Difference between revisions of "Book - The brain of the tiger salamander 19"

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Olfactory tracts of the third and higher orders, i.e., those separated by two or more synapses from the periphery, are generally  
Olfactory tracts of the third and higher orders, i.e., those separated by two or more synapses from the periphery, are generally  
designated by hyphenated compound words, as tr. olfacto-peduncularis; but, as mentioned above, many of these tracts are mixtures containing some axons of mitral cells. For further details of these connections see the summaries ('33&, p. 124; '27, p. 282).  
designated by hyphenated compound words, as tr. olfacto-peduncularis; but, as mentioned above, many of these tracts are mixtures containing some axons of mitral cells. For further details of these connections see the summaries ('33&, p. 124; '27, p. 282).
==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, 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
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
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
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
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, 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
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.
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
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.
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
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
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, tr.t.th.h.c.a.). — 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, tr.t.th.h.c.p). — 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, tr.t.th.h.c.p.),
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 tr.t.th.h.c.p. 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 {tr.t.th.h.c.p.) 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 (tr.th.p.c), 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, tr.th.h.d.c).
8. Tractus thalamo-hypothalamicus dorsalis cruciatus (tr.th.h.d.c). —
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
tr.th.h.d.c. 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 {tr.th.teg.d.c.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,
tr.th.teg.r.). This latter tract arises from both dorsal and ventral
thalamus, but only the dorsal component of it is under consideration
here (fig. 21, tr.th.teg.d.r.). 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
11. Tractus thalamo-tegmentalis ventralis cruciatus (figs. 2C, 17,
tr.th.teg.v.c). — 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
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
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, oLp.tr.). — 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
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 (tr.th.p.c), 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).
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eyes in the salamander (Triturus viridescens) with return of vision, J. Exper.
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Stroer, W. F. H. 1939. Ueber den Faserverlauf in den optischen Bahnen bei
Amphibien, Proc. kon. Akad. Wetensch., Amsterdam, 42:649-56.
. 1939a. Zur vergleichenden Anatomie des primaren optischen Systems bei
Wirbeltieren, Ztschr. f. Anat. u. Entwcklngsgesch., 110:301-21.
. 1940. Das optische System beim Wassermolch (Triturus taeniatus), Acta
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Strong, O. S. 1895. The cranial nerves of the Amphibia, J. Morphol., 10: 101-230.
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, 1926a. Ueber die Sulci und Eminentiae des Hirnventrikels von Diemictylus
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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.
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 tr.hab.th.). 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 (tr.op.ax.). The lightly stippled area in the postoptic commissure, marked
tr.th.h.d.c, contains thin fibers from the dorsal thalamus, which decussate more ventrally
(fig. 2C, tr.th.h.d.c). 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, tr.th.h.d.c.) 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 {tr.th.h.d.c.) 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 {tr.th.teg.r.), 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
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 {c.ch.)
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
(fr.b.is.), 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
mo.st 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 the.se 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
com.ch.L, 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.hab., habenular commissure
com. hip., hippocampal commissure
com.i., commissura infima Halleri
com.po., commissura postoptica
com.post., 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 tr.th.ieg.d.c.
a.a., arteria auditiva
a.ac., area acusticolateralis
a.b., arteria basilaris
a.c.c, arteria carotis cerebralis
a.c.c.a., arteria carotis cerebralis, ramus
a.c.c. p., arteria carotis cerebralis, ramus
a.c.i., arteria carotis interna
a.ch.r.a., arteria chorioidea rhombencephali
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
as.pal.st., pallio-striatal association
as.st.pal., strio-pallial association
aur., auricle
B., division B of tr.th.ieg.d.c.
b.ol., bulbus olfactorius
b.ol.ac, 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 = tr.th.r.
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 f.lat.t.
d.f.m,.t.; d.f.med.t., decussation oi f.med.t.
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.
d.tr.st.ped., decussation of tr.st.ped.
d.tr.st.t., decussation of tr.st.t.
em.com,.p., eminentia commissurae posterioris
em.s.t., eminentia subcerebellaris tegmenti
= nuc.cb. + teg.is.
em.th., 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
f.lat.t., fasciculus lateralis telencephali; lateral
forebrain bundle
f.lat.t.d., dorsal fascicles oi f.lat.t. = tr.st.t.
f.lat.v., ventral fascicles oi f.lat.t. = tr.st.pcd.
f.m.t., fasciculus medianus tegmenti
f.med.t., fasciculus medialis telencephali;
medial forebrain bundle
f.med.t.d., dorsal fascicles oi f.med.t.
f.med.t. v., ventral fascicles oi f.med.t.
f.jpo., fasciculus postolfactorius
f.retr., fasciculus retrofiexus
f.sol., fasciculus solitarius
f.sol.pf., 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
n.vn., nervus vomeronasalis
111., neurilemma nuclei
np.gen., geniculate neuropil
np.pt., pretectal neuropil
nuc.ac, nucleus accumbens septi
nuc.amg., nucleus amygdalae
mic.amg.d.l., nucleus amygdalae dorso
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.
nuc.is.c, 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
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.
nuc.oLp.tr., nucleus of olfactory projection
nuc.p.t., nucleus posterior tecti
nuc.po., nucleus preopticus
nuc.po.a., nucleus preopticus, pars anterior
nuc.po.p., nucleus preopticus, pars posterior
nuc.pt., nucleus pretectalis
nuc. Sep., nucleus septi
nuc.sep.m. = nuc.m.s.
nuc.tr.ol.h., nucleus of tractus olfacto
nuc.tub.p., nucleus tuberculi posterioris;
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
ol.p.tr., 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.
p.d.th., pars dorsalis thalami
p.d.th.m., pars dorsalis thalami, area medialis
p.d.th.p., pars dorsalis thalami, area posterior
p.g.c, periglomerular cells
p.hip., primordium hippocampi
p.i.d.; p.i.th., 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.
p.v.th., pars ventralis thalami
p.r.fh.a., pars ventralis thalami, area anterior
p.r.th.p., pars ventralis thalami, area posterior
par., paraphysis
pars.d.L, pars dorsolateralis of hemisphere =
pars.v.l., pars veutrolateralis of hemisphere =
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
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
r.V.mes.do., 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
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
rec.pin., 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
s.is., sulcus isthmi
s.l.b.o., sulcus limitans of olfactory bulb
s.l.h., sulcus limitans hippocampi
sd.s., sulcus limitans septi
s.lat.mes., sulcus lateralis mesencephali
s.m., sulcus medius thalami
S.O., sinus obliquus
s.po., sulcus preopticus
s.rh., sulcus rhinalis
s.shab., sulcus subhabenularis
s.st., sulcus striaticus
s.st.c, 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
st.gr., 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
str.med., stria meduUaris thalami
str.t., stria terminalis
t.c, transitional cells
t.f., taenia fornicis
t.th., 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
teg.is.m., tegmentum isthmi, pars magnocellu
teg. v., tegmentum trigemini
teg.VII., tegmentum facialis
fh., thalamus
tr.a., dorsal correlation tract of Kingsbury
tr.amg.hab., tractus amygdalo-habenularis
tr.amg.th., tractus amygdalo-thalamicus
tr.b., ventral correlation tract of Kingsbury
tr.b.in., 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
tr.c.th.m., tractus cortico-thalamicus medialis
tr.cb.teg., tractus cerebello-tegmentalis
tr.hab.t., tractus habenulo-tectalis
tr.hab.th., 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
tr.inp.b.v., tractus interpedunculo-bulbaris
tr.mam.inp., tractus mamillo-interpeduncu
tr.mam.ped., tractus mamillo-peduncularis
tr. mam. teg., tractus mamillo-tegmentalis
tr.mam.th., 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. hab.lat., tractus olfacto-habenu
laris lateralis
tr.ol.h.m.; ir.ol.hab.vied., tractus olfacto
habenularis medialis
tr.ol.hip.m., tractus olfacto-hippocampalis
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.ac.p. = tr.op.b.
tr.op.ax., axial bundle of tractus opticus
tr.op.b., basal bundle of tractus opticus
ir.op.l; tr.op.lat., tractus opticus lateralis, or
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
tr.pt. hy., tractus pretecto-hypothalamicus
tr.pt.tec, tractus pretecto-tectalis
tr.pt.th., 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.
tr.st.ped., tractus strio-peduncularis =
tr.st.pt., tractus strio-pretectalis
tr.st.t.; tr.st.teg., tractus strio-tegmentalis =
tr..st.tec., tractus strio-tectalis
tr.st.th., 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
tr.t.pt., 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
tr.t.th.h.c.a., tractus tecto-thalamicus et
hypothalamicus cruciatus anterior
tr.t.th.h.c.p., tractus tecto-thalamicus et
hypothalamicus cruciatus posterior
tr.th.r., tractus tecto-thalamicus rectus =
tr.teg.b., tractus tegmento-bulbaris
tr.teg.inp., tractus tegmento-interpedimcularis
tr.tcg.is., tractus tegmento-isthmialis
tr.teg.p.; tr.teg.ped., tractus tegmeiito-peduncularis
tr.th.h., tractus thalamo-bulbaris
tr.th.f., tractus thalamo-frontalis
tr.th.h.d.c, tractus thalamo-hypothalamicus
dorsalis cruciatus
tr.th.hab., tractus thalamo-habenularis
tr.th.mam., tractus thalamo-mamillaris
tr.th.p.; tr.tk.ped., tractus thalamo-peduncularis
tr.th.p.c; tr.th.ped.c., tractus thalamo-peduncularis cruciatus
tr.th.p.d., tractus thalarao-peduncularis dorsalis
tr.th.t., tractus thalamo-telctalis
tr.th.teg.d.c, tractus thalamo-tegmeutalis dorsalis cruciatus, with divisions A and B
tr.th.feg.r., tractus thalamo-tegmentalis rectus
tr.th.teg.v.c, tractus thalamo-tegmentalis ven
tralis cruciatus
tr.th.teg.r.r., 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
v.az.sep., vena azygos septi
r.l., ventriculus lateralis
r.m.a., velum meduUare anterius
r.par., paraphysial veins
r4-, fourth ventricle
z.lim.lat., zona limitaus lateralis
z.lim.med., zona limitans medialis
(1) to (10), tegmental fascicles

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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

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

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

Chapter XIX The Cerebral Hemispheres Subdivisions of the Hemisphere

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

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

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

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

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

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


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


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

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

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

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

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

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

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


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

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

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

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