibers, some strictly commissural between the tegmental fields and larger numbers passing from the tegmentum obliquely rostrad or caudad, with or without decussation, to other parts of the motor field.

The list below includes all the tracts which have been identified in this complex, and in addition to these there are diffuse connections through the neuropil with all neighboring regions. This summary is based upon what has been seen in both Amblystoma and Necturus.

Brachium conjunctivuni (p. 176 and fig. 10). — This is one of the largest and most compact components of the complex, though rarely impregnated in our material. From the cerebellum to its decussation it accompanies the isthmic sulcus, as is well shown in figure 71.

Tertiary visceral-gu.s-tatory tract (p. 169 and figs. 8, *23). — These fibers accompany those of the brachium conjunctivuni and are indistinguishable from them except in elective impregnations.

Tegmento-inter peduncular and inter pedunculo-teginental fibers (pp. 199, 201 and figs. 19, 60-68, 79, 80, 83, 84).— These very numerous fibers, passing in both directions between the interpeduncular nucleus and the isthmic and trigeminal tegmentum, are diffusely spread in the neuropil of the gray and the deeper layers of alba.

Tractus tegmento-pedimcularis (p. 215 and fig. 18). — These fibers pass from the dorsal and isthmic tegmentum to the peduncle, dispersed at all depths of gray and white substance. Some of them decussate in the ventral commissure in the vicinity of the fovea isthmi. They accompany similar fibers of tr. tecto-peduncularis (fig. 24).

Tractus tegmento-isthmialis (p. 283 and fig. 21).— These fibers from the dorsal to the isthmic tegmentum form a large part of the dorsal tegmental fascicles of group (7), but many of them are mingled with more dispersed fibers of tr, tecto-tegmentalis.

Tractus tecto-tegmentalis (fig. 21).— Fibers from all parts of the superior and inferior colliculus stream backward into the isthmic tegmentum. Most of the former enter dorsal tegmental fascicles of group (7) in company with longer fibers of tr. tecto-bulbaris rectus (p. 225), but many of them descend dispersed in the neuropil (fig. 79).

Tractus pedunculo-tegmentalis crticiatus.— These, as described above (p. 278), enter the ventral median tegmental fascicles of group (1) from the peduncle and, after decussation here, pass to the isthmic tegmentum by way of f . tegmentalis profundus.

Tractus 7namillo-tegmentalis .—^ome fibers from the dorsal hypothalamus reach the isthmic tegmentum accompanying the superficial tr. mamillo-cerebellaris (p. 170). Others take a deeper course in ventral tegmental fascicles of group (3) and f . tegmentalis profundus, as described on page 279.

Chapter XXI The Commissures


THE commissures are in two series, dorsally and ventrally of the ventricles. The fibers which cross the mid-plane are of two sorts: (1) some are strictly commissural, connecting corresponding regions of the two sides; (1) most of them are decussations, like the optic chiasma, connecting dissimilar regions. Those of the dorsal series include fibers of correlation which arise and terminate within the sensory zone and also connections between the sensory zone and other zones. Those of the ventral series are concerned in the main with motor co-ordination on the two sides of the bod3% some passing from sensory and intermediate zones to the motor zone, others lying wholly within the motor zone, and both sorts being accompanied by uncrossed fibers.

In addition to the crossed systems of correlation and co-ordination to which reference has just been made, it is a noteworthy fact that the main lines of fore-and-aft ascending and descending conduction in the brains of all vertebrates decussate, so that the adjusting centers for organs on the right side of the body are in the left side of the brain and vice versa. This applies especially to the apparatus of somatic adjustment, but not so generally to the visceral systems. Most of the fibers of the ascending secondary visceral-gustatory tract {tr.v.a.) and of the olfacto-hypothalamic tracts are uncrossed. The proprioceptive systems are both crossed and uncrossed.

The reason for the decussation of the major conduction pathways of the somatic sensori-motor systems has puzzled neurologists for many years, and fantastic theories have been expressed, some bearing the names of great masters — Wundt, Flechsig, Cajal, and others. This extensive literature has been reviewed by Jacobsohn-Lask ('24), who, instead of elaborating a theory in terms of the highly specialized human brain, like his predecessors, reviews the entire history of the evolution of the nervous system, from its first appearance in coelenterates, in relation to the bilateral symmetry of the body. None of these speculations have yielded satisfying conclusions.

A survey of the commissures and decussations of all vertebrate brains shows that some of them, like the posterior commissure and optic chiasma, hold constant positions throughout the series, while others vary widely in position and composition. It is evident that in the latter cases the site of crossing of a particular system of fibers is determined by the topographic arrangement of the parts to be connected. These arrangements are widely diversified in the lower groups of vertebrates and the crossing fibers tend to take the shortest available pathways. But when the course of a particular decussating tract has been established in an ancestral species, the original site of the decussation may be retained, despite great changes in the relative sizes of parts of the brain in phylogenetic descendants of the ancestral form, so that the tract may take a circuitous course to reach its decussation, as illustrated by some components of the postoptic complex in fishes. This conservatism may be accounted for by the fact that the decussating fibers may have collateral connections at any part of the course both before and after crossing. These considerations suggest that great caution should be observed in using the actual sites of crossing of the various systems of fibers as criteria of homology. Some of them are very stable; in other instances fibers with similar origins and terminations may cross in surprisingly different places, as illustrated by the various courses taken by fibers of the hippocampal commissure.

Primitively the roof plate of the brain, unlike the floor plate, was membranous, and the locations of commissures which invade it are determined by the functional requirements of the organs differentiated below it in the various species of animals. In the telencephalon the topographic arrangements of parts are extremely variable. In all cases there is a wide interruption of the commissures at the site of the stem-hemisphere fissure, paraphysis, dorsal sac, and their derivatives. In the diencephalon the habenular commissure is separated by the pineal recess from the commissura tecti, and the latter is continuous spinal ward with the posterior commissure and the com. tecti of the mesencephalon. In cyclostomes the middle part of the mesencephalic tectum is a membranous chorioid plexus, and the com. tecti is here interrupted.

At the posterior end of the tectum its commissure is continuous with a thin sheet of crossed and uncrossed fibers in the anterior medullary velum, and this, in turn, with the massive cerebellar commissures. Between the cerebellum and the calamus scriptorius the


roof is a membranous chorioid plexus except in some fishes, in which the large acousticolateral areas fuse over the ventricle, with some decussating fibers. In the region of the calamus there is a dorsal crossing of visceral sensory fibers (com. infima of Haller) and of somatic sensory fibers between the funicular nuclei. This dorsal commissure, reduced in size, is present through the entire length of the spinal cord.

The ventral series of commissures is concentrated in the anterior commissure of the telencephalon, the chiasma ridge of the diencephalon, the so-called ansulate commissure of the midbrain; and posteriorly of this it extends continuously as the ventral commissure of the rhombencephalon and spinal cord. The ventral decussations of Necturus from the tuberculum posterius to the spinal cord were described in 1930 ('30, p. 89) and the other commissures in papers before and after that date.

The arrangement of the telencephalic commissures of the Amphibia differs radically from that of all animals higher in the scale. As seen in median section (fig. 2), the anterior and hippocampal commissures do not cross in or above the lamina terminalis but in a high anterior commissure ridge which projects upward from the floor posteriorly of the interventricular foramina. This ridge is separated from the lamina terminalis by the wide precommissural recess, or aula, which is the vestibule of the interventricular foramina (fig. IB). Both pallial and subpallial parts of the hemispheres contribute fibers to the commissures in this ridge, and many fibers from the olfactory and hippocampal areas also cross in the habenular commissure.

The peculiar arrangement of the anterior and hippocampal commissures, the lamina terminahs, and the related chorioid plexuses of Ambly stoma has been described and illustrated ('35). It is explained by the topography at the di-telencephalic junction and the presence of unusually wide interventricular foramina. The explanation of this topography, in turn, is to be^ sought in the phylogenetic ancestors of existing Amphibia. In Protopterus as described by Rudebeck ('45), pallial formation is confined to the lateral wall of the hemisphere, and the hippocampal commissure passes down behind the foramen to cross in close association with the anterior commissure in essentially the same way as in amphibians. In view of the close similarity of development of the brains of amphibians and lungfishes, it is probable that the ancestral amphibian resembled Protopterus in this region.

The arrangement of myelinated fibers in all the commissures of Amblystoma as seen in the median plane is shown in figure 2C. The components of these commissures will now be summarized, with references to more detailed descriptions.


Commissura hippocampi. — These fibers converge from all parts of the hippocampal area to its ventral border, from which they descend behind the foramen to cross in the dorsal part of the anterior commissure ridge (figs. 96-99). Many of these fibers join a mixed longitudinal fascicle, termed "fimbria," which borders the primordium hippocampi for its entire length at the ventromedial margin. Another fascicle, composed exclusively of commissural fibers, assembles at the ventrolateral border of the primordium. Most of these fibers are unmyelinated. Weigert sections show a few brilliantly stained myelinated fibers scattered among them.

Accompanying this commissural bundle as it leaves the hippocampus are other similar fibers of tractus cortico-thalamicus medialis and tr. cortico-habenularis medialis. Some of the latter cross in the habenular commissure and return to the hippocampal area of the opposite side, this being the com. pallii posterior (figs. 32, 34, 71, 72, 76).

Commissura hahenularum {com. superior). — The analysis of the stria medullaris as detailed in chapter xviii reveals the following components of the habenular commissure: (1) The most anterior member is the com. pallii posterior (fig. 20, no. 8). (2) Posteriorly of this the com. superior telencephali includes components 3, 4, 5, and 6 of the diagram, viz., tr. olfacto-habenularis anterior, tr. cortico-habenularis lateralis, and tr. amygdalo-habenularis. (3) Still more posteriorly are crossing fibers of uncertain connections, some of which may come from other components of the stria medullaris. There are doubtless strictly commissural fibers connecting the habenulae of the two sides. The commissural fibers from the hemisphere are accompanied by uncrossed fibers. It is possible that these tracts are accompanied by crossed and uncrossed fibers, which pass from the habenular nuclei forward into the hemispheres, but this has not been demonstrated. Commissura tecti dieiicephali.— Most of these fibers are commissural, connecting the two pretectal nuclei. Some tecto-habenular and habenulo-tectal fibers decussate here.


Commissura posterior. — This is the primary pathway from the anterior part of the optic tectum to the motor zone of the midbrain. Its decussating fibers appear very early in embryogenesis, accompanied by uncrossed fibers from the tectum and eminence of the posterior commissure. In the adult many of the commissural fibers end in this eminence. Crossed and uncrossed fibers spread widely in the posterior part of the thalamus, the nucleus of the tuberculum posterius, and the dorsal tegmentum. The largest fascicles connect with the big cells of the nucleus of Darkschewitsch (p. 217).

Commissura tecti mesencephali. — This thin sheet of crossing fibers is continuous between the posterior commissure and the anterior medullary velum. It contains thin and thick fibers (many of the latter myelinated) which spread widely through all layers of the tectum. Most of these seem to be commissural between the tecti of the two sides, but no satisfactory analysis has been recorded. Some fibers of the mesencephalic root of the V nerve apparently decussate here, but, if so, the number is small.

Commissures of the anterior medullary velum. — Most of the fibers in the velum are longitudinal — tr. tecto-cerebellaris — and some of these may decussate here. The most constant and noteworthy component is the decussation of the IV nerve roots ('36, p. 342; '42, p. 255). The velum contains cells and fibers of the mesencephalic V root, and some of these may decussate here. Our preparations give no clear evidence of crossed fibers of this root; if present, the number is certainly not large.

Cerebellar commissures. — Larsell's analysis of these commissures is confirmed. The two systems are quite distinct. (1) The com. cerebelli is related with the median body of the cerebellum, including decussating fibers of tr. spino-cerebellaris, sensory root fibers of the V nerve, secondary trigeminal fibers from the superior sensory V nucleus in the auricle, and commissural fibers between these nuclei and between the two sides of the corpus cerebelli. (2) The com. vestibulo-lateralis cerebelli is a more dispersed system of fibers related with the vestibular and lateral-line centers of adjustment in the auricles. It is composed of root fibers of the VIII nerve and secondary fibers of both vestibular and lateral-hne systems. There are doubtless also commissural fibers between the two auricles. None of these fibers make significant connections with the median body of the cerebellum through which they pass. Their terminal relations are with auricular tissue which is the primordium of the floccular part of the mammahan flocculonodular lobe.

Commu-sura infima Halleri. — This is a decussation of the fascicuH soHtarii at the calamus scriptorius, containing both root fibers and secondary fibers of the visceral-gustatory system and doubtless also commissural fibers between the two commissural nuclei of this system.

Commifisure of the funieular nuclei. — Intimately associated with the preceding are commissural fibers between the nuclei of the dorsal funiculi in the calamus region, with which decussating fibers are mingled. This dorsal commissure of somatic sensory fibers is extended, reduced in size, downward through the entire length of the spinal cord. There is some evidence that the visceral sensory com. infima is also represented in the cord (p. 125).

This completes the summary of the dorsal commissures. We now turn to the ventral series, beginning, as before, at its anterior end.


The complex com. anterior occupies the entire anterior commissure ridge except its dorsal border. Its largest components are the partial decussations of the medial forebrain bundles below and the lateral forebrain bundles above (figs. 25-28). Associated with these fibers are others, including the com. amygdalarum (which is part of the stria terminalis system, p. 256), some fascicles of the nervus terminalis, and a dispersed decussation between the olfactory fields of the anterior parts of the hemispheres ('396, fig. 21).

The crossing fibers of the anterior commissure ridge are enveloped by a thin layer of gray which expands laterally as the large bednuclei of the anterior commissure. The thin floor of the long preoptic recess between the anterior commissure ridge and the chiasma ridge contains the longitudinal fibers of tr. preopticus, some of which decussate here.


All fibers of the optic nerves decussate in the chiasma opticum. The crossing occupies the anterior border of the chiasma ridge (figs. 2B and 2C). At the posterior border of the chiasma there is some mingling of optic fibers with those of the postoptic commissure, but in some of our Golgi preparations the optic fibers are electively impregnated and can be separated from the others ('41, '42).



The complex com. postoptica is represented in mammals by the supra-optic commissures, but its composition is so different in urodeles and mammals that exact homologies cannot be established. Further analysis of intervening species is requisite before these relationships can be clarified.

In Amblystoma this complex includes decussating fibers derived from the superior and inferior colliculi, the entire diencephalon, the amygdala, and the subpallial olfactory field of the cerebral hemisphere. Many of these tracts have collateral connections along their courses, both before and after crossing, and are accompanied by uncrossed fibers. There are also strictly commissural fibers connecting some of these regions (for evidence of such fibers in the frog see '25, p. 480). These decussating systems connect, after crossing, with extensive fields of the intermediate and motor zones, including the strio-amygdaloid area, preoptic nucleus, hypothalamus, ventral thalamus, geniculate neuropil, dorsal tegmentum, peduncle, isthmic tegmentum, and bulbar tegmentum. The posterior part of the postoptic commissure is comparable with the com. tuberis of some other vertebrates and consists mainly of decussating fibers from the ventral part of the hypothalamus to the peduncle and interpeduncular nucleus. The direction of conduction of most of these fibers has not been clearly determined, though most of the larger systems evidently converge from the sensory zone into the motor field.

The fibers of few of these components are assembled in wellorganized tracts ; most of them are so dispersed and commingled that analysis is very difficult. Some of them are myelinated, and these tend to be assembled in recognizable tracts. These myelinated tracts, as seen in Weigert sections, were the first to be described, but their distribution after crossing baffled analysis. These tracts are accompanied by much larger numbers of unmyelinated fibers in more dispersed arrangement. The courses of some of these have been revealed by elective Golgi impregnations, and other systems have been clarified by study of the sequence of their development as published in a series of papers from 1937 to 1941. In view of the complexity of these connections and the technical difficulties encountered in their study, it is not surprising that the earlier descriptions were incomplete and not free from error. It is believed that now it is possible to present an analysis of this complex which, though still incomplete, reveals its major features.

This generalized arrangement as seen in urodeles is probably primitive and may be taken as the point of departure in the study of the postoptic systems of more specialized brains of both fishes and higher vertebrates. These commissures have been analyzed in Necturus ('41a, p, 513), where they are still more generalized. Our present knowledge of these systems of Amblystoma was summarized on pages 219-28 of the paper of 1942, with diagrams illustrating the connections of the principal tracts. All known components are assembled in the following list. Here some of the earlier names of tracts have been replaced by more accurate terms; some others are retained, though now known to be inappropriate or inadequate. In this list there are included, first, the groups of fibers which descend to the chiasma ridge from the tectum and pretectal nucleus, followed by those descending from the thalamus, next, the systems arising in the hypothalamus, and, finally, a heterogenous group with hypothalamic connections. For additional details about some of these tracts in preceding chapters consult the Index.

1. Tr actus tecto-thalamicus et hypothalamicus cruciatus anterior (fig. 12, — This anterior tectal fasciculus is a mixture of myelinated and unmyelinated fibers from the dorsomedial part of the tectum opticum and the pretectal nucleus which descend across the thalamus in company with the more anterior fascicles of the optic tract. After partial crossing in the antero ventral part of the chiasma ridge, its fibers spread in the neuropil of the chiasma ridge and hypothalamus. Some of them may reach beyond this region. This cumbersome name was applied in my earlier papers to a mixed fascicle which had not been analyzed. It can now be replaced by the names of the several tracts of which it is composed. Some of these are uncrossed (notably tr. pretecto-thalamicus) , and some fibers of two of them decussate in the chiasma ridge, nos. 2 and 3 below. The arrangement of these components as seen in horizontal sections is shown in figures 25-36.

2. Tractus tecto-hypothalamicus anterior (tr.t.hy.a.). — This is the tectal component of the preceding fasciculus; it is evidently an optic pathway to the hypothalamus (p. 224). It passes through the pretectal nucleus and is accompanied by tecto-pretectal fibers. It probably is physiologically related with no. 3.

3. Tractus pretecto-hypothalamicus (fig. 15, tr.ptJiy.). — This tract as


it leaves the pretectal nucleus is accompanied by a large tr. pretectothalamicus (p. 234 and figs. 35, 36).

4. Tr actus tecto-ihalamicu.s ei hypothalamicus cruciatus posterior (fig. 12, — This, like no. 1, is a mixed fascicle which has been analyzed. Both these names may now be discarded in favor of shorter terms — anterior and posterior tectal fascicles. The posterior fascicle arises chiefly from the nonoptic nucleus posterior tecti and the adjoining ventrolateral margin of the optic tectum, the latter region in Necturus receiving few terminals of the optic tract ('41a, p. 516). This fascicle is probably activated primarily by lemniscus, rather than optic, fibers or by a combination of the two. The tracts of which it is composed have collateral connections with the geniculate neuropil and ventral thalamus both before and after crossing. These fibers descend from the tectum parallel with those of the lateral optic tract and internally of them. So far as known they have a common origin in the tectum, but, after crossing, they take widely divergent courses. The tracts, consequently, are named according to their terminal distribution. The most important components are the following, nos. 5 and 6.

5. Tractus tecto-hypothal amicus posterior. — These are finer fibers which terminate in the postoptic neuropil and neighboring regions of the hypothalamus. They are more clearly seen in Necturus ('41a, p. 516) than in Ambly stoma.

6. Tractus tecto-tegmentalis cruciatus (fig. 12, tr.t.teg.c). — The course and distribution of the thicker fibers of the posterior tectal fascicle were not clarified until they were identified in larval stages, in which they were electively impregnated because they mature precociously. These were first recognized in early feeding larvae ('39, p. 106) and later in adult Necturus ('41a, p. 516) and Amblystoma ('42, p. 222). Some erroneous descriptions of these fibers in my earlier papers have been corrected ('39, p. 110).

The more heavily myelinated fibers of this tract decussate in the dorsal part of the chiasma ridge (figs. 2C, 12, 16, 25,, and after crossing they enter tegmental fascicles of groups (8) and (6), and in smaller numbers they are dispersed in other fascicles. The dispersed fibers spread in the peduncle. Those which enter fascicles numbered (8) take a longer and more dorsal course, distributing to the dorsal, isthmic, and trigeminal tegmentum, some of them extending as far as the level of the V nerve roots. The entire course of these fibers can be followed in the horizontal sections, figures 25-35, where they are marked before their decussation and (8)

beyond the crossing.

There are two strong systems of crossed tecto-peduncular and tecto-tegmental fibers, both of which are drawn in figure 12. The system just described descends chiefly from the nonoptic part of the tectum { and, after crossing, spreads in the peduncle by way of tegmental fascicles (6) and throughout the tegmentum by way of fascicles (8). The second system is tr. tecto-peduncularis cruciatus (tr.t.p.c), which arises in the optic part of the tectum, crosses in the commissure of the tuberculum posterius, and then spreads out in the peduncle. This well-myelinated tract is accompanied by similar fibers from the pretectal nucleus and dorsal thalamus. These two tectopeduncular systems evidently have quite different physiological significance.

Attempts to analyze the postoptic components arising in the thalamus were unsuccessful until the sequence of development of these fibers was revealed by embryological studies. These findings were then confirmed by elective Golgi impregnations of older larvae and adults. Some errors in the earlier descriptions have been corrected, and now it is possible to give a fairly complete account of both the crossed and the uncrossed fibers which diverge from the thalamus. Since the direct and crossed fibers are evidently intimately related physiologically, both series are included in the following description. Efferent fibers from the thalamus are arranged in two sharply contrasted series, which arise, respectively, from the dorsal thalamus and the ventral thalamus.

The efferent series from the dorsal thalanuis includes uncrossed fibers to the tectum, habenula, cerebral hemisphere, ventral thalamus, hypothalamus, and peduncle which need not be further considered here; but some of the other uncrossed tracts, which evidently are in reciprocal physiological relation with the crossed tracts, should be specifically mentioned. Decussating fibers from the dorsal thalamus are in two groups. The first includes thick myelinated fibers of tr. thalamo-peduncularis cruciatus (, which, as already mentioned, joins tr. tecto-peduncularis cruciatus {tr.t.p.c.) to decussate in the commissure of the tuberculum posterius as described under that caption below. The second group is a much larger number of thin unmyelinated or lightly myelinated fibers which decussate in the postoptic commissure and will next be described.


7. Tractus thalamo-hypothalamicus et peduncnlaris cruciatus {tr.ih.h.p.c.). — In the earlier descriptions of both Aniblystoma and Necturus this name was given to a large collection of unmyelinated and lightly myelinated fibers which descends in disj^ersed arrangement from the dorsal thalamus to the postoptic commissure. Their distribution beyond the decussation could not be clearly followed, and some of those descriptions now require correction. As elsewhere pointed out ('42, p. 223), this name should now be discarded because at least three quite distinct tracts are represented here and relatively few of these fibers have any connection with the peduncle. The three tracts represented in this complex (nos. 8, 9, and 10) have a common origin in the dorsal thalamus, chiefly its middle sector, and, after crossing, take widely different courses. Their decussation is posterior to that of the tectal components of the commissure in a band which is narrow dorsally and spreads ventrally through a wide area of the neuropil of the chiasma ridge (fig. 2C,

8. Tractus thalamo-hypothalamicus dorsalis cruciatus ( — These fibers, most of which are unmyelinated, cross in the middle region of the chiasma ridge (figs. 2C, 15, 25) and then spread in the hypothalamus. In figures 27-33, 95, 102, and 103 the symbol refers to the mixture of fibers of tracts 8, 9, and 10 in their descending course from the thalamus to the commissure. The fibers of nos. 9 and 10 cross dorsally of those of no. 8, though there is mingling of the fibers of the three tracts with one another and with those of surrounding decussations.

The remaining fibers of this complex, after crossing, are distributed to the tegmentum in two tracts which take parallel courses, one superficially, the other at the border of the gray. These are designated components A and B, respectively. In figures 15 and 21 the components A and B are not separately designated.

9. Tractus thalamo-tegmentalis dorsalis cruciatus A { — This tract was first described in the early feeding larva ('39, p. 116) and later in the adult ('42, p. 224). After decussation, these unmyelinated fibers ascend from the chiasma ridge parallel with the course of the descending uncrossed limb of this commissure and more superficially. Their further course is shown in figures 26-34, here marked A . In the dorsal tegmentum they turn spinal ward along the ventrolateral border of the tectum. Here they lie close to the pial surface and immediately ventrally of the lateral optic tract (fig. 94, A). In this part of their course they join an uncrossed tract with similar origin from the dorsal thalamus and similar distribution in the tegmentum — tr. thalamo-tegmentalis rectus (figs. 15, 31-34, 94, This latter tract arises from both dorsal and ventral thalamus, but only the dorsal component of it is under consideration here (fig. 21, In some Golgi preparations there is evidence that axons from the dorsal thalamus may divide, with branches entering both the uncrossed and the crossed thalamo-tegmental tracts ('42, p. 224). Evidently, the crossed and uncrossed tracts are reciprocally related physiologically.

10. Tractus thalamo-tegmentalis dorsalis cruciatus B. — This is a deep component of the same system as the preceding, receiving nearly all the myelinated fibers of no. 7 of this list. In the chiasma ridge these fibers separate from the others of the group and cross at the dorsal border of the postoptic commissure (marked B in figs. 26 and 95). Beyond the decussation they scatter widely, and most of them enter the well-myelinated tegmental fascicles of group (8), within which they may descend as far as the V nerve roots.

The two crossed tracts to the tegmentum, nos. 9 and 10 of this list, seem to have the same origin and about the same field of distribution, except that one of them {A) terminates in the superficial tegmental neuropil and the other {B) arborizes in the deep neuropil. The physiological properties of these zones of neuropil evidently are different.

11. Tractus thalamo-tegmentalis ventralis cruciatus (figs. 2C, 17, — These thick fibers (many of them well myelinated) converge into the postoptic commissure from all parts of the ventral thalamus. In this part of their course they are not fasciculated but are scattered among other similar fibers so that their courses could not be followed until they were studied embryologically. They mature early and may be impregnated with reduced silver at stages when few other fibers respond to this treatment. We also have good elective Golgi impregnations of them in later larval stages ('39, pp. 98, 120; '396, p. 546; '42, p. 225).

These fibers cross in the posterodorsal part of the chiasma ridge, mingled with those of other systems. The thickest of the myelinated fibers are crowded together at the dorsal margin of the postoptic complex. After crossing, they spread widely in the peduncle and tegmentum. Most of those from the posterior part of the ventral thalamus enter ventral tegmental fascicles of group (4), and some of these may descend in the f . longitudinalis medialis. Many fibers from the


middle and anterior parts of the ventral thalamus reach the tegmentum through fascicles of groups (6) and (8) . These crossed fibers are accompanied by many others that take similar courses without decussation.

The hypothalamic components of the postoptic commissure include, in addition to the terminals of extrinsic tracts already described, a few well-defined tracts and several other less-well-known connections.

12. Tractus hypothalamo-peduncularis et tegmentalis (figs. 18, 23, tr.hy.ped.; fig. 21, tr.hy.ieg.). — This is the most noteworthy component originating within the hypothalamus. Its fibers assemble from the whole of the ventral part of the hypothalamus and comprise the main pathway from this region to the motor field of the peduncle and tegmentum. Some of them decussate in the posterior part of the chiasma ridge ; others are uncrossed ; still others decussate in the commissure of the tuberculum posterius. They connect by terminals or collaterals with the dorsal part of the hypothalamus, ventral part of the peduncle (including the neuropil of the area ventrolateralis pedunculi), and isthmic tegmentum. The last-mentioned group includes thick fibers, some of which are well myelinated, which enter ventral tegmental fascicles (4); and some of these may take long courses in the f. longitudinalis medialis (for further description see p. 280, and '42, p. 226).

13. Olfactory projection tract (fig. 19, — This name has been given to a thin strand of unmyelinated fibers which pass in both directions between the strio-amygdaloid area and a specific nucleus at the posterior border of the chiasma ridge ('21, p. 247; '27, p. 304; '36, fig. 5). Some of these fibers decussate here.

14. Tractus pedunculo-hypothalamicus. — A large component in the posterodorsal part of the postoptic commissure was provisionally given this name, though the exact connections of its fibers could not be determined ('42, p. 227).

15. Medial forebrain bundle. — The fibers of the medial forebrain bundle are interlaced with all components of the postoptic commissure laterally of the chiasma ridge. Many of these fibers of both descending and ascending systems enter the postoptic neuropil and participate in its formation. This implies a transfer of more or less of this activity to the opposite side of the brain, but no details of specific decussational or commissural pathways have been revealed.

Postoptic neuropil. — In the mid-plane, the postoptic decussations occupy most of the chiasma ridge, and these fibers are enveloped on all sides except ventrally by a gray layer, the bed-nucleus of the postoptic commissure. This nucleus is expanded laterally. From this gray layer, richly arborized ependymal elements and dendrites of neurons are spread among the decussating fibers. Similar long dendrites enter it from all surrounding parts, including the nucleus magnocellularis, from which tr. hypophysius arises ('42, fig. 51). The entire chiasma ridge is also permeated with dense axonic neuropil which is continuous with that of surrounding parts. This neuropil receives terminals and collaterals of axons of hypothalamic neurons, medial forebrain bundles, tractus preopticus, and most of the decussating systems. The chief outflow from it seems to be by axons of the nucleus of the postoptic commissure directed into tr. hypothalamo-peduncularis. Other fibers enter the medial forebrain bundle (for further details see '42, p. 219; Necturus, '336, p. 251; 'Mb, p. 383).

This neuropil is clearly one of the major adjusting centers of the urodele brain. Situated in the center of the great olfacto-visceral field, its connections indicate that it may be activated from every correlation center of the cerebrum. Undoubtedly it plays an important part in all general visceral activities. It is equally evident that there is no provision here for localization of specific functions. This is probably the undifferentiated primordium from which some of the specialized hypothalamic nuclei of mammals have been elaborated.


The commissure of the tuberculum posterius was defined in 1917 (]). 224) as the ventral mesencephalic decussations between the infundibulum and the fovea isthmi. Posteriorly of the latter the ventral tegmental commissure extends backward without interruption through the rhombencephalon and spinal cord. In Necturus there is a very short interruption of the ventral commissural system at the fovea isthmi ('30, p. 89), but in most urodeles this gap does not appear.

The composition and arrangement of these ventral commissures are very diversified in different vertebrates. The hypothalamic connections at the anteroventral end of the commissure of the tuberculum posterius of Amblystoma are in some other vertebrates widely separated as the retro-infundibular decussations, or decussatio hy


pothalamicus posterior. The remainder of the commissure of the tubercuhim and the anterior part of the ventral tegmental commissure comprise the ansulate commissure of the literature.

In Amblystoma I have recognized four chief components of the commissure of the tubercuhim posterius ('36, p. 305 and fig. 2), arranged as shown in sagittal section in figures 81, 104; in horizontal sections in figures 28, 29, 30, 31 (component 4 being here marked, tr.t.b.c.l.); and in transverse section in figure 94 (components 2 and 3 shown here, components 1 and 4 being shown in a neighboring section, '36, fig. 11). The connections of these components are as follows :

1. This is the partial decussation of unmyelinated fibers from the dorsal (mamillary) part of the hypothalamus to the interpeduncular nucleus ('36, p. 338 and figs. 3, 8, 11-13). Most of them enter ventral tegmental fascicles of group (2) as marked tr.mam.inp., on figures 19, 27-30, 71, 81, 92, 103.

2. This component contains partly myelinated fibers from the dorsal hypothalamus to the peduncle and tegmentum, accompanied by many uncrossed fibers and by fibers conducting in the reverse direction from the peduncle to the hypothalamus (p. 278; figs. 8, 21, tr.mam.feg., 23, tr.mam.ped.). Most of them spread in the alba of the peduncle or enter ventral tegmental fascicles of group (3). Another system is tr. hypothalamo-peduncularis et tegmentalis from, the ventral part of the hypothalamus to the peduncle and tegmentum (fig. 21, ir.hy.teq. and fig. 23, tr.hy.ped.). There is interchange of all these fibers with those of component 1 and with other tegmental fascicles. They are marked tr.mam.ped., tr.mam.teg., and (3) on figures 18, 21, 23, 27-31, 79, 82, 92, 94, 103.

3. The third component contains heavily myelinated fibers from the tectum, pretectal nucleus, and dorsal thalamus to the peduncle, accompanied by many uncrossed fibers — tr. tecto-peduncularis cruciatus {tr.t.p.c.) and tr. thalamo-peduncularis cruciatus (, as elsewhere described (p. 223; '42, p. 267). These are shown on figures 12, 15, 18, 22, 29-36, 94, 103.

4. The dorsal component of this commissure is larger than the others and is composed chiefly of tecto-bulbar and tecto-spinal fibers from the anterior part of the tectum opticum, marked tr.t.b.c.l. on the figures. Its fibers after oblique decussation enter ventral tegmental fascicles of group (1) (see p. 277 and figs. 12, 27-36, 93, 94).

In addition to these four well-defined components, there are unmyelinated fibers which descend from the tectum and dorsal tegmentum through the gray and deeper layers of the alba to the region of the nucleus of the III nerve (fig. ^2, tr.t.ped.). Some of these tectopeduncular fibers decussate in the ventral commissure both before and behind the fovea isthmi. In Necturus some fibers of the nervus terminalis probably decussate in the commissure of the tuberculum posterius (McKibben, '11), and this may be true in Amblystoma.


The ventral decussations of Necturus were analyzed in the paper of 1930 (p. 89), and those of Amblystoma are similar. For the details the reader is referred to that description and to a later contribution on larval Amblystoma ('396).



This list includes onh' the works cited in the text. It is not a systematic bibliography of the amphibian nervous system. Additional references are cited in the author's papers, in the comprehensive work by Ariens Kappers, Huber, and Crosby ('36), and in numerous other publications.

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ibid., 92:93-119. . 1943a. Deficient efferent innervation of the extremities following removal

of neural crest in Amblystoma, ibid., 94:319-49. Youngstrom, K. a. 1938. Studies on the developing behavior of Anura, J. Comp.

Neurol., 68:351-79.

. 1940. A primary and a secondary somatic motor innervation in Ambly • stoma, ibid., 73:139-51. . 1944. Intramedullary sensory type ganglion cells in the spinal cord of

human embryos, ibid., 81:47-53.




All of the 113 figures are of adult or late larval Amblystoma tigrinum, except figures 86B, 86C, 111, 112, and 113 of Necturus and figure 107 of Rana pipiens. The indicated magnifications of the drawings show wide diversity in actual sizes of the specimens due to two things: in the first place, the fresh specimens vary greatly in size, and, in the second place, shrinkage during preparation may amount to as much as one-fourth of the original linear dimensions. Figures 2-85 are original drawings, and figures 1 and 86-113 are selected from previous publications, with minor alterations in some of them.

The internal structure of the brain of Necturus has been illustrated by sections drawn at close intervals in the three conventional planes ('30, '336) and by many drawings of detail in several other publications. The general plan of the brain of Amblystoma is so similar to that of Necturus that comparisons are readily made. The sections of Amblystoma have been under investigation for nearly forty years, and the exigencies of the study and of publication have required concentration upon particular topics rather than description of the brain as a whole, and no comprehensive atlas of sections has been prepared. It is the aim of this book to supply a general view, but, unfortunately, the figures necessary to illustrate it are scattered in many publications. Some figures of especial value for general orientation are included here, and references are given to others in the literature.

Three series of Weigert sections of adult A. tigrinum, prepared by the late Dr. P. S. McKibben in 1910, were selected as standards of reference. These are his numbers IIC, transverse. IC, horizontal, and C, sagittal. All were fixed in a formalinbichromate mixture, imbedded in paraffin, cut serially at 12 n, and stained on the slides. These show minimum distortion of form, excellent histological preservation, and brilliant stain of myelinated fibers, with decolorization arrested at a stage which leaves all cell bodies clearly stained. The transverse series was chosen as the type specimen, and this has been quite fully illustrated, though these drawings were published at different times. To assist the reader who may wish to assemble an atlas of this specimen, a list of all available figures of it was published ('35a, p. 241), with serial numbers of the sections pictured, and to this list the following numbers may now be added:


98 Olfactory bulb. '246, fig. 5

570. . . . Nucleus posterior tecti. '36, fig. 15

595 ... . Cerebellum. Figure 91

635 .... Posterior border of V nerve roots. Figure 90

730 .... Level of IX nerve roots. Figure 89

929 .... Anterior root of first spinal nerve. Figure 88

956. . . . Calamus scriptorius. '446, fig. 4

975 .... Commissura infima. Figure 87

990 .... Nuclei of dorsal funiculi. '446, fig. 6

Figure 2C of this work is the median section of the middle part of the brain stem, reconstructed from this series of sections; and figures 1-4 of the paper of 1935 are similar reconstructions, showing chorioid plexuses and blood vessels.

A paper model of this transverse series was prepared, X 75, each section being drawn on cardboard seventy-five times the thickness of the section and then cut out along external and ventricular surfaces. These sheets, when properly stacked, show the external and ventricular configuration, and upon them details of internal structure were drawn in colored inks. The relevant data about this specimen were published ('35a) in connection with the preparation of the diagram of the median section, shown here as figure 2C (cf. fig. IB). Scales accompanying the diagram give the section numbers, so that any section figured can be accurately located on the projection. This outline has been used as the basis for many diagrams of internal structure in this work and previous papers. Figures 2A and '2B are similar diagrams, made from the series of horizontal sections illustrated in figures 25-36. A wax model was made from the sagittal Weigert series C (figs. lA and 85).

Two series of transverse sections of the adult forebrain based mainly on Golgi sections were published in 1927; six of these are here reproduced as figures 95-100. Six sagittal sections of the middle part of the adult brain stem by Rogers' reduced silver method were published in 1936, figures 17-22; and three of these are shown here as figures 102, 103, and 104. Seven pictures of the chiasma region from the sagittal Weigert series C are in figure 11 of 1941. A series of twelve horizontal Cajal sections of the adult follows in this book (figs. 25-36). Serial sections of larvae of 38 mm., prepared by Cajal's reduced silver method, have been illustrated (horizontal, '39b, figs. 2-20; transverse, '14a, figs. 4-14, and '39a, figs. 2-14). Somedetails of the development of the external form of the brain and the internal structures are reported in the papers of 1937-41. For meninges and blood vessels see pages 24-27, my papers ('34c^ and '35), Roofe ('35), and Dempster ('30).


Figure lA, B, and C'.^Lateral and median views of the brain of adult A. tigrinum, reproduced from '24a, figures 1, 2, and 3.

A. — Drawn from a wax model made from sagittal Weigert sections, no. C, collected at Chicago, 111. X 15. The lateral wall of the cerebral hemisphere has been cut away to open the lateral ventricle; compare figure 85 drawn from the same model.

B. — Median section of a specimen from Colorado. X 10. The dissection was prepared by Dr. P. S. McKibben and drawn by Katharine Hill. Brains of several specimens were exposed, fixed in situ in formalin-Zenker, then removed from the head, washed in water, and hardened in graded alcohols before being cut in the mid-sagittal plane. The shrinkage in alcohol accentuates the ventricular sculpturing.

C. — Key drawing to accompany figure IB.

Figure 2 A, B, and C. — Three drawings of the median section of the brain stem of the adult prepared by graphic reconstruction from sections.

A. — This median section is reconstructed from the series of horizontal Cajal sections ilhistrated in figures 25-36. X 23. Compare figure 2C made from a specimen differently prepared.

The shape of the two specimens is somewhat different, owing chiefly to slight dorsoventral compression of the midbrain and other distortions of the Cajal specimen. Despite these defects, the correspondence of the two median sections is fairly close. The figures of the two specimens are drawn on the same plan, with the important exception that in the drawing of 2C tlie ventricular sulci are projected upon the median plane, while in 2A and B the outhiies of the chief cellular areas are thus projected. In general, the sulci mark tlie boundaries of these areas, but this correspondence is not exact, and there is great individual variation in both these features. In the posterior part of the tegmentum isthmi a vertical dotted line marks, somewhat arbitrarily, tlie boundary between its central nucleus and a posterior sector composed of larger cells and transitional to the large-celled component of the trigeminal tegmentum.

Two drawings based on this reconstruction are shown, with emphasis on different features. In figure 2A the larger subdivisions of this part of the brain stem are demarcated so as to assist


the reader in comparing the amphibian topography with the conventional mammalian analysis. Median structures, that is, the cut surfaces of the section, are outlined with full lines, paramedian structures in dotted lines (compare the gross section, fig. IB; Necturus, '17, fig. 63; and McKibben, '11, fig. 5). Heavy broken lines mark the boundaries of the larger divisions as here defined, and dot-and-dash lines mark the subdivisions of the diencephalon.

B. — The same outline, bearing names of some of the smaller subdivisions. The scales at right and left indicate the approximate planes of the sections shown in figures 25-36. These levels are not exact, for unequal shrinkage during preparation produced some irregularities which have been smoothed in the diagram. The scales at top and bottom show section numbers of the type specimen, no. IIC, from which figure 2C was drawn, as indicated on that figure. Here again the correspondence is not exact, though sufficiently close to facilitate orientation of the published transverse sections with reference to the two median sections.

C- — This is the median section of the type specimen, IIC (p. 321), copied from 1935a, figure 1, with some changes in the lettering. X 30.

Figure 3. — Outline drawing of the dorsal surface of the adult brain. X 10. The paraphysis and chorioid plexus of the fourth ventricle have been removed. The courses of the .sensory fibers of the trigeminal and dorsal spinal nerve roots and of the spino-bulbar, spino-cerebellar, and spinal lemniscus secondary tracts are diagrammatically indicated. The general bulbar lemniscus {Im. of the other figures) arises from the entire sensory zone of the medulla oblongata and ascends approximately parallel with the spinal lemniscus.

Figures 4 and 5. — Two diagrams illustrating the extent of the sensory and motor zones as seen in horizontal sections of the adult brain (chap. v). X 15. The outlines are from a series of Cajal sections cut in a slightly different plane from those illustrated in figures 25-36, with the anterior end more dorsal.

Fig. 4. — This passes through the hippocampal commissure and shows on the left side the extent of the motor zone, as here defined, by oblique hatching and the sensory zone in the olfactory area by hatching in the reverse direction. The plane of section is about the same as that of figures 28 anteriorly and 27 posteriorly. It passes below the tuberculum posterius at the cerebral flexure, above which the motor zone of the peduncle and isthmus is continuous between the medulla oblongata and the ventral thalanms (fig. 30).

Fig. 5. — This shows the extent of the sensory zone at a more dorsal level, the plane being about that of figures 35 anteriorly and 34 posteriorly. The undifferentiated anterior olfactory nucleus encircles the base of the olfactory bulb, and through it passes the very large fasciculus postolfactorius (cf. fig. 105), from wliich tlie various olfactory tracts of the liemisphere are distributed (p. 55; '27, p. 283).

Figure 6. — Selected examples of long pathways of conduction leading toward the skeletal musculature of the trunk and limbs, seen as projected upon the lateral aspect of the brain. X 10. The only afferent systems drawn are the olfactory and optic, and from these receptive fields only a few lines of descending conduction are indicated as typical representatives of through fore-and-aft transmission.

Figure 7. — Diagram of the central courses of the sensory components of cranial nerves V to X of larval Amblystoma seen as projected upon the lateral surface of the medulla oblongata. X 36. The drawing is based on figure 3 of 1914. The general cutaneous component is drawn in dashed lines, the vestibular in thick unbroken lines, the visceral-gustatory in dotted lines, the three lateral-line VII roots in thick dash lines, the two lateral-line X roots in dot-and-dash lines, and correlation tracts a and b in thin continuous lines. Some fibers of the ascending roots of the general cutaneous and vestibular systems decussate in the cerebellum, and some visceral root fibers of the f. solitarius decussate in the commissura infima at the commissural nucleus of Cajal.

Figure S.— Diagram of the central connections of the visceral-gustatory system seen as projected upon the lateral surface of the adult brain. X 10. A probable direct connection from the superior visceral-gustatory nucleus (nuc.i'is.ti.) to the hypothalamus is indicated by the dotted line.

Figure 9. — Diagrammatic transverse section of the larval medulla oblongata near the level of the IX nerve roots, showing four types of neurons of the sensory zone. X 100. Drawn from preparations illustrated in 1914. Figure 89 shows a section of the adult brain in about the same plane. The arrangement of the fascicles of nerve root fibers is indicated. Neuron 1 is in synaptic relation with all components of these cranial nerves. Neuron 2 makes its chief connection with visceral-gustatory fibers of the f. solitarius and less intimate connection with vestibular and trigeminal fibers. Neuron 3 connects only with root fibers of the trigeminus. Neuron 4 connects with the trigeminus and also with the reticular formation and motor zone. Similar elements have been seen to connect also with fascicles of the VIII and other nerve roots. Axons of all these types decussate in the ventral commissure and may ascend in the general bulbar lemniscus. The axon of neuron 2 divides, one branch ascending in the secondary visceral tract {tr.r.a.) of the same side and the other crossing to the opposite side. Some similar neurons connect only with the f. solitarius and have unbranched axons entering tr.r.a. only. Numberless permutations of the various types of connection here shown have been observed.

Figure 10. — Diagram of the chief aflFerent connections of the body of the cerebellum and of the brachium conjunctivum seen as projected on the median section of the brain (chaps. iv and xii). X 18. The more lateral vestibular connections are not drawn. The outline is that of figure 2C, and dotted lines mark ventricular sulci and the boundaries of the chief subdivisions of the brain wall.

Figure 11. — Diagram of the chief afferent tracts to the tectum (pp. 48, 220).

Figure 12. — Diagram of the chief efferent tracts from the tectum. Many shorter connections are omitted (p. 223).

Figure IS. — Diagram of the connections of the mesencephalic nucleus of the V nerve and of some tecto-bulbar and tegmento-bulbar tracts probably concerned with feeding reflexes (p. 140).

Figure H.—DiagTaxn of the chief afferent tracts to the dorsal thalamus and of some other connections of the optic tracts (pp. 49, 236).

Figure 15. — Diagram of the chief efferent tracts from the dorsal thalamus (pp. 49, 237) and pretectal nucleus (p. 39).

Figure 16. — Diagram of the chief afferent tracts to the ventral thalamus (p. 239).

Figure 17.- — Diagram of the chief efferent tracts from the ventral thalamus (p. 240).

Figure 18. — Diagram of the chief connections of the "peduncle" (nucleus of the tuberculum posterius, pp. 50, 217). Many shorter connections are omitted. For the connections of the ventrolateral neuropil see chapter iii and figure 23.

Figure 19. — Diagram of the diencephalic connections of the amygdala (pp. 52, 248) and of some of the connections of the interpeduncular nucleus (chap. xiv).

Figiire 20. — Diagram of the connections of the habcnula. In the stria medullaris thalami the components are arranged in the fore-and-aft order in which they ascend, as also are the decussations in the habenular commissure (compare the horizontal sections, figs. 25-36). Efferent fibers from the habenida enter three tracts (fr./iab.t.,f.refr., and The afferent fibers are numbered as in the analysis of the stria medullaris in chapter xviii:

1. Tr. olfacto-habenularis medialis

2. Tr. olfacto-habenularis lateralis

3. Tr. olfacto-habenularis anterior, ventral division

4. Tr. olfacto-habenularis anterior, dorsal division

5. Tr. cortico-habenularis lateralis

6. Tr. amygdalo-habenularis

7. Tr. septo-habenularis

8. Tr. cortico-habenularis medialis

9. Tr. cortico-thalamicus medialis

10. Tr. olfacto-thalamicus

11. Tr. thalamo-habenularis

12. Tr. tecto-habenularis

Not shown in this figure is the tr. pretecto-habenularis and a probable tr. strio-habenularis.

Figure 21. — Diagram of the chief afferent connections of the tegmentum isthmi (p. 179). Most of the tracts here indicated end in both the isthniic and the trigeminal tegmentum. For the connections of the interpeduncular nucleus and its neuropil see chapter xiv.

Figure 22. — Diagram of the most direct connections between the retina and the peduncle. Optic tracts and efferent paths from the peduncle are drawn in full lines, internuncial connections in broken lines, and thick myelinated fibers in heavier lines (chap. xvi).

Figure 23.- — The chief nonvisual afferent connections of the ventrolateral peduncular neuropil (chap, iii) seen as projected upon the median section. Most of these fibers are unmyelinated, of thin or medium size. The visceral-gustatory system has more fibers than any of the others.

Figure 2!^. — Diagrammatic thick transverse section through the middle of the optic tectum and the oculomotor nucleus, illustrating typical connections between the tectum and the cerebral peduncle. The gray is outlined by a broken line, and the ventral border of the tectum by dotted lines. On the right side a typical neuron of the peduncle is drawn, with axon entering the ventral tegmental fascicles (f.r.t.). Four outlying cell bodies are seen at the border of the ventrolateral peduncular neuropil (a.rl.p.). On the left side is a neuron of the oculomotor nucleus, the dendrites of which connect with terminals of the basal optic tract in the ventrolateral neuropil. Compare figure 93.

Figures 25 to J6.— These semidiagrammatic drawings are made from sections selected from a horizontal series of the adult brain prepared by P. S. McKibben, Cajal's reduced silver method after fixation in alcohol. X 35. For approximiite planes of section see figure 2B. The sections are exactly horizontal right and left.

In this specimen the cell bodies are not blackened, and no dendrites are visible. All nuclei of cells are stained, and the gray pattern is clearly shown. There is scanty impregnation of the neuropil. In the cerebral hemispheres only the thickest axons are impregnated; elsewhere the thick and medium fibers are brilliantly differentiated. In some places where the boundaries of fascicles and tracts are obscure, interpretation has been aided by comparison with other specimens by methods of Cajal, Rogers, Golgi, and Weigert.

Horizontal sections are not so favorable for analysis of the tegmental fascicles (chap, xx) as are those cut in transverse and sagittal planes, and it is difficult to follow individual bundles as they recurve around the tuberculum posterius in the peduncle and tegmentum; but, by comparison with sections prepared by other methods and cut in various planes, the courses of most of the tracts and of the tegmental fascicles of groups (1) to (10) can be followed. The limits of the numbered groups of fascicles are not always clear, but their identification by number on the drawings is believed to be substantially correct.

Fig. 25. — Through the ventral part of the anterior commissure ridge and the dorsal border of the chiasma ridge. Only a few of the thicker fibers of the medial forebrain bundle are impregnated. These thread their way through the decussating fascicles of the chiasma ridge and spread out in the ventral part of the hypothalamus. The dense neuropil of these regions is not impregnated. All the optic fibers decussate ventrally of this level, except the most dorsal fibers of the axial tract ( The lightly stippled area in the postoptic commissure, marked, contains thin fibers from the dorsal thalamus, which decussate more ventrally (fig. 2C, Farther dorsally (fig. 26) these fibers, after crossing, separate into fascicles A and B (p. 299).

Fig. 26. — Ten sections more dorsally, the section passes through the dorsal fascicles of the medial forebrain bundle, tr. olfacto-peduncularis, and above the chiasma ridge. The mixed system of thalamo-hypothalamic and tegmental fibers (fig. 25, has separated into superficial (.4) and deep {B) tracts for the tegmentum, as described on page 299. The course of the superficial tract (A) can be followed in figures 27-34; most of the deeper fibers (B) join the dorsal tegmental fascicles of group (8).

Fig. 27.— This cuts the decussation of the dorsal fascicles of the lateral forebrain bundle in the anterior commissure and the motor roots and nucleus of the V cranial nerve. The fibers descending from the dorsal thalamus to the postoptic commissure { lie deep in the alba. The same fibers after decussation appear in the superficial tract, A, and in the deeper fascicles of group (8). Rostrally of the V roots, most of the thick fibers of the ventral commissure belong to tr. tegmento-bulbaris cruciatus. Among these the f. longitudinalis medialis is assembling from fascicles of groups (4), (5), and (6) (p. 281). Medially and dorsally of these are finer fibers of the dorsal division of tr. interpedunculo-bulbaris (tr.inp.b.d.).

Fig. 28. — Through the hippocampal commissure and the middle of the sensory root of the V nerve. The section cuts the ramus communicans posterior of "the arterial system (r.c.p.) and the decussation of the posterior division of tr. tecto-bulbaris cruciatus {tr.t.h.c.2.), also the most ventral fibers of the commissure of the tuberculum posterius.

Fig. 29.— This passes immediately below the cerebral flexure at the tuberculum posterius and the ventral border of the peduncle. Mingled with the crossing fibers of tr.f.b.c.2. are the most ventral crossing fibers (unmarked) of the anterior division of tr. tecto-bulbaris cruciatus. The central nucleus of the isthmus (nuc.i.s'.c.) is here well defined, and laterally of it are terminals of the dorsal tegmental fascicles of groups (7) to (10).

Fig. 30. — This section cuts the ventral border of the junction of the peduncle with the isthmus, a locus marked by the superficial origin of the III nerve. The anterior division of tr. tecto-bulbaris cruciatus (tr.t.b.c.l.) is decussating within the ventral tegmental fascicles of group (1) (p. 277). More posteriorly, the section cuts the dorsal border of the sensory V root and the visceral sensory roots of the VII and IX nerves, showing the entire prevagal part of the f. solitarius (pp. 148, 166 and figs. 37, 38).

Fig. 31. — Through the nucleus of the III nerve and the most dorsal component of the commissure of the tuberculum posterius containing decussating fibers of tr. tecto-b'jlbaris cruciatus 1. Farther forward the section cuts the ventral thalamus at its widest part and passes through the ventral border of the eniinentia thalami. Large numbers of thick fibers pass from the ventral thalamus to the peduncle and isthniic tegmentum {, both superficially and deeper in the alba. Similar fibers (not drawn) arise from the large cells of the peduncle, which comprise a primordium of the mammalian interstitial nucleus of the f. longitudinalis niedialis. Many of the thalamic and most of the pedimcular fibers enter ventral tegmental fascicles of group (5) (see p. 281). Tiiis section cuts tegmental fascicles of group (3) at the dorsal convexity of their course as they recurve around the tuberculum posterius.

Fig. 32. — Section passing just above the interventricular foramen and through the middle of the eminentia thalami. Accompanying fibers of the hippocampal commissure are those of tr. cortico-habenularis medialis for the stria medullaris and tr. cortico-thalamicus niedialis (primordium of the column of the fornix). Thick fibers stream backward from the ventral thalamus in tr. thalamo-tegmentalis rectus, some superficially, some at middle depth, and some at the inner border of the alba, the latter entering dorsal tegmental fascicles of group (8). Similar fibers arise from the larger cells of the peduncle, but to simplify the pictures these have not been drawn in figures 31 and 32. Most of them enter tegmental fascicles of group (5), and some of these continue spinalward in f. longitudinalis medialis. The more ventral large cells of the peduncle (fig. 31) correspond with the interstitial nucleus (Cajal) of f. longitudinalis medialis, the more dorsal of these cells (fig. 32) to the nucleus of Darkschewitsch (p. 217 and figs. 6, 18). For further details see the analysis of the tegmental fascicles in chapter xx and references there given.

At the level of figure 32, tegmental fascicles of groups (4), (5), and (10) are cut at the dorsal convexity of their courses through the peduncle, and here they are insinuated among the fibers of the anterior and posterior divisions of tr. tecto-bulbaris cruciatus, descending from the tectum toward their decussations. Farther back in the posterior isthmic neuropil the bulbar and spinal lemniscus systems {Im. and Im.sp.) are turning upward and forward to ascend in the dorsal tegmentum (figs. 33, 34). The wide auricle contains terminals of vestibular and lateral-line root fibers, other bulbar connections, and nucleus cerebelli.

Fig. 33. — This passes through the floor of the stem-hemisphere fissure, cutting the most dorsal fibers of tr. cortico-habenularis medialis as they enter the stria medullaris and the dorsal border of the eminentia thalami. Fibers of tr. thalamo-frontalis are seen emerging from the ventral border of the dorsal thalamus. Large cells of the nucleus of Darkschewitsch occupy the dorsal border of the peduncle. Laterally of these, tegmental fascicles (7) and (9) are recurving over the peduncle, and two sections farther dorsally, fascicles of group (8) are cut at the top of their convexity. More posteriorly the section passes through the dorsal border of the central nucleus of the isthmic tegmentum, laterally of which a few fibers of the brachium conjunctivum are impregnated. Some of these are seen to arise from the nucleus cerebelli. Externally of this nucleus is the posterior neuropil of the isthmus containing dendrites from the superior visceral-gustatory nucleus, terminals of the ascending secondary visceral tract {tr.v.a.), and many other components. Behind the auricle the section passes through the ventral part of the lateral recess of the ventricle and spinalward of this through the "dorsal island" of Kingsbury.

Fig. 34. — Ten sections farther dorsally, all components of the stria medullaris can be identified in the ventral habenular nucleus. The tr. habenulo-thalamicus (tr.hab.ih.) contains fibers passing in both directions between this nucleus and the thalamus and regions posteriorly of it. The bulbar lemniscus (Im.) and the spinalJemniscus (Im.sp.) traverse the dorsal tegmentum to reach the dorsal thalamus. More posteriorly the section passes through the gray of the superior visceral-gustatory nucleus {nuc.vis.s.) and the junction of the body of the cerebellum { with the auricle.

Fig. 35. — Through the ventral borders of the dorsal habenular nucleus, nucleus pretectalis, and tectum. Posteriorly it passes through the superior medullary velum, showing the decussation of the IV nerve roots, and through the body of the cerebellum dorsally of the cerebellar commissure. Fibers of com. vestibulo-lateralis cerebelli are crossing at this level. Compare 1942, figures 78 and 79, cut in about the same plane, Weigert and Golgi.

Fig. 36. — Through the com. posterior and the tectum at its widest part. In figures 35 and 36 the brachia of the superior and inferior colliculi are designated tr. tecto-thalamicus rectus {

Figures 37, 38, and 39. — These semidiagrammatic sketches show some details of the medulla oblongata from an advanced larva. The sections are obliquely horizontal, with the right side and the anterior end more dorsal. X 30. The series contains 27 thick Golgi sections, numbered from dorsal to ventral surfaces, and the data here assembled are selected from sections 11-24. The outlines of sections 13, 17, and 20 are drawn, and upon these there are sketched some details from these and neighboring sections. The impregnation is scanty on a clear ground, with long courses of individual fibers clearly shown. There is no impregnation of the lemniscus systems and but little of fibers descending from above the isthmus, so that the intrinsic bulbar structures impregnated are well defined. In figures 37 and 38 the boundary between gray and white layers is marked by a broken line; in figure 39 the gray of the interpeduncular nucleus is stippled.

Fig. 37. — On the left side this section cuts through the base of the auricle and the floor of the lateral recess. The right side is more dorsal and includes the superior visceral-gustatory nucleus and its neuropil, within which are collaterals from tr. tecto-bulbaris rectus (p. 225). On the left side, V root fibers arborize in the ventral part of the isthmic neuropil, and some of them continue into the cerebellar commissure. Almost the entire course of the postfacial f . solitarius is projected from several sections.

Fig. 38. — This section on the left cuts the superficial origins of the V, VII, and VIII nerve roots and, farther forward, of the III root. The sensory V fibers are seen to bifurcate with more slender ascending branches, most of which arborize in a neuropil, wliich is the primordiuni of the superior sensory V nucleus of mammals. The prefacial f. solitarius fibers are smooth and unbranched to their terminals rostrally and internally of the entering V root. Some impregnated fibers of the vestibular root (r. VIII.) extend forward to end within and rostrally of the superior V neuropil, but these are not drawn. Internally of tlie trigeminal neuropil and confluent with it are terminals of ascending crossed and uncrossed fibers termed tr. bulbo-isthmialis (, which is continuous with tr. spiuo-bulbaris {tr.sp.b.). These fibers ascend and descend from all levels of the medulla oblongata, many of them first decussating as internal or external arcuate fibers. Many of them bifurcate into ascending and descending branches with or without decussation. They accompany the spinal lemniscus and comprise a mixed bulbo-spinal and spino-bulbar system.

Fig. 39. — This section, parallel with the ventral surface, illustrates the courses of some of the external arcuate fibers, many of which are impregnated on the right side and none on the left. Useful landmarks are provided by the decussation of Mauthner's fibers (fib.M.) and two blood vessels, which, farther dorsally, are related with the roots of the V (b.r.V.) and VII (b.r.VlI.) nerves. No fibers of lemniscus and secondary visceral systems are impregnated. The visible fibers evidently are concerned chiefly with bulbar and spinal adjustments.

Figure .1,0. — Detail from a horizontal Golgi section of a late larva, showing the entrance of the sensory V root and three neurons of the motor V nucleus. X 50. The dendrites extend laterally to engage collaterals of the sensory root fibers, and in the adjoining section ventrally other dendrites of these cells ramify downward into the ventral alba, engaging collaterals and terminals of the descending and ascending fibers of the ventral funiculus, here unstained. Nothing except the elements drawn is impregnated in this region. Presumably, the axons of these cells enter the motor V root, the unimpregnated myelinated fibers of which are here darkened by the Golgi fluid.

Figures 41 to 44- — These semidiagrammatic sketches are made from horizontal Golgi sections of an advanced larva, with elective impregnation of the spinal lemniscus and a few other details. X 30. The series contains 26 thick sections, numbered from ventral to dorsal. Sections 2, 3, and 4, illustrating innervation of the hypophysis, have been published ('42, figs. 56, 57, 58). Here sections 7, 10, 12, and 14 are drawn, with some additions in each case from intervening sections.

Fig. 41. — The section passes through the V and VHI roots, with impregnation of superficial fascicles of descending fibers of both roots and ascending VHI fibers, some of which extend as far as the cerebellum. No ascending sensory V fibers are impregnated. The loci of the III, IX, and X roots farther dorsally are indicated by blood vessels which accompany these roots. The thick arcuate fibers from the region of the calamus scriptorius enter a mixed tract marked Im.sp., which here contains bulbo-spinal, spino-bulbar, spino-cerebellar, and spinal lemniscus fibers. Most of the thin unmyelinated fibers here impregnated come from lower levels of the spinal cord, and their further ascending course is shown in figures 42, 43, and 44. A few thick fibers of the ventral funiculi are impregnated, some of which decussate in the ventral commissure below the auricle.

Fig. 42. — At this level, thick arcuate fibers in the vicinity of the first spinal roots descend in the calamus region and after crossing join the spinal lemniscus. Some of these bifurcate with a thick ascending branch and a slender descending. Farther forward under the auricle the lemniscus fibers turn dorsalward in the ventral part of the isthmic neuropil, and here many of them end. This is tr. bulbo-isthmialis of figures 38 and 39. The lemniscus fibers which continue rostrad have collateral endings here. Other elective Golgi preparations from this lot show the origin of spinal lemniscus fibers from the nuclei of the dorsal funiculus ('446).

Fig. 43. — This section cuts the III nerve root and its nucleus and, in the isthmic neuropil, dispersed fibers of the spinal lemniscus and ascending VIII root. Medially of these are axons emerging from the gray of the trigeminal tegmentum, which turn forward and arborize within the gray of the isthmic tegmentum (p. 185).

Fig. 44. — At this level the few remaining impregnated fibers of the spinal lemniscus are turning forward across the dorsal tegmentum to reach the dorsal thalamus. The crude drawing gives a very inadequate picture of the delicacy of these widely branched terminal arborizations. The more numerous fibers of this tract, which arborize in the tectum, are not impregnated in this preparation (compare fig. 101 and '396, fig. 26). For the patterns of vascular supply sketched in the left thalamus see page 27.

Figure 45. — Terminals of the dorsal lateral-line root of the facial nerve, from the right side of a horizontal Golgi section of a late larva. X 50. Only three fibers of this root are impregnated, and each of these ramifies through almost the entire extent of the "dorsal island" of the area acusticolateralis immediately spinalward of the auricle, which is the exclusive central field reached by fibers of this root (compare fig. 33).

Figures ^6 and ^7. — Details from horizontal Golgi sections of an advanced larva. X 75.

Fig. 46. — A neuron of the anterodorsal part of the trigeminal tegmentum in the ventral part of the auricle (p. 185). The plane of section is approximately that of figure 31.

Fig. 47. — A small neuron at the anteroventral border of the body of the cerebellum, where it merges with the nucleus cerebelli. The slender dendrites extend outward into the dorsal part of the isthmic neuropil, and the axon is directed forward and downward into the brachium conjunctivum. Fibers of the trigeminal component of the cerebellar commissure (com.cb.) are approaching their decussation immediately dorsally of this level, as also are more scattered fibers of the vestibulo-lateral component (com.cb.L).

Figure ^8. — Detail from a transverse Golgi section previously pictured ('42, fig. 43). X 50. The small neuron of the dorsal tegmentum has a short axon, which ramifies in the deep neuropil of the gray near the cell body and also dorsalward in the alba of the anterior end of the nucleus posterior tecti. On the vascular loops {b.v.) seen here see page 27.

Figure 49. — A typical neuron of the dorsal tegmentum from a horizontal Golgi section of an advanced larva. X 75. The ependymal surface is marked by a thin line at the left, the pial surface by a thicker line at the right, and the outer border of the gray by a broken line. The axon arises from the dendrite and is directed spinalward (downward in the figure) and probably enters tegmental fascicle (7). A similar neuron >Ji s-itu is shown in figure 44 of the paper of 1942.

Figure 50. — Horizontal section through the decussation and spiral endings of the f. retroflexus (p. 197). Golgi method. X 50. The semidiagrammatic drawing is based on several adult specimens, in each of which the fasciculus of both sides is massively impregnated from the habenula to the interpedvuicular neuropil. All details drawn are visible in two sections of the specimen outlined (no. 2229), wliich is, however, poorly preserved. Several otlier series of sections show the same relations more clearly, though the oblique planes of some of the sections are less convenient. The considerable lateral obliquity of one of these gives an interesting view of the interior of the spiral. In these specimens the spiral endings are impregnated for about twothirds of the distance between the decussation and the level of the V'roots.

Figures 51 and 52. — Two adjoining horizontal Golgi sections of a late larva, illustrating the decussation and spiral terminals of the f . retroflexus, which is impregnated on only one side. The right side is more ventral. X 50.

Figures 53 and 54. — Two adjacent thick horizontal Golgi sections from an advanced larva. X 50.

Fig. 53. — At the level of the fovea isthmi (f.i.) and the decussation of the f. retroflexus. Laterally of the interpeduncular neuropil are impregnated fibers of tr. olfacto-peduncularis, with endings by open arborizations in this neuropil; compare figure 59, cut in a similar plane a little more dorsally.

Fig. 54. — Ventrally and posteriorly of the decussation only a few isolated fibers of f. retroflexus are impregnated, and tr. olfacto-peduncularis is terminating in the ventral interpeduncular neuropil in the same area as the spiral. The tufted endings seen here are not derived from either of these tracts.

Figure 55. — Detail of the spiral course of fibers of the f . retroflexus below the decussation, from a horizontal Golgi section of a late larva. X 50. In the interpeduncular neuropil nothing but these three fibers is impregnated, so there is no possibility of confusion.

Figures 56, 57, 5S.- — Three horizontal Golgi sections of an adult brain, in which the right f . retroflexus is unstained and the left fasciculus is abundantly impregnated from the habenula to its decussation. X 50. Some details of the decussation are added to figure 58 from the section adjoining it ventrally. The impregnation fails spinalward of the decussation. The left fasciculus shows an atypical division into two bundles as it enters the alba of the peduncle (p. 262).

Figure 59.- — An obliquely horizontal section through the superficial origins of the III roots, advanced larva. X 50. Here it is in about the plane of figure 30, but sharply inclined to the horizontal plane, with the posterior end more ventral and the anterior end more dorsal than that level. Impregnated fibers of the olfacto-peduncular tract pass the region of the fovea isthmi and then turn medially. Most of them end in the interpeduncular neuropil with open arborizations, though some extend farther spinalward (compare figs. 53, 54). From this neuropil slender axons of the dorsal interpedunculo-bulbar tract descend near the mid-plane and soon turn laterally to end in a dense axonic neuropil at the ventral border of the caudal end of the tegmentum isthmi and rostral end of the tegmentum trigemini. Here they engage dendrites of the smaller neurons of this region. Other preparations show that some of these fibers extend spinalward as far as the IX nerve roots.

Figures 60 to 6^. — These drawings present additional details from a series of transverse Golgi sections (no. 2246), which has already been quite fully described and illustrated in the sector of the brain stem between the interventricular foramen and the nucleus of the IV nerve ('27, pp. 271, 278, figs. 22-40). Those figures were drawn from sections selected from nos. 55-97 of the series. Section 88 was subsequently drawn on a larger scale to show some details of structure, including the arrangement of the tegmental fascicles, at the level of the III nerve roots ('42, fig. 44). Figures 60-64 extend the series spinalward (X 50), with special reference to the interpeduncular connections of the tegmentum, which are here well impregnated. These tegmento-interpeduncular fibers comprise one component of the complex f. tegmentalis profundus (p. 286), and the incomplete references to them made in 1927 can now be clarified and rectified. In this specimen there is no impregnation of the f . retroflexus or tr. olfacto-peduncularis. A few neurons of the interpeduncular nucleus are incompletely stained, but apparently no axons from these cells are impregnated. The dense interpeduncular neuropil here seen is composed almost exclusively of axons from the tegmentum and tr. mamillo-interpeduncularis. Terminals of the latter, when separately impregnated, are seen to be of more open texture than the dense vertically arranged terminal tufts of the tegmento-interpeduncular fibers (fig. 60). The two types of terminals are closely interwoven with each other and with tufted terminals derived from axons of neurons of the interpeduncular nucleus (not here impregnated) .

Fig. 60. — This section lies between the nuclei of the III and IV nerves, i.e., between the levels of figures 92 and 93, at about the level of figure 13 of 1936 and two sections rostrally of figure 39 of 1927. The drawing is a composite, containing some details from the two adjoining sections. Axons from both dorsal and isthmic tegmentum descend in the f. tegmentalis profundus, some terminating in the underlying interpeduncular neuropil and some of thicker caliber decussating in the ventral commissure (fr.ieg.b.), where collaterals separate from them to end in tufts of the interpeduncular neuropil. In figure 39 of 1927 these decussating fibers are called tr. tegmento-peduncularis, and the same fibers are here called tr. tegmento-bulbaris. Both designations are correct. The larger number of these fibers after decussation turn spinalward, some sending collaterals forward also. Others turn rostrad into the peduncle.

Fig. 61.— This section passes through the nucleus of the IV nerve, and the three neurons impregnated probably belong to this nucleus. Approximately the same plane is illustrated in several published figures ('25, figs. 9, 19; '36, fig. 14; '42, fig. 43). At this level the myelinated fibers of the ventral median fascicles (1) are turning laterally to enter tr. tecto-bulbaris cruciatus in the position of the broken lines under the gray of the interpeduncular nucleus. The thick fibers of tr. tegmcnto-bulbaris shown in figure 60 are here reduced in number, and the thin fibers of tr. tegmento-interpeduncularis are more numerous. These terminals and those of tr. mamillo-interpeduncularis enter into the dense axonic interpeduncular neuropil.

Pig 62. This detail is from the lower part of the interpedimcular nucleus rostrally of the

V nerve roots, not far from the plane of figure 91. There is impregnation of a typical large neuron of the trigeminal tegmentum and a smaller element at the border of the interpeduncular nucleus. In the interpeduncular neuropil the visible fibers are nearly all derived from the overlying tegmentum, with perhaps some terminals of tr. mamillo-interpeduncularis. At this level the ventral fascicles (4), (5), and (6) have united to form a compact f. longitudinalis medialis, and the dorsal fascicles spread out laterally into the isthmic neuropil and bulbar tegmentum (compare the horizontal sections, figs. 29-32).

Fig. 63. — Detail of floor-plate structure immediately caudad of the V nerve roots. Though the interpeduncular nucleus is not considered to extend as far spinalward as this level, the lateral and dorsal interpeduncular axonic neuropil is still rather dense (compare figs. 79, 81). The fibers impregnated here are derived chiefly from the overlying tegmentum, and some of them descend as far as the VII roots accompanying the dorsal and ventral tr. interpedunculobulbaris, which is not impregnated in this specimen. Two typical ependymal elements of the bulbar tegmentum are drawn.

Fig. 64. — This is a similar sketch of the ventral median raphe at the level of the VII nerve roots. A remnant of the tegmento-interpeduncular neuropil persists, and this extends no farther spinalward, though other preparations show that axons from the interpeduncular nucleus reach at least as far as the IX nerve roots. The ependyma here is more compact and mossy than at the level of the trigeminus.

Figures 65 and 66.— Two transverse sections through the interpeduncular region at the level of transition between the isthmic and the trigeminal tegmentum of the adult. X 50. Each figure is a composite of two adjacent sections, so that four consecutive sections are represented in the two drawings.

Sections taken farther forward from this well-impregnated series have been shown ('42, figs. 45-47). Between the levels of the III and IV nuclei, tr. tegmento-interpeduncularis is selectively stained, with tufted endings in the interpeduncular neuropil similar to those shown in figures 61 and 6:2. At the level of figure 65 only a few of these fibers are impregnated and also a few neurons of the interpeduncular nucleus with dendrites extending downward into the interpeduncular neuropil.

In figure 66 the neurons of the interpeduncular nucleus are seen to have thick dendrites extending laterally to ramify widely in the alba of the isthmic and trigeminal tegmentum and thinner dendrites directed ventrally into the interpeduncular neuropil. Both dendritic and axonic arborizations enter the glomerulus-like tufts; but, in order to clarify their relations, only the dendritic component is drawn on the left side and the axonic component on the right (compare figs. 83, 84).

Figure 67.- — Transverse section through the middle of the interpeduncular nucleus of a halfgrown larva. X 75. The broken line marks the outline of the gray substance. The Golgi impregnation is scanty, showing in this region only a few decussating fibers of tr. tegmento-bulbaris on a clear ground. The more dorsal of the two fibers shown is sketched from the adjoining section spinalward. These thick axons probably arise from unimpregnated cells of the isthmic tegmentum, as shown in figiu-e 68. In the ventral commissure, where these fibers decussate, slender collaterals leave them to ramify in the interpeduncular nucleus and its neuropil.

Figure 6S. — A diagram based on Golgi sections of a larval Arablystoma from the same lot as figure 67. X 75. The sections are obliquely transverse. The section outlined is in about the same plane as figure 67 on the right and passes through the auricle on the left. The details are from this and the two adjoining sections. Everything drawn was observed, but the assembly is schematic. Thick axons of neurons of the isthmic tegmentum {tr.feg.b.) converge into the ventral commissure, where slender collaterals separate from them to arborize in tufted form in the interpeduncular neuropil.

Figures 69 a/id 70. — These two drawings illustrate details of the interpeduncular region at and immediately rostrally of the nucleus of the IV nerve (compare the diagram, fig. 68). X 50. The impregnation of the transverse Golgi sections of this adult brain is exceptionally good, and it has been quite fidly illustrated (';*7. figs. ^4-30; U, figs. 24-31).

Fig. 69. — This section is adjacent posteriorly to the one shown in figure 30 of 1942, to which reference may be made for the topographic relations. In both figures the bundles of myelinated fibers of the ventral and ventromedian tegmental fascicles are outlined with broken lines. In the interpeduncular region of the section here shown, there is no impregnation of any nervous elements except a few fibers of tr. tegmento-bulbaris at their decussation in the ventral commissure. Within the commissure slender collaterals descend into the interpeduncular neuropil, where some of them have tufted endings.

Fig. 70.^This is drawn from the second section spinalward of figure 69, through the nucleus of the IV nerve, one neuron of which is impregnated. Thick myelinated axons of the IV nerve root (darkened by the Golgi fluid) ascend from this nucleus in the position indicated by broken lines. Typical ependymal elements of the ventral raphe are drawn.

Figure 71. — Obliquely longitudinal section of the adult brain taken not far from the midsagittal plane, with the dorsal and anterior sides inclined somewhat laterally, so that almost the entire length of the f. retroflexus appears in a single thick Golgi section. Golgi method. X 30. The drawing is semidiagrammatic, with some details added to the section outlined from neighboring sections and from the opposite side.

At the point where tr. cortico-habenularis medialis joins the stria medullaris, numberless fine collaterals separate from it to enter the eminentia thalami. The section cuts the optic nerve at its junction with the brain. Peripherally, each optic nerve between the foramen and the brain shows several hundred impregnated unmyelinated fibers, most of which lose the stain before entering the brain. The surviving impregnated fibers decussate in chiasmatic bundles 1 (ch.l.) at the extreme anteroventral angle of the chiasma and ascend toward the tectum as the most rostral fibers of the marginal optic tract, losing the stain before reaching the tectum ("42, p. 232).

The abundant impregnation of the f . retroflexus extends only as far as the decussation, below which its fibers are unstained. As these fibers turn medially toward the decussation, many of them separate and descend uncrossed along the lateral margin of the interpeduncular neuropil. The locus of the sidcus isthmi on the ventricular siu-face is projected upon the drawing as a thick broken line. In the posterior hp of this fissure there is massive elective impregnation of the brachium conjunctivura and the locus of the decussation of these fibers more medially is marked by three crosses. Before and after crossing, many of these fibers turn spinalvvard in the superficial alba of the isthmic tegmentum.

Figures 72 and 7S. — Two semidiagrammatic drawings from an obliquely longitudinal series of Golgi sections of an adult brain. The sections are cut at an angle of about 30° from the sagittal plane, with dorsal and posterior sides more lateral.

Fig. 72. — These details are assembled from several consecutive sections to illustrate some components of the complex of fibers at the di-telencephalic boundary. X 25. Only some of the thicker unmyelinated axons are impregnated in this preparation. The ventral border at the

left is near the median plane, and posteriorly of this place the section is inclined laterally. At the posteroventral border of the primordium hippocampi some thick fibers are seen to converge at the thalamic junction, and Weigert sections show that a few of them are myelinated. The larger number of these fibers descend to the hippocampal commissure, and a smaller number a.scend in the stria medullaris thalami as tr. cortico-habenularis medialis. Others descend obliquely posteroventrad into the ventral thalamus, passing through and posteriorly of the gray of the eminentia thalami^ — tr. cortico-thalamicus medialis. This is the precursor of the mammalian columna fornicis, but here apparently none of these fibers reach the mamillary region of the hypothalamus. These thick fibers are accompanied by far more numerous very thin fibers, some of which arise as collaterals from the decussating fibers of the hippocampal commissure (fig. 71). An impregnated neuron of the bed-nucleus of the anterior commissure ridge is sketched from the most lateral of the sections here represented. Its dendrites spread dorsally and ventrally among the hippocampal fibers; the course of the axon is not revealed.

The well-impregnated fibers of tr. thalamo-frontalis arise from the cells of the dorsal thalamus shown in figure 73. In the chiasma ridge, chiasmatic bundles 2 and 3 of optic fibers are impregnated, and their terminals in the lateral part of the tectum are seen at the top of the figure. The details of this optic connection have been pubHshed ('42, p. 235). The fibers of f . tegmentaHs profundus seen at the right are ascending from their decussation in the ventral commissure to spread superficially over the lateral aspect of the isthmic tegmentum. Most of these fibers belong to the brachium conjunctivum.

Fig. 73. — Section more medially, showing details of the origin of the f. retroflexus from the habenula. X 50. The drawing is a combination of three adjoining sections. Only the thicker axons of the fasciculus are here impregnated. These surround a dense fascicle of thinner unstained fibers. The fibers here impregnated arise from neurons of the habenula, from others more posteriorly in the pars intercalaris, and probably also from some of the impregnated neurons of the pars dorsalis thalami. None of these stained fibers reach the interpeduncular nucleus. They spread out in the alba of the posteroventral part of the peduncle, some turning forward into area ventrolateralis pedunculi. The dendrites of the impregnated neurons of the dorsal thalamus extend dorsally among terminals of tr. tecto-thalamicus rectus and also for long distances posteroventrally, accompanying the f . retroflexus into the area of the primordial geniculate bodies (pp. 221, 238). Sections taken more laterally show abundant impregnation of axons from this group of cells which enter tr. thalamo-frontalis (fig. 72).

Figures 74 to 78. — Five sagittal sections of the adult brain, illustrating the composition of the stria medullaris thalami (p. 256 and fig. 20). X 37. The sections were prepared by the reduced silver method of Cajal and cut at 10 n slightly oblique to the sagittal plane, with the dorsal and rostral sides more lateral (compare "36, figs. 17-79, where the dorsoventral obliquity is in the reverse direction). In these sections the thickest axons, both myelinated and unmyelinated, are black, and the thinner fibers range through gray to brown or yellow, so that components of the stria are well dififerentiated.

Fig. 74. — This cuts through the di-telencephalic junction near its lateral border, passing through the olfacto-peduncular tract below, and above this through the dorsal and ventral fascicles of the lateral forebrain bundle. The artery (r.h.m.) lies in the floor of the stem-hemisphere fissure. Here four components of the stria medullaris converge, two of them- — tr. olfactohabenularis anterior and lateralis — passing up from the ventral surface externally of the lateral forebrain bundles.

Fig. 75. — In the region of the stem-hemisphere fissure the section passes medially of the lateral forebrain bundles, showing the connection of tr. olfacto-habenularis medialis and tr. septo-habenularis with the stria. Ventrally the plane of section is not far from that of figure 103; dorsally it is much more lateral.

Fig. 76. — At this level tr. cortico-habenularis medialis enters the stria and ascends as its rostral member. All components of the stria are distributing fibers in the habenular neuropil, posteriorly of which several of the tracts retain their identity. Fibers of tr. habenulothalamicus which recurve spinalward over the gray of the dorsal thalamus have been seen to connect only with the ventral habenular nucleus. This tract includes also thalamo-habenular fibers. The boundary of the gray of the eminentia thalami is marked by a broken line.

Fig. 77. — A large proportion of the fibers of all components are here dispersed in the habenular neuropil. A residue of fibers is accumulated posteriorly in a commissural bundle, within which the following components can be identified: tr. cortico-habenularis lateralis and medialis, tr. olfacto-habenularis anterior, tr. amygdalo-habenularis.

Fig. 78. — In this section, 300 ^ from the mid-plane, the limits of the gray of the dorsal habenular nucleus (corresponding with the medial nucleus of mammals) and of the pretectal nucleus are marked by broken lines. Most of the habenidar neuropil has disappeared. Some fibers from this residue enter the habenular commissure, but most of the crossing fibers are derived from recognizable tracts. The thickest fibers cross at the anterior end of the commissure. These apparently are derived chiefly from tr. amygdalo-habenularis, with additions from other components. The thinner fibers of the commissural bundle include clearly some residue of tr. cortico-habenularis lateralis and medialis and tr. olfacto-habenularis anterior.

Figures 79 and SO.— Two adjoining obliquely sagittal Golgi sections of the adult brain, illustrating some components of the interpeduncular neuropil. In this specimen the finest axons are richly impregnated, especially the deep neuropil of the gray substance, which is continuous throughout the tectum, peduncle, and dorsal, isthmic, and bulbar tegmentum. In the area illustrated it receives fibers from the fields dorsally and anteriorly, and from it axons descend to the isthmic and bulbar tegmentum. All cell bodies are imbedded in this dense fibrillar reticulum. On these small-scale drawings it is impossible to portray the delicacy and complexity of this tissue, and the attempt is made to indicate the general trend of its coarser fibers.

Fig. 79. — This section is median in the floor of the ventricle from the fovea isthmi to the level of the IX nerve roots. X 40. The ventricular floor from the fovea to the tuberculum posterius is projected from two adjacent sections, as indicated by a dotted line. This section is very thick and is so inclined dorsoventrally as to include above the ventricular floor a thin slice of the nucleus of the tubercidum posterius, tegmentum isthmi, and rostral end of the trigeminal tegmentum of one side, ^'entrally of the ventricular floor in the isthmus region the section cuts the interpeduncular nucleus and neuropil close to the mid-plane on the opposite side. The level of the VIII nerve roots is indicated by the decussation of Mauthner's fibers, from which one of the fibers is seen descending. The other fiber appears in the adjacent section 24. From the latter section tlie portion of tr. interpedunculo-bulbaris dorsalis (ir.inp.b.d.) spinalward of this decussation is added to the drawing, showing that some fibers of this tract descend as far as the level of the IX nerve roots. None of its fibers have been seen to extend farther spinalward. The ventral division of this tract {Ir.inp.h.r.), on the contrary, goes much farther, perhaps as far as the spinal cord.

At the fovea isthmi several ependymal elements are impregnated, and a small artery here enters the foveal pit at the ventral surface. These ependymal elements are crossed by unimpregnated myelinated fibers of the ventral tegmental fascicles {f.m.f.{l)), as indicated by dashed lines. A single small neuron of the nucleus of the tuberculum posterius is impregnated. Its dendrites spread downward among slender tortuous axons of tr. mamillo-peduncularis, which form a dense neuropil in the gray of the peduncle. In this plane no fibers of tr. mamillo-interpeduncularis are stained, but they are abundant farther laterally.

Unimpregnated cell bodies of the interpeduncular nucleus are clearly visible, arranged as indicated by the dotted outlines. Among these and ventrally of them is dense neuropil, very inadequately shown in the drawing. These fibers are derived in part from tr. mamillo-interpeduncularis and from the overlying tegmentum, and in larger part they are axons of the cells of the interpeduncular nucleus. These axons take tortuous courses, mainly directed dorsalward into the isthmic and trigeminal tegmentum and spinalward into the dorsal and ventral interpedunculo-bulbar tracts. From them arise numberless collaterals which ramify in the interpeduncular neuropil and enter the glomeruli (fig. 83).

Fig. 80.— The adjoining section. X 75. The ependymal floor of the ventricle is slightly to one side of the mid-plane, ventrally of which the section is inclined laterally. Heavy broken lines mark the upper and lower limits of the zone of unimpregnated cell bodies of the interpeduncular nucleus. Scattered cells of this nucleus are distributed in the underlying neuropil except its ventral part. Above this zone of den.sely crowded cells are the less crowded cells of the isthmic and trigeminal tegmentum, none of which are impregnated. One neuron of the interpeduncular nucleus is impregnated with dendrite directed ventrally into the neuropil. Fragments of other dendrites, with tufted terminals in glomeruli, are spread throughout tlie neuropil, some of which are drawn (compare figs. 65, 66, 81-84). There is no other impregnation in the ventral interpeduncular neuropil containing the unimpregnated spiral endings of the f. retroflexus, but dorsally of this there are stained axons of various sorts among the dendrites. These are not drawn, except those lying between the cells of the interpeduncular nucleus and the myelinated fibers of the ventral commissure, the most dorsal bundles of which are outlined with dotted lines. Most of immyelinated fibers enter tr. interpeduncvdo-bulbaris dorsalis. The interpeduncular neuropil is broadly connected with the overlyiiig tegmental neuropil by fibers passing in both directions. In this plane (not drawn) there are a few fibers of tr. niamillo-interpeduncularis and more of them farther laterally.

Figure ,Si .—Obliquely sagittal Golgi section of an adult brain taken close to the mid-plane. X 60. Some other sections of this specimen have been published (fig. 101; '25, figs. 14, 40). The plane of section of this figure is similar to that of figure 79, but somewhat more oblique. The ventral surface is nearly median in the isthmus, and dorsally and anteriorly of this the plane is inclined laterally. Since the sections are very thick, the dorsal ventricular border of the tuberculum posterius (indicated by the dotted line), and a slice of the lateral wall dorsally of it are included in the section outlined. The detail is drawn from this section and the adjacent section laterally. The four components of the commissure of the tuberculum posterius (p. 302) and the dorsal fascicles of the ventral commissure posteriorly of the fovea isthmi are outlined with dotted lines. The decussation of the anterior division of tr. tecto-bulbaris cruciatus in the ventral median tegmental fascicles {J.m.t.{l)) is indicated by dashed lines (compare '36, fig. 2).

A neuron of the dorsal (mamillary) part of the hypothalamus and the dendrite of another are impregnated. Their axons branch, and one branchlet enters tr. mamillo-interpeduncularis. This tract partially decussates in component 1 of the commissure of the tuberculum posterius (the retroinfundibular commissure of the literature), and its slender unmyelinated fibers arborize in the rostral part of the interpeduncular neuropil chiefly laterally of the plane here shown (compare '36, figs. 3, 8, 20). Two ependymal elements are drawn. These lie near the ventral median raphe under the cerebellum (compare figs. 63, 64, 70).

Unimpregnated cell bodies of the interpeduncular nucleus are scattered under the ependymal surface and among the myelinated median fascicles and ventral commissure bundles. A well-impregnated dendrite of one of these is drawn; it has a tufted terminal near the ventral surface (compare figs. 62, 65, 66, 80, 82). There is scanty and very clear impregnation of some of the coarser fibers of the interpeduncular neuropil. Those in its anterior part are derived chiefly from tr. mamillo-interpeduncularis and probably some from tr. olfacto-peduncularis, which is partially impregnated more laterally (fig. 101). More posteriorly the visible fibers all seem to be axons of cells of the interpeduncular nucleus which assemble to descend in the dorsal and ventral interpedunculo-bulbar tracts.

Figure 82.— k semidiagrammatic drawing of a thick Golgi section from an obliquely sagittal series of an adult brain. X 37. The section is nearly median posteriorly (at the right), and anteriorly it is much more lateral, including the f. retroflexus in the peduncle. Some fibers of this fa.sciculus descend uncrossed into the interpeduncular neuropil. Mingled with the uncros.sed fibers of the f. retroflexus are thicker and smoother fibers of tr. olfacto-peduncularis, some of which enter the interpeduncular neuropil. Dorsally of these is the mamillo-peduncular tract, comprising ventral tegmental fascicle (3), and still farther dorsally are fibers of tegmental fascicle (6), which come from the postoptic commissure. These tegmental fascicles contain both myelinated and unmyelinated fibers, and both sorts are here darkened by the Golgi treatment. Between the two tegmental fascicles just mentioned are the darkened thick myelinated fibers of the anterior division of tr. tecto-bulbaris cruciatus (f.m.1.{l)), some of which decussate near the fovea isthmi dorsally of the olfacto-peduncular fibers and others descend in the ventral median fascicles. One neuron of the interpeduncular nucleus is well impregnated and dendrites of several others, all of which exhibit the characteristic dendritic tufts. No ependyma, spiral fibers, or axonic tufts are impregnated in the interpeduncular neuropil. The scanty axonic neuropil is probably composed exclusively of terminals of uncrossed fibers of the f. retroflexus and olfacto-peduncular tract.

Figure SJ.— Diagram of a neuron of the interpeduncular nucleus, seen as projected upon the median sagittal section. X 40. This is a composite drawing from observations made on many sections cut in various planes. Compare figures 19 and 84.

Figure 84. — Diagram of the composition of the interpeduncular glomeruli. X 45. This drawing, like figure 79, shows the median section of the floor plate and dorsally of this an oblique slice of the overlying tegmentum. Two neurons of the interpeduncular nucleus are drawn. Tufted collaterals from the axon of the anterior element engage dendritic glomeruli of the posterior element. Tufted axonic terminals enter glomeruli from small cells of the isthmic and trigeminal tegmentum and also from collaterals of tr. tegmento-bulbaris, which arises from large cells of the same tegmental areas. Compare figures 19 and 83.

Figure 85. — The lateral aspect of the adult brain, drawn from the same wax model as figure lA. X 15. This drawing shows the courses of the four most superficial components of the stria medullaris thalami seen as projected upon the lateral surface (fig. 20 and chap, xviii). Tractus olfacto-habenularis lateralis (2) goes directly dorsally from the preoptic nucleus. Tractus olfacto-habenularis anterior arises in ventral (3) and dorsal (4) divisions from the anterior olfactory nucleus. Tractus cortico-habenularis lateralis (5) arises from the primordial piriform lobe and joins no. 4 before entering the stria medullaris.

Figure 86A, B, and C— The brains of A. tigrinum and Necturus drawn to approximately the same scale.

A.— Lateral aspect of the brain of A. tigrinum and its arteries. After Roofe ('35). X 7.5. Drawn after fixation in 10 per cent formalin for six weeks.

B. — Lateral aspect of the cerebrum of Necturus. This and figure 86C are copied from 1933c, figures 3 and 4. X 8.

(2 — Median section of the brain of Necturus (compare fig. 2). In the thalamus and midbrain the section is median, and the dotted lines indicate ventricular sulci which mark the boundaries of the chief cellular areas. In the cerebral hemisphere the section is slightly to one side of the mid-plane, and the dotted lines indicate the boundaries of cellular areas with limits marked internally by ventricular sulci.

The section pictures which follow are here reproduced from previous publications (with minor alterations in some instances), and more complete descriptions will be found in the references cited.

Figures 87 to 91.- — Five transverse sections of the adult medulla oblongata. Method of Wiegert. X 37. These are copies, respectively, of figures 5, 3, 2, and 1 of 19446 and figure 16 of 1936, drawn from the type specimen, no. IIC.

Fig. 87. — Section taken immediately below the calamus scriptorius through the commissural nucleus and the com. infima of Haller.

Fig. 88. — Through the lower vagus region and the most anterior ventral rootlet of the first spinal nerve.

Fig. 89 .^At the level of the IX nerve roots. A rootlet of the \T nerve emerges in this section. The two lateral-line roots of the vagus enter immediately rostrally of this level.

Fig. 90. — Section taken immediately below the superficial origin of the V nerve roots, including the posterior part of the motor V nucleus.

Fig. 91. — Section through the cerebellum, auricle, and rostral end of the trigeminal tegmentum.

Figure 92. — Transverse section immediately spinalward of the nucleus of the IV nerve. X 37. Ventrally it passes through the posterior end of the infundibulum and dorsally through the junction of superior and inferior colliculi. The Arabic numbers in parentheses refer to the tegmental fascicles described in chapter xx. This is a copy of figure 14 of 1936, drawn from a reduced silver preparation, method of Rogers.

Figure 93. — Semidiagrammatic transverse section at the level of the III nerve roots. X 37. Copied from figure 14 of 1942. The layers of the tectum and some of its afferent tracts are shown on the left side and efferent tracts on the right.

Figure 9^. — Section through the rostral border of the commissure of the tuberculum posterius. X 37. Copy of figure 10 of 1936 and drawn from the same specimen as figure 92.

Figures 95 to 100.- — These are obliquely transverse sections (ventral side inclined spinalward), copied, with some alterations, from the paper of 1927, where full descriptions will be found. X 25. They are from the brain of a recently metamorphosed adult from Colorado, prepared by the Golgi method. The drawings are composite, the outlines and much of the detail being derived from the specimen mentioned, supplemented by additional details selected from 20 other specimens cut in the transverse plane and stained by the methods of Golgi, Weigert, and Cajal.

Fig. 95. — Through the habenular commissure and the posterior border of the po.stoptic commissure ('27, fig. 20).

Fig. 96. — Through the rostral border of the chiasma ridge and the middle of the eminentia thalami. By reason of the obliquity of the section, it passes rostrally of the habenulae ('27, fig. 15). The cellular area marked p.v.ih. is the nucleus of the olfacto-habenular tract (p. 248).

Fig. 97.- — Through the hippocampal commissure and the decussation of the lateral forebrain bundles in the anterior commissure ("27, fig. 12).

Fig. 98. — Immediately in front of the lamina terminalis. On the right side is a characteristic impregnation of the neuropil of the corpus striatum. In the preparation drawn, only the dendritic component of the neuropil is impregnated ("27, fig. 9).

Fig. 99. — Section about 0.1 mm. anterior to the last, through the mid-septal region. The head of the caudate nucleus is here at its maximum size. In the ventrolateral wall on the right side the neuropil of the corpus striatum, in the preparation here drawn, has only the axonic component impregnated (cf. fig. 108). These contorted and branched axons are interlaced with the dendrites shown in figure 98, and the boundary of this area of dense neuropil is sharply defined ('27, fig. 8).

Fig. lOO.^Section through the middle of the olfactory bulb and the extreme anterior border of the primordium hippocampi. It includes also the posterior border of the f. postolfactorius ("27, fig. 4).

Figure 101.- — Semidiagrammatic sagittal section of the adult brain, Golgi method. X 35. The section is oblique, with the dorsal side somewhat more lateral, cut in a plane which includes almost the whole length of the lateral forebrain bundles and tr. olfacto-peduncularis. In the tectal neuropil there are typical endings of the spinal lemniscus and tr. strio-tectalis (copied from '42, fig. 17).

Figures 102, 103, /O^.^Three sagittal sections from the brain of a specimen from Colorado prepared about two weeks after metamorphosis by the reduced silver method of Rogers. The sections are slightly oblique, with the dorsal surface and caudal end more medial. X 37. In this series of sections the courses of the tegmental fascicles can be clearly followed. These pictures are selected from a series published in 1936.

Fig. 102. — The section passes through the middle of the gray substance of the tectum opticum at its widest part and shows the courses of the lateral forebrain bundles and lemniscus systems ('36, fig. 17).

Fig. 103.- — Section taken 0.17 mm. more medially and showing the arrangement of the more medial tegmental fascicles ('36, fig. 18).

Fig. 104. — Section taken 0.16 mm. more medially and about 0.1 mm. from the mid-plane at the tuberculum posterius. It includes the interstitial nucleus of the f . longitudinalis medialis {nuc.tub.p.) and the nuclei of the III and IV nerves ('36, fig. 22).

Fig. 105. — Semidiagrammatic horizontal section through the dorsal border of the olfactory bulb and dorsal sector of the anterior olfactory nucleus of the adult. X 50. The medial border (at the right) is slightly inclined ventrally. The diagram is based on sections of a single specimen. The details pictured have all been observed, but the assembling is a .schematic composition ('34, fig. 1). The histological structure of the olfactory bulb as seen in horizontal sections is shown in figure 110. Figure 105 is a similar diagram at a more dorsal level. Two mitral cells (1) are drawn and a typical granule cell (2). Two elements of transitional type (3) in the stratum granidare send slender axons to the anterior olfactory nucleus, three neurons (//) of which are drawn. Farther back there are three typical neurons of the primordium hippocampi (6') and one of the primordium pirifornie (-5).

Fig. 106.^ — Detail of the neuropil of the gray substance of the dorsal sector of the anterior olfactory nucleus in the plane of figure 105 and from the same specimen ('34, fig. 2). Golgi method. X 100. Only the axonic component of the neuropil is here impregnated. The positions of the unimpregnated cell bodies are indicated by the dotted outlines.

Fig. 107. — This is a detail of the neuropil of the gray substance of the rostral part of the corpus striatum of the adult frog, Rana pipiens ('34, fig. 3). Golgi method. X 142. The section pictured is obliquely longitudinal, inclined about 45° from the horizontal plane, witli the ventricular border more ventral. The rostral end is above. The impregnation is similar to that shown in figure 106. One small neuron of the striatal gray is impregnated.

Figure 108. — Neuropil of the anterior olfactory nucleus and corpus striatum, as seen in an obliquely horizontal section of the adult brain. The right side is more dorsal ('42, fig. 15). Golgi method. X 50. The section pa.sses through the right hemisphere near the ventral surface, including the ventral sector of the anterior olfactory nucleus and the ventral border of the gray of the corpus striatum {c.s., here unimpregnated). Laterally of this gray is the striatal neuropil {c.s.n.), in which only the axonic component is impregnated (cf. fig. 99). Rostrally of the striatal gray is the extreme ventral border of the neuropil of the caudate nucleus, the structure of which is .shown in figure 109.

Figure 109.— An obliquely longitudinal section through the ventrolateral border of the adult hemisphere ('42, fig. 16). (lolgi method. X 80. The section is inclined to the sagittal plane, with the anterior and dorsal sides much more lateral. It cuts the accessory olfactory bulb and the anterior end of the caudate nucleus, whose gray {nucxaud.) is unimpregnated. Above and externally of this is a dense impregnation of the caudate neuropil with sharply defined borders. Some unimpregnated cell bodies are enmeshed within this neuropil, only the axonic component of which is impregnated. This neuropil is continuous with that of the remainder of the striatal complex shown in figure 108. Anteriorly of it, six neurons of the anterior olfactory nucleus are impregnated.

Figure llOA, B.— Diagrams of the structure of the olfactory bulb as seen in horizontal section of the adult brain.

A. The outline is taken from a section at the widest part of the olfactory bulb and the elements are drawn to scale from several Golgi sections, most of them from the same specimen ('246, fig. 1). X 50. Glomeruli are outlined with dotted lines. The approximate locus of figure hob' is indicated. The following types of neurons have been described: (1) periglomeridar cells; (2) mitral cells; (3) subglomerular tufted cells; (4) granule cells; (5) cells intermediate between granules and neurons of nucleus olfactorius anterior; (6) transitional cells of nucleus olfactorius anterior.

B._The layers of the olfactory bulb as seen in horizontal section after fixation in formalinZenker followed by Mallory's stain ('246, fig. 4). X 150.

Figure 111. — Lateral aspect of the forebraiii of Necturus (cf. fig. 86B), upou which some olfacto-somatic pathways are projected (*33e, fig. 1). X 8. Olfactory tracts are drawn in broken lines, descending tracts in solid lines, and the thalamic radiations and their connections are in red lines (p. 102). Connections with the epithalamus and hypothalamus are omitted.

Figure 112. — Median section of the brain of Necturus (cf. fig. 86C), upon which some of the descending pathways from the cerebral hemisphere to the hypothalamus and epithalamus are projected (p. 268) ('33p, fig. 2). X 8.

Figure 113. — Diagram illustrating the relations of projection and association fibers in the cerebral hemispheres of Necturus as seen in transverse section a short distance rostrally of the lamina terminalis (p. 102). The gray substance is outlined by a broken line. Descending fibers arc drawn on the left, ascending and pallial association fibers on the right ("34, fig. 7). X 16.

The following list includes all abbreviations on the figures and a few others used in previous publications. Reference may also be made to the list appended to the monograph on Necturus ('336, pp. 280-88), where synonyms of many of these terms are given, with references to the literature. For reference to published lists of tracts see page 270, and for nomenclature of blood vessels see the paper of 1935 and Roofe ('35).

c.s.v.n., neuropil of c.s.v. cb., cerebellum cell M., cell of Mauthner ch., chiasma opticum col.inf., celliculus inferior = nuc.p.t. col.swp., colliculus superior; tectum opticum com.amg., commissure of amygdalae coni.ant., anterior commissure com.cb., commissura cerebelli, commissura vestibulo-lateralis cerebelli com.cb.l.L, lateral-line component of com.cb.l. com.cb.V., trigeminal component of com.cb. com.cb.VIII., vestibular component of

com.cb.l. com.hab., habenular commissure com. hip., hippocampal commissure com.i., commissura infima Halleri com.po., commissura postoptica, commissura posterior com.t.d., commissura tecti diencephali com.t.m., commissura tecti mesencephali com.tub., commissura tuberis coni.tub.p., commissura tuberculi posterioris com.v., commissura ventralis

A., division A of

a.a., arteria auditiva, area acusticolateralis

a.b., arteria basilaris

a.c.c, arteria carotis cerebralis

a.c.c.a., arteria carotis cerebralis, ramus

anterior a.c.c. p., arteria carotis cerebralis, ramus

posterior a.c.i., arteria carotis interna, arteria chorioidea rhombencephali

anterior a. gen., area geniculata = np.gen. a.l.t. = a.vl.p. a.o., arteria ophthalmica a.s.a., arteria spinalis anterior a.s.l., arteria spinalis 1 a.s.l.r.a., arteria spinalis 1, ramus anterior a.s.l.r.p., arteria spinalis 1, ramus posterior a.p., angulus ventralis

a.vl.p., area ventrolateralis pedunculi = a.l.t. amg., amygdala amg.n., neuropil of amygdala as.amg.pal., amygdalo-pallial association as.pal.amg., pallio-amygdaloid association, pallio-striatal association, strio-pallial association aur., auricle

B., division B of

b.ol., bulbus olfactorius, bulbus olfactorius accessorius

6.«'.,blood vessel

b.v.III., v., VII., IX., X., blood vessels

associated with cranial nerve roots br.coL; br.col.s.; br. col.inf., brachia of superior

and inferior colliculus = br.conj., brachium conjunctivum

c.cb., corpus cerebelli

C.S., corpus striatum

c.s.d., corpus striatum, pars dorsalis

c.s.d.n., neuropil of c.s.d.

c.s.v. , corpus striatum, pars ventralis

d.b., diagonal band of Broca d.f.l.t., decussation oi d.f.m,.t.;, decussation oi d.f.retr., decussation oi f.retr. d.fib.M., decussation of Mauthner's fibers d.isL, dorsal island of Kingsbury d.r.IV., decussation of IV nerve roots d.rinf., decussatio retroinfundibularis; components 1 and 2 of com.tub.p., decussation of, decussation of,.p., eminentia commissurae posterioris em.s.t., eminentia subcerebellaris tegmenti

= nuc.cb. +, eminentia thalami em.V., eminentia trigemini ep., epiphysis

F., foramen interventriculare

f.d.t., fasciculi dorsales tegmenti

f.i., fovea isthmi

f.l.m., fasciculus longitudinalis medialis, fasciculus lateralis telencephali; lateral

forebrain bundle, dorsal fascicles oi =, ventral fascicles oi = f.m.t., fasciculus medianus tegmenti, fasciculus medialis telencephali;

medial forebrain bundle, dorsal fascicles oi v., ventral fascicles oi f.jpo., fasciculus postolfactorius f.retr., fasciculus retrofiexus f.sol., fasciculus solitarius, prefacial fasciculus solitarius f.teg.p., fasciculus tegmentalis profundus f.v.t., fasciculi ventrales tegmenti fib.arc, arcuate fibers jib.M., fiber of Mauthner jim., fimbria fiin.d., funiculus dorsalis

g.I., ganglion cells of nervus olfactorius gl.ol., glomeruli olfactorii

hab., habenula

hah.d., nucleus dorsalis habenulae

hah.d.n., neuropil of hah.d.

hab.v., nucleus ventralis habenulae

hab.v.n., neuropil of hab.v.

hem., cerebral hemisphere

hyp., hypophysis

hyp.d., hypophysis, pars distalis

hyp.g., hypophysis, pars glandularis

hyp.i., hypophysis, pars intermedia

hyp.n., hypophysis, pars nervosa

hyp.i., hypophysis, pars tuberalis

hyth., hypothalamus

7 to .Y, cranial nerve roots inf., infundibulum inp.n., interpeduncular neuropil is., isthmus rhombencephali

lam.t., lamina terminalis Im., general bulbar lemniscus Im.sp., lemniscus spinalis

M., mouth of paraphysis

m.c, mitral cells

m.l.e., membrana limitans externa

m.l.i., membrana limitans interna

n.Il.; n.op., nervus opticus n.ol., nervus olfactorius, nervus vomeronasalis 111., neurilemma nuclei np.gen., geniculate neuropil, pretectal neuropil, nucleus accumbens septi nuc.amg., nucleus amygdalae mic.amg.d.l., nucleus amygdalae dorso lateralis nuc. B., nucleus of Bellonci nuc.B.n., neuropil of Bellonci nuc.caiid., nucleus caudatus nuc.cb:, nucleus cerebelli nuc.covi.C, nucleus commissuralis of Cajal nnc.d.f.l.t., bed-nucleus of decussation of nuc.d.f.m.t., bed-nucleus of decussation of nuc.Dark., nucleus of Darkschewitsch nuc.ecmam. = a.ii.p. nuc.f.sol., nucleus of fasciculus solitarius II lie. 1 1 1., nucleus of oculomotor nerve niic.inp., nucleus interpeduiicularis nnc.inp.n., neuropil of nuc.inp., nucleus centralis isthmi nuc.IV., nucleus of trochlear nerve nuc.l.s., nucleus lateralis septi nuc.m.s., nucleus medialis septi nup.ol.a.; nuc.ol.ant., nucleus olfactorius

anterior nuc.ol.a.d.; nuc.ol.ant.d., nucleus olfactorius

anterior, pars dorsalis nuc.ol.ant.m., the same, pars medialis nuc.ol.ant.v., the same, pars ventralis nuc.ol.d.L, nucleus olfactorius dorsolateralis

= p.pir., nucleus of olfactory projection

tract nuc.p.t., nucleus posterior tecti nuc.po., nucleus preopticus nuc.po.a., nucleus preopticus, pars anterior nuc.po.p., nucleus preopticus, pars posterior, nucleus pretectalis nuc. Sep., nucleus septi nuc.sep.m. = nuc.m.s., nucleus of tractus olfacto habenularis nuc.tub.p., nucleus tuberculi posterioris;

pedmicle nuc.v.l, nucleus ventrolateralis of cerebral

hemisphere; amygdala nuc.V.m., motor nucleus of trigeminus nuc.V.mes., mesencephalic nucleus of trigeminus nuc.V.s., superior nucleus of trigeminus nuc.vis.n., neuropil of nuc.vis.s. nuc.iris.s., nucleus visceralis superior; superior

gustatory nucleus

o.s.c, organon subcommissurale, olfactory projection tract

P., paraph ysis

p.d.hi/th., pars dorsalis hypotliakmi

p.d.hytk.d., dorsal lobe of p.d.hijth.

■p.d.hyth.v., ventral lobe of p.d.hyfh., pars dorsalis thalami, pars dorsalis thalami, area medialis, pars dorsalis thalami, area posterior

p.g.c, periglomerular cells

p.hip., primordium hippocampi

p.i.d.;, pars intercalaris diencephali

p.p.d., primordium pallii dorsalis

p.pir., primordimii piriforme = nuc.ol.d.l.

p.v.hyth., pars ventralis hypothalami

p.v.hyth.a., anterior lobe of p.v.hyth.

p.v.hyth.p., posterior lobe of p.v.hyth., pars ventralis thalami

p.r.fh.a., pars ventralis thalami, area anterior, pars ventralis thalami, area posterior

par., paraphysis

pars.d.L, pars dorsolateralis of hemisphere = p.pir.

pars.v.l., pars veutrolateralis of hemisphere = c.s.

ped., pedunculus cerebri

pl.c.r., plexus chorioideus rhombencephali

po. (4),(6),(5), fibers from postoptic commissure entering tegmental fascicles

prorn.l., prominentia lateralis

prom.r., prominentia ventralis

r.c.c.b., ramus communicans cum arteria

basilaris r.c.p., ramus communicans posterior r.h., ramus hypothalamicus r.h.m., ramus hemisphaerii medialis r.h.v., ramus hemisphaerii ventralis r.III., root of oculomotor nerve r.IV., root of trochlear nerve r.IX., root of glossopharyngeal nerve r.IX.m., motor root of IX nerve r.IX.v.s., visceral sensory root of IX nerve r.L; reel., recessus lateralis rhombencephali r.mes.s., ramus mesencephali superior r.p.m., recessus posterior mesencephali r.po., recessus preopticus r.sp., root of spinal nerve r.sp.v.l., ventral root of first spinal i'lerve r.V., root of trigeminal nerve r.V.asc, ascending fibers of sensory V root r.V.m.S., posterior motor V root T.V.mes., mesencephalic root of V nerve, dorsal division of r.V.mes. r.V.mes.v., ventral division of r.V.mes . r.V. mot., motor root of V nerve r.V.sen., sensory root of V nerve r.V.sp., spinal root of V nerce r.V I., root of abducens nerve

r.V II., root of facial nerve r.VILl.L, lateral-line roots of VII nerve r.VII.l.l.d., dorsal lateral-line VII root r.VII.l.l.m., middle lateral-line VII root r.VII.l.l.v., ventral lateral-line VII root r.VII.m.; r.V II. mot., motor root of VII

nerve r.VII.m.2., posterior motor VII root r.VII.v.s., visceral sensory root of VII nerve r.V 1 1 1., root of VIII nerve r.VIII.d., dorsal root of VIII nerve r.VIII.v., ventral root of VIII nerve r.X., root of vagus nerve r.X.l.l., lateral-line roots of vagus nerve r.X.l.l.d., dorsal lateral-line root of vagus r.X.l.l.r., ventral lateral-line root of vagus r.X.v.s., visceral sensory root of vagus reed., recessus lateralis rhombencephali rec.op., recessus preopticus rec.op.l., recessus opticus lateralis rec.p.m., recessus posterior mesencephali rec.pcm., recessus precommissuralis, recessus pinealis

s., limiting sulcus of nuc.tnb.p.

s.d., sulcus dorsalis thalami

s.erh., sulcus endorhinalis

s.hyth., sulcus hypothalamicus

.s.hyth.d., sulcus hypothalamicus dorsalis

s.hyth.p., sulcus hypothalamicus posterior

s.ih., sulcus intrahabenularis, sulcus isthmi

s.l.b.o., sulcus limitans of olfactory bulb

s.l.h., sulcus limitans hippocampi

sd.s., sulcus limitans septi, sulcus lateralis mesencephali

s.m., sulcus medius thalami

S.O., sinus obliquus

s.po., sulcus preopticus

s.rh., sulcus rhinalis

s.shab., sulcus subhabenularis, sulcus striaticus, sulcus strio-caudatus

S.V., sulcus ventralis thalami

s.v.a., sulcus ventralis accessorius thalami

sac.d., saccus dorsalis

sac.v., hypophysis, pars nervosa

Sep., septum

sep.ep., septum ependymale

sep.m., septum mediale

sep.t.p., septum transversum paraphysis

st.amg.f., strio-amygdaloid field

st.ep., ependymal layer

st.glom., layer of glomeruli, granular layer

st.m.c, layer of mitral cells

st.mol., molecular layer = st.plx.

st.n., layer of nerve fibers st.plx., plexiform layer, stria meduUaris thalami str.t., stria terminalis

t.c, transitional cells

t.f., taenia fornicis, taenia thalami

t.v.q., taenia ventriculi quarti

teed., dorsal thickening of tectum

tect., tectum mesencephali

teg.d., tegmentum dorsale mesencephali

teg. is., tegmentum isthmi, tegmentum isthmi, pars magnocellu laris teg. v., tegmentum trigemini teg.VII., tegmentum facialis fh., thalamus

tr.a., dorsal correlation tract of Kingsbury tr.amg.hab., tractus amygdalo-habenularis, tractus amygdalo-thalamicus tr.b., ventral correlation tract of Kingsbury, tractus bulbo-isthmialis tr.b.sp., tractus bulbo-spinalis tr.b.t.L, tractus bulbo-tectalis lateralis tr.c.h.L; tr.c.hab.l., tractus cortico-habenu laris lateralis tr.c.h.m.; tr.c.hab.m., tractus cortico-habenu laris medialis, tractus cortico-thalamicus medialis tr.cb.teg., tractus cerebello-tegmentalis tr.hab.t., tractus habenulo-tectalis, tractus habenulo-thalamicus tr.hy.ped., tractus hypothalamo-peduncularis tr.hy.teg., tractus hypothalamo-tegmentalis tr.hyp., tractus hypophysius tr.inj.asc, tractus infundibularis ascendens tr.inp.b., tractus interpedunculo'-bulbaris tr.inp.b.d., tractus interpedunculo-bulbaris

dorsalis tr.inp.b.v., tractus interpedunculo-bulbaris

ventralis tr.mam.inp., tractus mamillo-interpeduncu laris tr.mam.ped., tractus mamillo-peduncularis tr. mam. teg., tractus mamillo-tegmentalis, tractus mamillo-thalamicus tr.ol., tractus olfactorius tr.ol.d., tractus olfactorius dorsalis tr.ol.d.l., tractus olfactorius dorsolateralis tr.ol.h.a.; tr.ol.hab.ant., tractus olfacto habenularis anterior tr.ol.h.a.v., ventral division of tr.ol.h.a. tr.ol.h.l.; tr.ol., tractus olfacto-habenu laris lateralis tr.ol.h.m.; ir.ol.hab.vied., tractus olfacto habenularis medialis

tr.ol.hip.m., tractus olfacto-hippocampalis medialis

tr.ol. pal.d. = tr.ol.d.

tr.ol. pal.l. = tr.ol.d.l.

tr.ol.ped., tractus olfacto-peduucularis

tr.ol. s., fasciculus olfactorius septi

tr.ol.r., tractus olfactorius ventralis

tr.ol.v.l., tractus olfactorius ventrolateralis

tr.op., tractus opticus = tr.op.b., axial bundle of tractus opticus

tr.op.b., basal bundle of tractus opticus

ir.op.l;, tractus opticus lateralis, or ventralis

tr.op.m.; tr.op. vied., tractus opticus medialis, or dorsalis

tr.op., mar., tractus opticus marginalis

tr.ped.mam., tractus pedunculo-mamillaris

tr.pc, tractus preopticus hy., tractus pretecto-hypothalamicus, tractus pretecto-tectalis, tractus pretecto-thalamicus

tr.sep.c, tractus septo-corticalis

tr.sep.hab., tractus septo-habenularis

tr.sp.b., tractus spino-bulbaris

tr.sp.cb., tractus spino-cerebellaris

tr.sp.t., tractus spino-tectalis = Im. sp., tractus strio-peduncularis =, tractus strio-pretectalis;, tractus strio-tegmentalis =, tractus strio-tectalis, tractus strio-thalamicus

tr.t.b., tractus tecto-bulbaris tr.t.b.c, tractus tecto-bulbaris cruciatus tr.t.b.p., tractus tecto-bulbaris posterior tr.t.b.r., tractus tecto-bulbaris rectus tr.t.cb.; tr.tec.cb., tractus tecto-cerebellaris tr.t.hab., tractus tecto-habenularis tr.t.hy.a., tractus tecto-hypothalamicus anterior tr.t.p.; tr.t.ped., tractus tecto-peduncularis tr.t.p.c, tractus tecto-peduncularis cruciatus, tractus tecto-pretectalis tr.t.sp.; tr.tec.sp., tractus tecto-spinalis tr.t.teg., tractus tecto-tegmentalis ir.t.teg.c, tractus tecto-tegmentalis cruciatus, tractus tecto-thalamicus et

hypothalamicus cruciatus anterior, tractus tecto-thalamicus et

hypothalamicus cruciatus posterior, tractus tecto-thalamicus rectus =

br.col. tr.teg.b., tractus tegmento-bulbaris tr.teg.inp., tractus tegmento-interpedimcularis, tractus tegmento-isthmialis

tr.teg.p.; tr.teg.ped., tractus tegmeiito-peduncularis, tractus thalamo-bulbaris, tractus thalamo-frontalis, tractus thalamo-hypothalamicus dorsalis cruciatus, tractus thalamo-habenularis, tractus thalamo-mamillaris;, tractus thalamo-peduncularis;, tractus thalamo-peduncularis cruciatus, tractus thalarao-peduncularis dorsalis, tractus thalamo-telctalis, tractus thalamo-tegmeutalis dorsalis cruciatus, with divisions A and B, tractus thalamo-tegmentalis rectus, tractus thalamo-tegmentalis ven tralis cruciatus, tractus thalamo-tegmentalis ven tralis rectus tr.v.a., tractus visceralis ascendens tr.r.d., tractus visceralis descendens tr.v.t., tertiary visceral tract tub. p., tuberculum posterius, vena azygos septi r.l., ventriculus lateralis r.m.a., velum meduUare anterius r.par., paraphysial veins r4-, fourth ventricle, zona limitaus lateralis, zona limitans medialis

(1) to (10), tegmental fascicles