View source for Talk:Paper - Certain developmental relations and fiber connections of the triangular gyrus in primates (1948)

CERTAIN DEVELOPMENTAL RELATIONS AND
FIBER CONNECTIONS OF THE TRIANGULAR
GYRUS IN PRIMATES

ISABEL LOCKARD?
Department of Anatomy, University of Michigan, Ann Arbor

TWENTY-FIVE FIGURES

INTRODUCTION

The foundation for most of our knowledge of the cortical
aspects of speech was laid by the work on aphasia of various
early observers such as Broca (1861a, 1861b), Jackson (Head,
15), Wernicke (1874), and Marie (’06a, ’06b, ’06c), and
more recently Goldstein (’26), Head (’26), Weisenberg and
McBride (’85) and Nielsen (’47). Apart from their studies
little attention has been given to the neurological patterns
involved either in normal speech or in speech disorders.

It is the purpose of this paper to present certain anatomical observations which are related to this subject. Fundamental to an understanding of either normal or pathological
speech is a knowledge of the development of the so-called
speech centers of the brain with their relations to the rest
of the developing cortex and, most especially, of their fiber
connections.

The results presented in this paper, although not complete,
provide a basis for further research on the speech centers.
It is planned to continue this line of investigation as suitable
human material becomes available.

‘A dissertation submitted in partial fulfillment of the requirements for the
degree of Doctor of Philosophy in the Department of Anatomy, University of
Michigan,

349
850 ISABEL LOCKARD

I wish to thank the Horace H. Rackham School of Graduate
Studies of the University of Michigan for the generous grant
which aided in this study. Most of all I'am indebted to
Professor Elizabeth C. Crosby, under whose careful direction
this paper was planned and executed.

LITERATURE

Four books were used extensively in the preparation of this
paper. They are ‘‘Contribution to the Surface Anatomy of
the Cerebral Hemispheres with a Chapter upon Craniocerebral Topography by V. Horsley’’ by Cunningham (1892),
‘‘Das Menschenhirn”’ and ‘‘Das Affenhirn’’ by Retzius (1896,
06), and ‘‘The Comparative Anatomy of the Nervous System of Vertebrates Including Man’’ by Ariéns Kappers,
Huber and Crosby (’36). Pertinent details obtained from
various papers in the literature are discussed in the text.

MATERIAL AND METHODS

In the study of the ontogenetic development of the neopallium, the brains of 19 human fetuses, of one newborn and
of one two-year-old child were used. This material was
made available at the University of Pittsburgh through the
courtesy of Dr. Davenport Hooker. The ages of the fetuses
were estimated according to the Boyd scale (’41) for the
crown-rump length (table 1). A margin of error was recognized as caused by variation in size of fetuses of a given
age, and probably by swelling as well, in that the material
was fixed in 10% formalin. An example of this unavoidable
inaccuracy is shown in fetuses F17 and F18 (figs. 8 and 10)
the ages of which were estimated at 27 and 293 weeks, respectively. Fetus F17 had the better developed brain and
perhaps was the older fetus.

The phylogenetic development of the triangular gyrus
was studied from the brains of 11 monkeys ranging from
Cercopithecus to Papio. Several of these brains were gifts
of Dr. Fred Mettler (Department of Neurology, College of
PRIMATE TRIANGULAR GYRUS 351

Physicians and Surgeons, Columbia University) to the Laboratory of Comparative Neurology, Department of Anatomy,
University of Michigan. The material, both human and
monkey, was supplemented extensively by the many fine
illustrations in Retzius’ atlases (1896, ’06).

For the study of the fiber connections, the brain of Macaca
mulatta was used. Two series, cut transversely (A at 201
and B at 30») and stained according to the Weil method, were
available for study from the Huber Neurological Collection.

A blotter reconstruction of the cerebrum was made from
the macaque series B, for use in the study of fiber connections.
Sections were traced at a magnification of 4.3 x. Twentytwo out of every 100 sections were used so that the complete
reconstruction approximated 4.3 times the length of the original brain. The model (figs. 24 and 25) was used to relate
gyri and sulci as seen in cross section to the surface of the
brain so that the fiber bundles under consideration could be
located accurately with respect to their cortical relation.

ONTOGENETIC DEVELOPMENT OF CERTAIN NEOPALLIAL REGIONS
Lateral wall and insula

Early in development the cerebrum is somewhat beanshaped with the frontal pole most anterior and the temporal
pole most posterior. The occipital pole is not evident as a
protuberance (see His-Ziegler model of brain of 13.6mm
human embryo). As the brain enlarges, the pallium envelops
the basal ganglia and the thalamus. This overlapping of the
more centrally-placed structures occurs in all directions, the
posterior part of the pallium showing the most conspicuous
change in position. The temporal lobe extends downward and
anteriorly, bringing the occipital pole into view. By approximately 16 weeks (F5) the temporal lobe has assumed a
position approaching that at term and in fetuses a few weeks
older (F11 and F15) it has completed its shift forward.
During this process, the oceipital lobe gradually descends into
a position comparable to that which it occupies at birth. It
352 ISABEL LOCKARD

is during this early period that the brain enlarges most
rapidly. As the fissural pattern appears, the expansion is
retarded, this being a period of differentiation (compare figs.
1-6 with figs. 6-12).

As the temporal Jobe shifts downward and anteriorly, it
forms the Sylvian fossa (Jordan and Kindred, ’32). The floor
of this fossa is the primordium of the insula. The cortex in
this region (nearest corpus striatum) is the first to thicken
and differentiate (Streeter, ’12; Kodama, ’26). By 9 or
94 weeks fascicles can be traced from insular cortex into the
internal capsule. Thus the insula, anchored firmly to the
underlying structures, retains its position as the temporal

TABLE 1
Data concerning fetuses used in study
APPROXIMATE AGE

IN WEEKS ACC. TO
BOYD SCALE

O-R LENGTH

1
NO, OF FETUS IN MM

Fl 54 103
F2 65 U1
F3 109 143
¥F4 112 144
F5 119 154
F6 123 154
F7 131 16
F8 134 164
F9 136 164
F10 152 18
Fu 162 19
F12 167 19
F13 182 204
Fu4 197 29
F15 210 23
F16 220 24
F17 246 27
F18 268 294
F19 280 314
F20 40

 

1The fetuses are numbered in order according to their estimated ages and
these numbers are used throughout the text and on the figures. For convenience
and completeness the brain (F20) of the baby at term is included in table 1.
PRIMATE TRIANGULAR GYRUS 353

lobe pivots around it and, in the course of further development, becomes buried under the opercula,

The inferior limiting sulcus of the insula appears at about
15 weeks. The posterior part of the suleus develops first (F3,
F6). Soon thereafter the superior limiting suleus is evident
(F6). These two sulci join secondarily to form a C-shaped
erease, the circular suleus, which outlines the insula (fig. 2).

As the opercula begin to overlap the insula, the Sylvian
fossa assumes a triangular shape (19 weeks, fig. 5). In the
frontal operculum a fairly sharp angle is formed at the place
where an anterior ramus of the lateral (Sylvian) fissure is
seen in older fetuses. At about this time, or a little later, the
insula begins to enlarge and the bulge which appears in its
anterior portion (fig. 5) makes it possible to distinguish an
anterior from a lateral surface. Concurrently the cortex anterior to the insula is enlarging and the anterior bounding
ramus of the insula appears (fig. 6). As the opercula approach each other, the lateral surface of the insula exhibits
a superior aspect under the parietal and frontal lobes and an
inferior aspect under the temporal lobe.

In the material studied the central suleus, which separates
the long from the short gyri of the insula, appears at about
22 to 24 weeks (fig. 7). In Cunningham’s series it was present
toward the end of the 5th month, and according to Connolly
(740) it occurs at 25 or 26 weeks. In fetuses F17 and F18 the
central sulcus of the insula is present only on the inferior
aspect of the lateral surface, suggesting that the lower part
of the suleus develops first. At this stage, the central sulcus
of the insula is in line with the fissure of Rolando (fig. 7;
Eberstaller, 1887; Cunningham, 1891 and 1892), although this
relationship is not evident in the adult. According to Cunningham (1891) the precentral sulcus of the insula appears
shortly after the central suleus and bears the same relationship to the precentral suleus of the frontal lobe. The postcentral sulcus does not appear before the middle of the 6th
month or later (Cunningham, 1892). In the material studied
there was no indication of insular sulci other than the central
304 ISABEL LOCKARD

in fetuses younger than 315 weeks. In the fetus of that age
(F19), as in the brain at term, all the insular sulci are present.

The lateral fissure closes in sections posteroanteriorly.
At 22 weeks the posterior ascending ramus (fig. 13). and by
313 weeks the portion between this ramus and a point underlying the central or precentral sulcus of the hemisphere is
closed. This condition persists until after birth (fig. 12),
the closure of the last and most anterior section of the fissure
being effected by the postnatal development of the frontal
and temporal association areas including their opercular
portions. At two years of age the lateral fissure is completely
closed.

Precentral and postcentral gyri

The pre- and postcentral gyri can be recognized first when
their delimiting sulci appear. In the material studied the
central and postcentral sulci are present at 22 weeks (F'14).
An earlier stage in the development of the central sulcus
at the time it has two parts (deseribed by Cunningham, 1892)
is demonstrable in the 23-week fetus (fig. 6) in which the postcentral and the superior and inferior precentral sulci are
also present, According to Streeter (712) the pre- and postcentral sulei appear soon after the central. The material
studied and that depicted by Retzius (1896) support this view.

The two parts of the central suleus of Rolando lengthen
and join secondarily to form one shallow sulens which, by
24 weeks (fig. 7), arches anteriorly and downward over the
lateral surface of the hemisphere. The central suleus continues to lengthen and by about 28 weeks the upper portion
has buckled, thus forming the superior genu with its convexity directed anteriorly and the inferior genu with its convexity directed posteriorly. The lower part of the sulcus
has remained comparatively straight except for a backward
bending of the inferior tip, occasionally seen in fetuses of this
age. Concurrent with this development in the contour of
the sulcus is its extension onto the medial surface of the
hemisphere (F17 and F19, fig. 11). Cunningham (1892) did
PRIMATE TRIANGULAR GYRUS 359

not observe this medial extension until the beginning of the
10th month. By term (fig. 12) the contour of the central sulcus
corresponds fairly well with that found in the adult brain.

There is a progression in development of the walls of the
suleus as well as of the surface contour. As the genua of
the central sulcus develop, corresponding ridges, perpendicwar to the surface of the brain, appear on the suleal walls.
The vertical ridges, however, are not dependent on the
surface buckling of the suleus, for they may appear before
the genua are evident on the surface (19). These ridges
are more pronounced at term than in younger fetuses. On
the anterior wall of the sulcus there are slight protusions
which cause the suleus to slant downward and somewhat
posteriorly. At two years of age these protusions are so exaggerated that the walls of the sulcus are markedly irregular
and its slant has correspondingly increased. The depth of the
central sulcus (about 18mm at the center) is approximately
double that at term.

Triangular gyrus

For many years Broca’s area (the triangular gyrus, together with the opereular and possibly the orbital portions
of the inferior frontal gryus) was regarded as the cortical
center for speech. Broea (1861a) was the first to report the
disorder known as motor aphasia. Although in his patient
the extent of brain damage was large, he attributed the loss
of speech to destruction of the cortex in the region which
bears his name. Later observers questioned the justification
of this concept. Marie (’06a, ’06b, ’06c) and Marie and Foix
(’17) believed that the inferior frontal gyrus did not play an
essential role in the function of language, but that the important language center is in the posterior part of the superior temporal gyrus (Wernicke’s area). Following their work
and that of Head (’26) there arose the belief that speech is
made possible by the integrative processes of various cortical
areas. Localized areas, lesions of which produce aphasia,
are currently regarded as nodal points in the fiber systems
356 ISABEL LOCKARD

associated with speech. Such a nodal point is the left triangular gyrus in right-handed people.

The region that will develop into the triangular gyrus and
the opercular portion of the inferior frontal gyrus can first
be recognized as such when a horizontally-placed sulcus.
makes its appearance. This sulcus, extending forward from
the superior limiting sulcus of the insula at its junction with
the anterior bounding ramus, appears during the 6th month
(fig. 6). It resembles the anterior horizontal ramus of the
lateral fissure and, according to Cunningham (1892), was regarded as such by Broca. Cunningham himself (1892) believed that the wedge-shaped pars triangularis grows into
the original single sulcus, thus producing both the anterior
ascending and anterior horizontal rami. The relationship
of the parts of this suleal complex at various stages of development he described as resembling the letters I, Y, V, and U.
In Kollmann’s well-known illustration of a fetal brain at the
end of the 7th month (’07, see his figure 621) a single
horizontally-placed ramus is labelled the anterior ascending
ramus. With this interpretation our material agrees.

To differentiate the anterior ascending from the anterior
horizontal ramus of the lateral fissure during their early
development, two features characteristic of these sulci were
used as criteria. At the age when both the anterior ascending
and the anterior horizontal rami are present and readily
distinguishable, the anterior ascending ramus is frequently
found to bifureate. The anterior horizontal ramus, however,
is rarely forked, even at term. A second feature useful in
the identification of the two anterior rami is the fact that
the inferior frontal suleus arches over the upper part of
the anterior ascending ramus and may swing down into the
triangular gyrus between the two anterior rami (fig. 8).
Although the inferior frontal sulcus is occasionally discontinuous, its relationship to the anterior ascending ramus is,
for the most part, constant. Usually the inferior frontal
sulcus has no such relation to the anterior horizontal ramus,
although occasionally it approaches this ramus. These two
PRIMATE TRIANGULAR GYRUS 357

criteria (the bifurcation of the anterior ascending but not of
the anterior horizontal ramus, and the relation of the inferior frontal suleus to the anterior ascending but usually
not to the anterior horizontal ramus) make it possible to
establish the identity of the anterior rami in many cases, even
though the sulci are not in their adult positions (fig. 14), or
in case only one ramus is present (fig. 15). On the basis of
these criteria it is possible to identify one or both of these
rami by their positions and relations, before the anterior
ascending ramus has bifureated or the inferior frontal sulcus
is well-defined (fig. 13).

In the course of development, the anterior ascending
ramus, originally horizontally-placed, shifts into an upright
position. This change occurs gradually and, in the hemispheres studied and in Retzius’ illustrations (1896), many
intermediate stages are evident. Some of these early stages,
before the horizontal ramus has appeared, are shown in figures 6, 13, and 15. As the insula enlarges, its anterior portion
displaces the Sylvian tip of the anterior ascending ramus
downward and anteriorly. Growth of the frontal cortex must
also play a part in determining the position of the sulcus.
In many brains at about the end of the 7th or beginning of
the 8th month the anterior ascending ramus is bifurcated,
although not uncommonly this sulcus is unforked in older
fetuses and also in adult brains.

Usually the anterior horizontal ramus does not appear
until the anterior ascending ramus has reached an angle of
approximately 45° with a plane running parallel to the base
of the brain, although the ascending ramus may still be
quite short. In some cases the anterior ascending ramus is
bifurcated before the anterior horizontal ramus is first evident, and in others it is not bifurcated even though the anterior horizontal ramus has appeared (cf. figs. 7 and 15,
anterior ascending ramus not labelled in fig. 7). When the
anterior horizontal ramus appears, it slants downward or
downward and anteriorly (fig. 14). Then, in the course of
development, it swings upward into a horizontal or slightly
358 ISABEL LOCKARD

ascending position (figs. 14, 7, 12), the angle formed by the
two rami remaining relatively constant.

Like the anterior ascending, the anterior horizontal ramus
varies as to time of appearance and rate of development.
In the material studied, it was absent on both sides in the
23-week fetus (fig. 6) but clearly evident on both sides in
the 24-week fetus (fig. 7). In the Retzius material (1896),
the anterior horizontal ramus was prominent in some brains
as early as the 7th month but in others it was no more than
an indentation of the surface at 9 months.

The anterior rami of the two sides do not necessarily develop concurrently. Thus, in fetus F18 (293 weeks) the two
anterior rami are present and approximately the same length
on the right side, the anterior ascending ramus forming an
angle of 44° with a plane parallel to the base of the brain.
On the left side, only one horizontally-placed ramus is present.
Whether there is correlation between the presence of such
unilaterally better developed anterior rami and dominance of
the corresponding hemisphere was not determined.

In summarizing the development of the triangular gyrus
it should be kept in mind that the relations are more nearly
constant than is the time at which various changes take place.
The triangular gyrus first becomes evident during the 6th
month when the horizontally-placed anterior ascending ramus
of the lateral fissure appears. At this time the primordium
of the triangular gyrus lies below the anterior ascending
ramus and in front of the insula. There is as vet no anterior
horizontal ramus. As the insula enlarges and extends farther
forward, the triangular gyrus is displaced to a position anterior to and above it. About midway in the course of this
change in position, the anterior horizontal ramus appears,
first being nearly vertical, then shifting with the triangular
evrus to a horizontal position. By term, the anterior ascending ramus has reached a vertical or nearly vertical position
and the anterior horizontal ramus, a horizontal position.

The anterior ascending ramus frequently bifureates at
about the time the anterior horizontal appears. Cunningham
PRIMATE TRIANGULAR GYRUS 3909

(1892) interpreted the branches of the anterior ascending
ramus as an early stage of the Y formation. Later, the two
true anterior rami form a Y, V, or U. Such a relationship
in the later stages represents the proportionate development
of the fronto-parietal and orbital opercula on the one hand
to that of the triangular gyrus on the other. These formations
probably indicate individual variations and play no outstanding part in the development of the triangular gyrus.

Auditory area

The superior temporal sulcus develops by the fusion of
two (or oceasionally three) separate segments. At 23 weeks
a posterior segment is present (fig. 6). By 24 weeks an
anterior portion, underlying the postcentral gyrus, has appeared. By 28 weeks the anterior and posterior segments have
joined and the superior temporal gyrus is demareated (figs.
8, 10).

At about the same period as the fusion of the parts of
the superior temporal sulcus, a faint suleus separates the
primordium of the anterior from that of the posterior
Heschl’s convolution (fig. 10). Coneurrently with the forward extension of the superior temporal sulcus, an anterior
Heschl’s convolution appears as a ridge on the upper surface
of the superior temporal gyrus, although the region of the
future posterior Heschl’s convolution remains flat (fig. 8).
The left side apparently precedes the right in development.
Oceasionally it is difficult to determine whether the ridge is
the anterior or the posterior convolution of Heschl. At term,
the anterior convolution lies behind the posteentral sulcus.
The first ridge to appear underlies the postcentral sulcus or
the posteentral gyrus, so it is probably the anterior convolution of Heschl. In the brain at term (fig. 12) two ridges,
representing the anterior and posterior convolutions, are
present.

There are certain differences between Heschl’s convolutions at ‘term and in the adult. At term the middle sulcus
360 ISABEL LOCKARD

is visible for about 3mm on the exposed surface of the superior temporal gyrus. In the adult this suleus does not reach
the surface of the lateral fissure. The suleus forming the
posterior boundary of the posterior convolution is shorter,
in proportion to the middle suleus, than it is in the adult. At
term it also reaches the surface of the lateral fissure but not
in the adult. A short convolution which lies posterolateral
to the posterior Heschl’s convolution is entirely visible on
the surface at birth, but is hidden within the lateral fissure
in the adult brain. At term Heschl’s convolutions make up
almost the entire upper surface of the superior temporal gyrus within the lateral fissure; in the adult there is a great
increase in the opercular cortex behind and to a lesser extent
in front of them. The differences between the auditory region
at term and in the adult indicate changes in the association
areas rather than in the projection area itself. The auditory
association area, located in the superior temporal gyrus,
continues to develop after birth and closes Heschl’s convolutions, their sulci and the little convolution completely
within the lateral fissure. Postnatal development of the auditory association area may also account for the increase in
the opercularized cortex posterior and anterior to Heschl’s
convolution. During their development, Heschl’s convolutions maintain a fairly constant position in relation to the
insula, the surface of the lateral fissure and the postcentral
gyrus. The auditory area develops late, therefore an arrest
in development at or near term might still interfere with
hearing. No morphological changes were noted which could
be related to the development of a localization pattern such
as that described by Woolsey and Walzl (742).

Visual area

Before the cortex has begun to differentiate, the wall of
the hemisphere is thin. The region overlying corpus striatum
is the first cortical area to thicken and differentiate. From
there growth of the wall gradually extends over the whole
PRIMATE TRIANGULAR GYRUS 361

pallium (Streeter, 12). At 11 weeks (F2) the medial wall
of the hemisphere is thin and smooth. By 144 weeks (fig. 1),
it has expanded ‘without thickening appreciably and has
become folded, forming bulges and intervening, radiallyarranged sulci on the medial surface. According to Kollmann
(1898) this folding of the thin medial wall may occur as
early as the 8th week but reaches its maximum development
during the third or 4th month. With the development of corpus callosum and the consequent thickening of the medial
wall, all or all but one or two of these sulci are obliterated
(about 163 weeks, fig. 3; about 18 weeks, according to Retzius’
illustrations, 1896). Kollmann (1898) also noted that most of
these sulci are transitory and that they disappear with the
development of corpus callosum. Some observers have regarded these transitory sulci as artefacts produced by fixation (Mall, ’03, Hochstetter-— quoted by Symington, ’01),
however, evidence to the contrary lies in the fact that in
malformed brains in which corpus callosum has failed to
develop these sulci are not transitory but remain as permanent sulci of the medial wall (Ariéns Kappers, Huber and
Crosby, ’36). In normal brains, the two posterior sulci of
this group may remain as primitive parieto-occipital and
calcarine fissures, as in fetus F9 (fig. 3). Kollmann (1898)
also stated that certain of the radially-arranged sulci may
persist and that they coincide with the developing parietooccipital and calearine fissures. In some hemispheres, however, these primitive sulci also are transitory, and the medial
wall is left smooth (F8). In other hemispheres the parietooccipital fissure remains and the calcarine disappears (fig. 4)
and in still others, the converse is true. Many examples of
these variations are to be found in the material used for
this study and in that illustrated by Retzius (1896).

In those hemispheres in which the early calcarine or parieto-occipital fissure, or both, are obliterated, the permanent
fissures may not appear until much later. Among the hemispheres illustrated by Retzius (1896) there were specimens as
362 ISABEL LOCKARD

old as 6 months in which there was no more than a suggestion
of one (the other being present) or both of these fissures. If
the early calcarine has disappeared, the permanent calcarinc
fissure develops in two parts—as a dimple on the medial
occipital wall (F'14) and as a notch on the posterior wall of
the parieto-occipital fissure near its lower border (fig. 4).
The dimple on the medial occipital wall elongates anteroposteriorly and becomes the posterior limb of the fissure.
The notch on the wall of the parieto-occipital fissure extends
backward to join that portion which has arisen independently.
If the two early fissures have been retained, they may or
may not join anteriorly; frequently there is a marked depression or hollow in the region where they come together (fig. 3).

During the course of their development these fissures retain
their original positions. However, if either the parieto-occipital or the calearine fissure (or both) has developed secondarily and is therefore shallow, it deepens as the adjoining
pallium increases in thickness. If both the early fissures have
been retained, the characteristic depression at the point where
they meet is obliterated by growth of the pallium in this region. Concurrently with this filling-in process, there is an extension of the fissural complex downward and anteriorly in
line with the parieto-occipital fissure. This extension resembles the anterior limb of the calcarine fissure but in those hemispheres in which the parieto-occipital fissure is deeper than
the calcarine, the floor of the extension is continuous with that
of the former rather than that of the latter. In those hemispheres in which the floors of the two fissures are of approximately the same depth, a slight bulge in the floor separates
the calearine from the rest of the sulecal complex.

A later stage is evident in the fetus F18 in which the
calearine fissure, more shallow than the parieto-occipital,
extends into the depth of the parieto-occipital, lying as a
sulcus along its posterior wall. In fetus F17 the superior
lip of the calearine fissure has increased and turns downward
into the parieto-occipital fissure where it ends. In this fetus,
PRIMATE TRIANGULAR GYRUS 363

as in fetuses F19 (fig. 11) and F20, the anterior growth of
the calearine fissure has thus cut off the parieto-occipital
fissure so that the extension becomes continuous with the
calearine fissure, forming its anterior limb, rather than the
remaining a part of the parieto-occipital fissure. In brains of
older fetuses, such as that of fetus F18, in which the parietooccipital fissure is deeper than the calearine it is likely
that the early parieto-occipital fissure was retained but that
the primitive ecalearine fissure was lost and later replaced.

The entire extension or anterior limb of the calcarine
fissure is visible on the medial surface (that is, not hidden by
the cerebellum) from the time of its appearance until term.
At two years of age an intermediate stage is reached. In the
adult, the anterior limb of the calearine fissure is usually
concealed by the cerebellum.

During the 7th month, although sometimes as early as
the 6th or as late as the 9th month according to Retzius’
illustrations (1896), the posterior tip of the calcarine fissure
widens (fig. 9). That part of the cortex which forms the
floor of the fissure in this area enlarges, fills in the space
between the two calcarine lips, and thus forms the bifurcation
so common to this fissure (fig. 11). It is this portion of the
visual cortex — between the two arms of the bifurcation —
which receives impulses set up in the macular portion of
the retina. This division of the calearine fissure, present
usually during the 7th month in man, develops with the differentiation of the fovea centralis which ‘‘begins to form
after the 6th month and is present in fetuses of 7} or 8
months’’ (Streeter, 712, p. 248).

During the last two months of intrauterine life, the calcarine fissure lengthens. If the fissure does not bifureate, or
if the arms of the bifurcation are short, the calcarine fissure
tends to be longer than it does if the arms are long. At about
term the tip of the calearine fissure extends beyond the
occipital pole onto the lateral surface of the brain for a variable distance.
364 ISABEL LOCKARD

Supramarginal and angular gyri

Lesions of the supramarginal gyrus, which usually impinge on the temporal isthmus and. therefore interfere with
associations of the temporal lobe, may produce an amnesic
aphasia (Nielsen, ’47). In such types of aphasia there is
difficulty in naming objects and frequently grave interference
with the whole significance of language. The angular gyrus,
bordering the visual psychic area, is regarded as a visual
correlation center and a lesion in that area produces a sensory
aphasia involving visual engrams. According to Nielsen (’47,
p. 248), the angular gyrus is ‘‘the area of revisualization of
the written word.’’ Like the triangular gyrus, the supramarginal and angular gyri represent nodal points in the
system of association fibers. Lesions involving these gyri
affect not only the cortex but also the related short association pathways and the underlying long association bundles
as well.

The supramarginal gyrus can first be recognized at about
24 weeks (fig. 7). It is marked off superiorly by the intraparietal suleus. The angular gryus cannot be identified at
this time. By about 28 weeks (fig. 10) both the angular and
the supramarginal gyri are present although they are not
easily distinguishable. These gyri rapidly become prominent.
The angular gyrus takes on a lobulated form (F19), which
is still present at term (fig 12). There are three lobules.
One of these is mostly posterior or inferior and the other
two are largely superior to the superior temporal sulcus.
By the second year this lobulation has disappeared.

Association areas

Parietal association area. The general association area of
the parietal lobe is located in the superior parietal Jobule
and in the superior part of the inferior parietal lobule. From
approximately 28 weeks until term the parietal lobe is composed predominantly of the postcentral gyrus and the supramarginal and angular gyri; the superior part of the inferior
PRIMATE TRIANGULAR GYRUS 363

parietal lobule is absent and the superior parietal lobule is
small and poorly defined. At the point where the parietooccipital fissure reaches the upper border of the brain, there
is a pronounced depression (figs. 8 and 12). With the postnatal enlargement of the parietal association areas, these
changes occur: (1) the supramarginal and angular gyri lie
farther from the upper border of the hemisphere but retain
their relation to the superior temporal and lateral fissures,
(2) the parieto-occipital depression disappears, and (38) the
parieto-occipital fissure cuts the upper border of the hemisphere at a point considerably more caudal than before birth.

Frontal association area. The Rolandic angle was measured as an indication of the development of the frontal
association area. However, no significant variation was
found. Such a lack of change is interpreted as indicating a
synchronous development of the parieto-occipital and the
frontal association areas.

Occipito-temporal association area. Sample measurements
of several brains were taken at the angle between the base of
the temporal lobe and a line from the occipital pole to the
posterior tip of the lateral fissure. It was thought that
changes in the angle would reveal any disproportionate
growth of either the occipito-temporal cortex within the
angle or the parieto-occipital cortex above and outside the
angle. No change in the angle was noted, so no detailed
measurements were taken.

From the observations made of the association areas, it
may be concluded that these various regions develop concurrently. It should be noted that at term they are not much
better developed than in much younger fetuses, although at
two years the association areas resemble those of the adult.

PHYLOGENETIC DEVELOPMENT OF CERTAIN NEOPALLIAL REGIONS
Triangular gyrus

Study of specimens of primate brains in the Laboratory
of Comparative Neurology at the University of Michigan and
366 ISABEL LOCKARD

of those illustrated by Retzius (’06) has shown that the
phylogenetic development of the inferior frontal gyrus in
monkeys and apes resembles in some respects the ontogenetic
development of the same area in man. It was noted that, in
subhuman primates, the fronto-orbital suleus is located anterior to the precentral sulcus, in a position corresponding to
the original single anterior ramus of the human lateral
fissure. This sulcus, located anterior to. the temporal lobe,
is not a true branch of the lateral fissure. It is, however, related to the lateral fissure in that (like the developing anterior
ascending ramus of man) its ventral end is continnous with
the anterior fissure bounding the insula (Elliot Smith and
others quoted by Ariéns Kappers, Huber and Crosby, ’36).
In higher apes the fronto-orbital suleus, like the anterior
ascending ramus of man, is frequently found to bifurcate
(fig. 18). Bolk (’10) reported this condition in the gorilla.
The inferior frontal sulcus maintains the same relationship
to the fronto-orbital sulcus in monkeys and apes that it does
to the anterior ascending ramus in man (figs. 16, 17, and 18).
Therefore, according to the criteria used in identifying the
anterior rami in the human fetal brain (see section on the
ontogenetic development of the triangular gyrus), the frontoorbital suleus of subhuman primates is homologous with the
anterior ascending ramus of the lateral fissure of man.

Bolk (710) was of the opinion that the fronto-orbital sulcus
itself was not homologous with either the anterior ascending
or the anterior horizontal ramus, but that the branches of its
bifurcated tip were homologous with the two anterior rami
of the lateral fissure. His views, then, for the gorilla correspond to those of Cunningham (1892) concerning the ontogenetic development of these rami. Elliot Smith believed
the anterior horizontal ramus developed as a lengthening
of ramus opercularis and that the anterior ascending ramus
was a new structure, produced through the growth of the
frontal operculum (Ariéns Kappers, Huber and Crosby, ’36).

In Cebus, there is no fronto-orbital sulens and no triangular
gyrus. It is as though the ventrolateral part of the frontal lobe
PRIMATE TRIANGULAR GYRUS 367

had been dissected away at a time when the fronto-orbital
sulcus (anterior ascending ramus) was still in a horizontal
position, leaving only the upper lip of the sulcus. In the
macaque and in Cercopithecus the fronto-orbital sulcus is
present (fig. 16). In these forms this sulcus slants upward
and forward. This condition is due not to precocity in the
development of the surrounding structures, but rather to
a lack of development of the frontal association area — especially of the cortex which, in man, lies above the anterior
ascending ramus. In the cortex anterior to and below the
fronto-orbital suleus is a rudimentary triangular gyrus. In
these monkeys and in the ape, the cortex buried in the lateral
fissure does not extend far enough forward to play a part
in determining the position of the triangular gyrus and its
bounding ramus (or rami) as it does in man. In these primates, the triangular gyrus is not so far removed from
insular cortex as first seems apparent. The cortex which, in
subhuman primates, forms a part of the lateral surface as
far anteriorly as the fronto-orbital sulcus, and which is
opercularized in man, may be regarded as unopercularized
insula (Ariéns Kappers, Huber and Crosby, ’36). In the
baboon, there is still no anterior horizontal ramus. However,
in the mandril (Papio porcarius, illustrated by Retzius, ’06)
a branch from the orbital sulci approaches the triangular
gyrus, This branch may be regarded as a forerunner of the
anterior horizontal ramus. Among the apes a greater development of this area is evident. In the orangoutan (Simia
satyrus, fig. 17) an anterior horizontal ramus is present.
This suleus, a branch from the orbital sulci, lies on the ventral
surface of the brain evidently because of a lack of development of the orbital operculum. In the chimpanzee (Troglodytes niger) the triangular gyrus is well developed (fig. 18)
and can be seen easily although the anterior horizontal ramus
is still not visible on the lateral surface. In figure 18 the
fronto-orbital sulcus has bifurcated and the inferior frontal
sulcus arches over it as in the human brain. These observations suggest that the triangular gyrus and its bounding
368 ISABEL LOCKARD

rami develop in much the same way phylogenetically as ontogenetically, the fronto-orbital sulcus — or anterior ascending
ramus — preceding the anterior horizontal ramus. Such differences as occur between monkeys and apes on the one hand
and man on the other are due primarily to the increase in the
human frontal association areas.

DESCRIPTION OF THE FIBER CONNECTIONS

In the functioning of the cortex for speech, as in its other
activities, the relations of outstanding significance are its
fiber connections to and from other cortical and basal areas.
In the following pages some of these intereommunicating
systems will be reported. This is not a complete account of
these connections, many more series cut in a variety of planes
and stained by various techniques, supplemented by experimental and clinical material and oscillograph records, will be
necessary before enough information has been gathered to
make even a reasonably complete story. This paper marks
the beginning of what we hope to be a continuing project.
It presents some facts necessary as ground work for ablation
and stimulation experiments and analyses of complex Golgi
and silver material.

The occipital lobe in the monkey has been shown by Poliak
(732) to be concerned with visual functions. Here the retinal
quadrants are projected according to a pattern with the
superior quadrants (inferior visual fields) projected to the
upper half and the inferior retinal quadrants to the lower
half of the area. From the upper part of the occipital cortex,
beginning with the upper lip of the calearine fissure, a bundle
of fibers passes along the superficial border of the white
matter in the superior half of the occipital lobe (where impulses from the inferior visual fields are projected) to reach
area 18. From area 18 fascicles (see A, fig. 19) pass to that
part of area 19 which is rostral to the lower half of area 17
and 18, that is the more ventral portion of area 19. The stimulation experiments of Crosby and Henderson (’48) indicate
PRIMATE TRIANGULAR GYRUS 369

that this area is concerned with conjugate eye movement
downward, and downward and to the opposite side.

Crosby and Henderson (’48) have also shown that from
the under lip of the ealearine fissure fascicles pass to adjoining portions of area 18. From area 18, fibers accumulate
along the outer border of the white matter of the hemisphere.
They are joined gradually as they pass forward by fascicles
from the remainder of area 18 on the inferior surface of the
occipital lobe until they constitute a fairly large bundle. This
bundle turns dorsorostralward, still following the superficial border of the white matter, and then frays out in the
most caudal and dorsal part of area 19. Crosby and Henderson (’48) have found that this part of area 19 on stimulation
gives upward movement of the eyes.

Where the bundles from the inferior occipital region and
those from the superior occipital region approach each
other — and arising apparently from the intermediate regions — are fascicles which distribute to area 19 between
those from the superior and the inferior occipital fields (see
B and C, fig. 20). The region reached by this intermediate
bundle, on stimulation gives conjugate deviation of the eyes
toward the opposite side (Crosby and Henderson, ’48).

The auditory area in the monkey is situated on the upper
part of the exposed surface of the superior temporal gyrus,
and on its opereular portion (figs. 23 and 24). It is not then
so completely buried on the opercular face of this gyrus as
it is in man, in which case, according to Poliak (’32), ‘only
a few fibers reach the convexity of the superior temporal
convolution on the free surface of the temporal lobe.’’ In
the monkey, as in other mammals, projection fibers from
the medial geniculate nucleus of the diencephalon pass sublenticularly to the auditory area. Accompanying this bundle
are fascicles, apparently of cortico-geniculate and corticotectal character, which pass to the medial geniculate nucleus
and the superior colliculus. There are also bundles, presumably of cortical origin, which continue caudalward behind
370 ISABEL LOCKARD

the medial geniculate nucleus to the capsule, at least, of the
inferior colliculus (Crosby and Henderson, ’48).

From auditory cortex association fibers pass to the caudal
part of the middle temporal gyrus which is below the region
of distribution of the fibers from the upper occipital lobe and
near the juncture of areas 19, 20, 21 and 22 (see G, fig. 23).
A second interconnection of the auditory cortex with the
middle temporal gyrus is provided by a two-neuron chain
which undergoes synapse in the superior temporal gyrus,
above the deep part (X, fig. 23) of the superior temporal
suleus and below the auditory projection area. There are also
fibers which connect the auditory area and the middle temporal gyrus by-passing this region of synapse in their course
downward and medially (see H, fig. 23). From the middle
temporal gyrus (lower part of areas 19 and 21), and from
the underlying inferior temporal gyrus fascicles pass out
of the cerebral hemisphere to lower centers. These bundles
carry three general types of fibers, cortico-pontine, corticopulvinar (Walker, ’38), and the internal cortico-tectal system
of Huber and Crosby (Huber and Crosby in Huber et al.,
’43, for various mammals; Crosby, ’46, for the monkey; also
Mettler, ’32, for the cat). The differential origin of the three
systems has not been satisfactorily established but the presence of these association fibers from the auditory cortex
suggests that the external cortico-tectal tract is part of
a discharge path from auditory cortex to centers of the midbrain where it may make efferent connections.

Short fibers relate the auditory area with the association
areas adjoining it. Superiorly the fibers of this bundle pass
to the inferior parietal lobule both along the lateral fissure
and within the fissure (see G, fig. 23), and to the cortex of
the superior lip of the lateral fissure deep to the pre- and
postcentral gyri. There are fascicles from these areas into
the lower part of the corresponding motor or sensory areas.
Since the inferior part of the cental sulcus is directed downward and somewhat posteriorly, more of these fibers are
associated with the motor than with the sensory area. In
PRIMATE TRIANGULAR GYRUS 371

the two series studied, many of these bundles passing from
the region above the lateral fissure to that below it, do so
without synapse in the insula on the right side. On the left
side, however, in the region where the fibers join the cortex
deep to the motor area, the converse is true (see E and F,
fig. 22). It is evident then that the auditory area is connected
with that portion of the motor area which controls the
muscles of the larynx, tongue, and lips — that is, the musculature involved in vocalization and articulation. To a lesser
extent the auditory area is related to the corresponding part
of the sensory area. This connection is by way of a 3- or
4-neuron chain from the auditory projection area to adjoining
association cortex and from there to the cortex adjacent to
the motor or sensory area, The impulses may pass from the
auditory association area to the insula and from the insula to
the cortex bordering on the motor or sensory areas. There are
also direct bundles from the auditory projection area to the
sensory area.

The superior longitudinal and the inferior fronto-occipital
fasciculi interconnect various cortical areas of the hemisphere from its frontal to its occipital pole. The superior
longitudinal fasciculus lies above and medial (fig. 21), and
the inferior fronto-occipital fasciculus lies below and medial
to the claustrum (fig. 23). As these bundles go forward
their fibers fan out and intermingle with the rest of the
white matter. Such is their position that deep lesions of
the triangular gyrus must almost necessarily’ involve them.
The superior longitudinal fasciculus contains fibers which
interconnect the insula, upper and anterior part of the
claustrum (see D, fig. 21), and the frontal cortex, including
area 4 and probably the triangular gyrus, with the occipital
and temporal cortices. These anterior areas are also interconnected with one another through this fasciculus but the
bundle is largest in the region of area 4 so it is logical to
assume that there are many interconnections between these
anterior areas and those lying posterior to the central sulcus.
The inferior fronto-occipital fasciculus carries fibers which
372 ISABEL LOCKARD

interconnect frontal and occipito-temporal cortex. The indications are that the part of the occipital cortex on which are
projected impulses from the upper retinal quadrants is related to the frontal cortex through the superior longitudinal
fasciculus and the part which receives impulses from the
lower retinal quadrants through the inferior fronto-occipital
fasciculus.

It is evident then that the triangular gyrus is a nodal
point in a series of association systems. Destruction in this
area will profoundly affect the richness of associations in
regions distant from the site of the lesion and impoverish the
general association pattern of the cerebral hemisphere.

DISCUSSION

The organs used for speech serve a dual function — that
of speech and some vegetative process. Similarly the cortical
control of speech is brought about through the adaptation
of cerebral association fibers rather than through the activity
of any cortical areas specially designed for purposes of
language. Consequently, if such association pathways are interrupted, there are disturbances in the thinking processes
underlying speech. It is therefore possible for a loss of articulate speech to result from lesions in parts of the hemisphere
other than Broca’s area, particularly on the dominant side,
if such lesions interfere seriously with the association pathways. As Nielsen (’47) has pointed out, in order to talk
you must have something to say.

The pattern of association pathways has been considered
in the section dealing with fiber’ bundles in the brain of
Macaca mulatta. Tracts were found which relate visual projection with visual psychic areas, and auditory projection
with auditory psychic and general association areas. These
areas are in turn connected with the association regions of
the frontal cortex, claustrum and insula, and with the motor
area. The triangular gyrus, fibers of which also contribute
to this pattern, overlies the bundles which connect the frontal
cortex with the other areas just mentioned. Articulate specch
PRIMATE TRIANGULAR GYRUS 373

is a highly coordinated type of motor expression based upon
impulses reaching the sensory areas, colored by impulses
involving the association areas and then passed over to the
motor centers. Lesions of the triangular gyrus destroy the
fibers related to that area and may also interrupt the pathways to the frontal cortex, and therefore greatly impoverish
the fiber systems upon which speech depends.

Phylogenetically, as ontogenetically, the understanding of
speech appears before its motor expression. Orton (’35) has
pointed out that many animals (as dogs) learn the meaning
of words, although, on the motor side their communication
is solely on an emotional basis. Hines (’42) observed that the
macaque uses a propositional type of speech as well as the
emotional type. Garner (’07), who studied animals in their
natural habitat, discussed the differences in speech of a number of monkeys and apes and regarded some as exceedingly talkative. Although no detailed study of this kind was
carried on for this paper, observations were made of the
speech and behavior of a young macaque and its mother.
Much of the sound emitted by the monkeys obviously was
no more than noisy screaming but it was evident that in their
more quiet moments there was some communication by vocalization between the two.

Before discussing articulation in the monkey some reference to its development in the human should be made. It
has been suggested that vowel sounds are based on swallowing movements and that underlying consonant sounds are
swallowing, sucking and chewing movements (Shohara, 732,
35). Swallowing and sucking are established prenatally —
swallowing by 13 weeks, and sucking by 29 weeks (Hooker,
44). Thus, at birth, the foundation has been laid for all
vowel and some consonant elements. Although the order in
which sounds develop is not rigidly fixed, vowels tend, with
some overlapping, to appear before the consonant sounds
(West, Kennedy and Carr, ’37). Initial consonants are used
in syllables before final consonants, and single consonants
precede consonant combinations such as bl, er, et cetera. It
374 ISABEL LOCKARD

should be stressed that the ability to articulate sounds is present before their utilization for speech purposes, just as the
vegetative processes on which speech sounds are based precede the ability to articulate those sounds. There is, however,
a considerable overlap in the development of these activities.
Some of the sounds dependent upon swallowing and sucking
movements. appear before the child learns to chew. Later,
with the maturation of cortical association bundles, the child
begins to use words although he may not have mastered all
the more difficult sounds or sound combinations.

Detailed studies of the speech sounds of the monkey were
made by Paget (’30) and of the chimpanzee by Learned (725).
In our work it was observed that the speech pattern of the
baby macaque and its mother was altered not only by a
variety of sound elements but also by variation in the pitch
and the duration of sound. The sound elements consisted
mostly of nasalized vowels and the nasal consonants: 9 (ng),
m, and n. These consonants, among the first acquired in man,
are based on swallowing (9) and swallowing and sucking
movements (m, n) (Meader, ’40). No use of the soft palate
for the production of speech sounds could be detected although Paget (’30) listed ‘‘a soft-palatal release’’ among the
consonant sounds of monkeys, and Learned (’25) included
g and k among the consonant sounds of the chimpanzee.
Single, initial consonants were often used by the baby macaque and its mother although there were no final consonants
or consonant combinations. Learned (’25) noted that combinations and final consonants were both employed by the
chimpanzee.

It is evident that the sounds used by the monkey are those
which appear early in human speech. The nasality of the
monkey sounds also indicates a correlation with developing
human articulation as many believe that soft-palatal action
in articulation is one of the later human acquisitions. The use
of only initial, single consonants by the macaque suggests
a further correlation with an early stage in the human development of articulation. It is interesting to note that
PRIMATE TRIANGULAR GYRUS 375

the chimpanzee, using final consonants and consonant combinations (Learned, ’25), has a greater articulatory range
than does the macaque, and that he is more nearly like the
human in this respect.

The rudimentary speech of the macaque has, for its anatomie basis, a complex associational pattern in which visual,
auditory, and other sensory stumuli come into relationship
with one another and discharge toward motor centers. The
frontal lobe component of this pattern is small in monkeys.
In man, with the great increase in the association areas, especially those of the frontal lobe, there is an increase in the
frontal lobe component to the triangular gyrus and additional
connections with other associational areas of the hemisphere.
Consequently, the anatomical pattern underlying speech activities is greatly enriched in man. This augmentation of
interconnections finds functional expression in the much
varied and complex human speech.

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ABBREVIATIONS

A, fibers connecting upper part area
18 and rostral part area 19

ang.g., angular gyrus

a.as.r., anterior ascending ramus of
lateral fissure

a.b.r.I., anterior bounding ramus of
insula

a.ca.f., anterior limb of calearine fissure

a.H.ev., anterior Hechl’s convolution

a.hy., anterior horizontal ramus of
lateral fissure

awar., auditory area

B, fibers connecting intermediate portions of area 18 and area 19

C, fibers connecting intermediate portions of area 18 and area 19

ca.f., calearine fissure

ca.n., calearine notch

e.s., central sulcus (of Rolando)

¢.s.1., central suleus of insula

cir.s., circular sulcus

el, claustrum

D, fibers from superior longitudinal
fasciculus to claustrum

E, fibers connecting frontal operculum
and insula

e.cap., external capsule

F, fibers connecting superior temporal
gyrus and insula

fr.or.s., fronto-orbital suleus

fr.pa.op., fronto-parietal operculum

G, fibers connecting superior temporal
gyrus and middle temporal gyrus

H.cv., Heschl’s convolutions

ifr.o.fas., inferior fronto-oecipital fasciculus .

ifr.s., inferior frontal sulcus

ipa.lob., inferior parietal lobule

I., insula

lf., lateral fissure

lv., lateral ventricle

m.t.g., middle temporal gyrus

m.t.s., middle temporal sulcus

or.s., orbital suleus

pa.o.f., parieto-oceipital fissure

pa.o.d., parieto-occipital depression

po.s., postcentral suleus

p.ca.f., posterior limb of calearine fissure

p.asr., posterior ascending ramus of
lateral fissure

pr.s., precentral sulcus

pm.H.ev., primordia of Heschl’s convolutions

pu., putamen

su.lo.fas., superior longitudinal fasciculus

su.t.g., superior temporal gyrus

su.t.s., superior temporal suleus

sm.g., supramarginal gyrus

t.co.pon., temporo-cortico-pontine tract

trg.g., triangular gyrus

tru.s., transitory sulci

X, region of synapse for fibers connecting auditory area and middle temporal gyrus
PRIMATE TRIANGULAR GYRUS PLATE 1
ISABEL LOCKARD

  
  

Flt !Quwks.

 
 
 

a aL FIl 1Qwks.

   

lfrs.
aas.G

FI6 24 wks.

   
 
  

ang.gSm.Q.
ut. Ss

FI7 27 wks.

1 to 12 Drawings made from photographs and seale drawings of the material. (x 1)

379
PRIMATE TRIANGULAR GYRUS
ISABEL LOCKARD

PLATE 2

 

F2O0 40wks.

380
PRIMATE TRIANGULAR GYRUS PLATE 3
ISABED LOCKARD

13. 4.

p.as.r
aas.r ifr.
\Yy } aas.r
OF a.hr
6th mo.@arly)
\5. 6or 7t" mo,

 

Troglodytes nig.

13 to 15 Drawings adapted from illustrations in ‘‘Das Menschenhirn’’ by G. Retzius
(1896). (X 2/3)

16 to 18) Drawings adapted from illustrations in ‘‘Das Affenhirn’’ by G. Retzius (06).
(X 2/3)

381
PRIMATH TRIANGULAR GYRUS
ISABWL LOOK ARD

PLATE 4

 

19 to 23 Photomicrographs of selected portions of transverse sections through brain
of Macaca mulatta. Weil technique.

19 to 21 Fascicles labeled include connections between upper part area 18 and
area 19 (A), intermediate portions of area 18 and area 19 (B and C), and fibers
from superior longitudinal fasciculus passing to the claustrum (D). Key drawings
19’ 20’ and 21’ are included for orientation. (x 42)

22 and 23 Fascicles labeled include connections between frontal operculum and
insula (11), insula and superior temporal gyrus (F), parietal operculum and superior
temporal gyrus (G) and middle and superior temporal gyrus (H). (x 10)
PRIMATE TRIANGULAR GYRUS PLATE 5
ISABEL LOOKARD

 

 

383
384

 

PRIMATE TRIANGULAR GYRUS
ISABEL LOCKARD

PLATE

6
PRIMATE TRIANGULAR GYRUS PLATE 7
ISABEL LOCKARD

 

385
PRIMATE TRIANGULAR GYRUS PLATE 8
ISABEL LOCKARD

 

 

2s

24 Latero-ventral view of left side of reconstruction of brain of macaque, showing some of the cortical areas described in text. (XX 1/3)

25 Rostral view of typical block (cross section) of reconstruction at level indictated by arrows on figure 24. (X 1/3)

386