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

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Chapter 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

270

THE SYSTEMS OF FIBERS 271

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.

THE BASAL FOREBRAIN BUNDLES

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, tr.th.f.) correspond with the thalamo-striatal system. The dorsal group of fascicles, the lateral forebrain bundle (i. lateralis telencepha\\,f.lat.t.), 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, f.med.t.), 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 {f.lat.t.v. 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 (f.lat.t.d. and tegmen

THE SYSTEMS OF FIBERS 273

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, tr.st.tec; '42, p. 262) and the associated tr. strio-pretectalis (figs. 14, 101, tr.st.pt.). 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 (ol.p.tr.), as described on page 242.

The most noteworthy ascending system of fibers of the lateral bundle is tr. thalamo-frontalis {tr.th.f.). 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

274 THE BRAIN OF THE TIGER SALAM.\NDER

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

THE SYSTEMS OF FIBERS 275

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 TEGMENTAL FASCICLES

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

THE SYSTEMS OF FIBERS 277

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.

THE SYSTEMS OF FIBERS 279

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

THE SYSTEMS OF FIBERS 281

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

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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, tr.th.teg.r.). 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

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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,f.lat.t.d. 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, f.lat.t.v. 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, tr.th.teg.r.). 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, tr.th.teg.v.r. 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.

FASCICULUS TEGMENTALIS PROFUNDUS

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

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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.