Lesson
No 22
Diencephalon
comprises the thalamencephalon and hypothalamus. Thalamencephalon consists of thalamus opticus, epithalamus and metathalamus.
Hypothalamus formed by front optic
part and back (olfactory) part.
Thalamus opticus
is a paired body, which consists of gray substance. In the front it carries the
anterior tubercle. The posterior
extremity pulvinar is expanded,
directed backward and lateralward, and overlaps the superior colliculus. Each
thalamus about
Coronal section of Thalamus, lateral and third ventricles
Epithalamus
consists of the pineal body (Epiphysis),
and the habenulae with trigonum habenulae, the posterior commissure. Pineal body is
connected with thalamus by the habenulae.
The Metathalamus comprises the geniculate bodies, a medial and a
lateral. The medial geniculate body
(corpus geniculatum mediale) lies
under cover of the pulvinar of the thalamus. The inferior brachium from the inferior
colliculus attaches to the medial geniculate bodies. The lateral geniculate body (corpus geniculatum laterale) is an oval
elevation on the lateral part of the pulvinar. The superior brachium from the superior
colliculus attaches to the lateral geniculate bodies.
Hind- and mid-brains; postero-lateral
view.
Anterior part of the Hypothalamus consists of the optic chiasm and tuber cinereum with infundibulum
that carries the hypophysis. Posterior
part consists of the mammillary bodies and
subthalamic region that carries the corpus subthalamicum (nucleus of Luis).
The third ventricle, the cavity of
the diencephalon has 6 walls:
·
lateral walls formed by medial
surface of the thalamus
·
lower wall (floor) formed by
hypothalamic region. There are infundibuli recess and optic recess
·
anterior wall formed by terminal
lamina, columna fornicis and anterior cerebral commissura
·
anterior wall is formed by the
habenular commissure and posterior commissure. There is suprapineal reces
·
upper wall (roof) formed by tela
choroidea of the III ventricle with plexus choroideus
There is interventricular foramen
(of Monro) between anterior thalamic tubercle and columna fornicis. Foramen
communicates the III ventricle with the lateral ventricles of cerebrum.
Cerebral aqueduct connects the III ventricle with the cavity of the IV
ventricle.
Thalamus
situated in middle place on midsagittal dissection of the brain
The pineal body (corpus pineale; epiphysis) is a small, conical,
reddish-gray body lies in the depression between the superior colliculi and has
an endocrine role. Habenulae extend
from the epiphysis to the right and left thalamus. The pineal recess of the
third ventricle is located near base of the epiphysis. The epiphysis covered by
capsule externally, the septa separate glandular parenchyma into lobuli.
Special glandular pinealocytes and gliocytes are the cells of the epiphysis.
Often there is “sand” in the gland of adults. The epiphysis produces hormone which
inhibits the hypophysis activity until puberty age and takes part in regulation
of the metabolism.
The hypophysis (pituitary endocrine gland) is a reddish-gray, somewhat
oval mass. It is attached to the end of the infundibulum, and is situated in
the fossa hypophyseos of the sphenoidal bone, where it is retained by a
circular fold of dura mater, the diaphragma sellae. The hypophysis consists of
anterior (adenohypophysis) part and posterior (neuorohypophysis) part. Adenohypophysis has three portions:
anterior (or pars distalis), pars intermedia and pars tuberalis. Neuorohypophysis
has pars nervosa and infundibulum. Adenohypophysis (pars distalis) secretes somatotropin,
adrenocorticotropin, thyrotropin, folliculotropin, prolactin and luteotropin.
Pars intermedia produces melanocytestimulating hormone. Neuorohypophysis
secretes vasopressin and oxytocyn both of which are produced in
the hypothalamus.
The hypophysis cerebri in position.
Shown in sagittal section.
Median sagittal through the hypophysis
of an adult monkey. Semidiagrammatic.
The diencephalon is connected above and in front with
the cerebral hemispheres; behind with the mid-brain. Its upper surface is
concealed by the corpus callosum, and is covered by a fold of pia mater, named
the tela chorioidea of the third ventricle; inferiorly it reaches to the base
of the brain.
The diencephalon comprises: (1) the thalamencephalon;
(2) the pars mamillaris hypothalami; and (3) the posterior part of
the third ventricle. For descriptive purposes, however, it is more
convenient to consider the whole of the third ventricle and its boundaries
together; this necessitates the inclusion, under this heading, of the pars
optica hypothalami and the corresponding part of the third ventricle—structures
which properly belong to the telencephalon.
The Thalamencephalon.—The
thalamencephalon comprises: (1) the thalamus; (2) the metathalamus
or corpora geniculata; and (3) the epithalamus, consisting of the
trigonum habenulæ, the pineal body, and the posterior commissure.
Dissection showing the ventricles of the
brain.
The Thalami (optic thalamus)
are two large ovoid masses, situated one on either side of the third ventricle
and reaching for some distance behind that cavity. Each measures about
The anterior extremity is narrow;
it lies close to the middle line and forms the posterior boundary of the
interventricular foramen.
The posterior extremity is
expanded, directed backward and lateralward, and overlaps the superior
colliculus. Medially it presents an angular prominence, the pulvinar,
which is continued laterally into an oval swelling, the lateral geniculate
body, while beneath the pulvinar, but separated from it by the superior
brachium, is a second oval swelling, the medial geniculate body.
The superior surface is free,
slightly convex, and covered by a layer of white substance, termed the stratum
zonale. It is separated laterally from the caudate nucleus by a white band,
the stria terminalis, and by the terminal vein. It is divided into a
medial and a lateral portion by an oblique shallow furrow which runs from
behind forward and medialward and corresponds with the lateral margin of the
fornix; the lateral part forms a portion of the floor of the lateral ventricle,
and is covered by the epithelial lining of this cavity; the medial part is
covered by the tela chorioidea of the third ventricle, and is destitute of an
epithelial covering. In front, the superior is separated from the medial
surface by a salient margin, the tænia thalami, along which the
epithelial lining of the third ventricle is reflected on to the under surface
of the tela chorioidea. Behind, it is limited medially by a groove, the sulcus
habenulæ, which intervenes between it and a small triangular area,
termed the trigonum habenulæ.
The inferior surface rests upon and
is continuous with the upward prolongation of the tegmentum (subthalamic
tegmental region), in front of which it is related to the substantia
innominata of Meynert.
Coronal section of brain immediately in
front of pons.
The medial surface constitutes the upper
part of the lateral wall of the third ventricle, and is connected to the
corresponding surface of the opposite thalamus by a flattened gray band, the
The lateral surface is in contact
with a thick band of white substance which forms the occipital part of the
internal capsule and separates the thalamus from the lentiform nucleus of the
corpus striatum.
Structure.—The
thalamus consists chiefly of gray substance, but its upper surface is covered
by a layer of white substance, named the stratum zonale, and its lateral
surface by a similar layer termed the lateral medullary lamina. Its gray
substance is incompletely subdivided into three parts—anterior, medial, and
lateral—by a white layer, the medial medullary lamina. The anterior part
comprises the anterior tubercle, the medial part lies next the lateral wall of
the third ventricle while the lateral and largest part is interposed between
the medullary laminæ and includes the pulvinar. The lateral part is
traversed by numerous fibers which radiate from the thalamus into the internal
capsule, and pass through the latter to the cerebral cortex. These three parts
are built up of numerous nuclei, the connections of many of which are
imperfectly known.
Coronal section of brain through
intermediate mass of third ventricle.
Connections.—The
thalamus may be regarded as a large ganglionic mass in which the ascending
tracts of the tegmentum and a considerable proportion of the fibers of the
optic tract end, and from the cells of which numerous fibers (thalamocortical)
take origin, and radiate to almost every part of the cerebral cortex. The
lemniscus, together with the other longitudinal strands of the tegmentum,
enters its ventral part: the thalamomammillary fasciculus (bundle of
Vicq d’Azyr), from the corpus mammillare, enters in its anterior tubercle,
while many of the fibers of the optic tract terminate in its posterior end. The
thalamus also receives numerous fibers (corticothalamic) from the cells of the
cerebral cortex. The fibers that arise from the cells of the thalamus form four
principal groups or stalks: (a) those of the anterior stalk pass
through the frontal part of the internal capsule to the frontal lobe; (b)
the fibers of the posterior stalk (optic radiations) arise in the
pulvinar and are conveyed through the occipital part of the internal capsule to
the occipital lobe; (c) the fibers of the inferior stalk leave
the under and medial surfaces of the thalamus, and pass beneath the lentiform
nucleus to the temporal lobe and insula; (d) those of the parietal
stalk pass from the lateral nucleus of the thalamus to the parietal lobe.
Fibers also extend from the thalamus into the corpus striatum—those destined
for the caudate nucleus leave the lateral surface, and those for the lentiform
nucleus, the inferior surface of the thalamus.
Hind- and mid-brains; postero-lateral
view.
The Metathalamus comprises the geniculate
bodies, which are two in number—a medial and a lateral—on
each side.
The medial geniculate body (corpus
geniculatum mediale; internal geniculate body; postgeniculatum) lies under
cover of the pulvinar of the thalamus and on the lateral aspect of the corpora
quadrigemina. Oval in shape, with its long axis directed forward and
lateralward, it is lighter in color and smaller in size than the lateral. The
inferior brachium from the inferior colliculus disappears under cover of it
while from its lateral extremity a strand of fibers passes to join the optic
tract. Entering it are many acoustic fibers from the lateral lemniscus. The
medial geniculate bodies are connected with one another by the commissure of
Gudden, which passes through the posterior part of the optic chiasma.
The lateral geniculate body (corpus
geniculatum laterale; external geniculate body; pregeniculatum) is an oval
elevation on the lateral part of the posterior end of the thalamus, and is
connected with the superior colliculus by the superior brachium. It is of a
dark color, and presents a laminated arrangement consisting of alternate layers
of gray and white substance. It receives numerous fibers from the optic tract,
while other fibers of this tract pass over or through it into the pulvinar. Its
cells are large and pigmented; their axons pass to the visual area in the
occipital part of the cerebral cortex.
The superior colliculus, the pulvinar, and
the lateral geniculate body receive many fibers from the optic tracts, and are
therefore intimately connected with sight, constituting what are termed the lower
visual centers. Extirpation of the eyes in a newly born animal entails an
arrest of the development of these centers, but has no effect on the medial
geniculate bodies or on the inferior colliculi. Moreover, the latter are
well-developed in the mole, an animal in which the superior colliculi are
rudimentary.
The Epithalamus comprises the trigonum
habenulæ, the pineal body, and the posterior commissure.
The trigonum habenulæ is a
small depressed triangular area situated in front of the superior colliculus
and on the lateral aspect of the posterior part of the tænia thalami. It
contains a group of nerve cells termed the ganglion habenulæ.
Fibers enter it from the stalk of the pineal body, and others, forming what is
termed the habenular commissure, pass across the middle line to the
corresponding ganglion of the opposite side. Most of its fibers are, however,
directed downward and form a bundle, the fasciculus retroflexus of
Meynert, which passes medial to the red nucleus, and, after decussating with
the corresponding fasciculus of the opposite side, ends in the interpeduncular
ganglion.
The pineal body (corpus pineale;
epiphysis) is a small, conical, reddish-gray body which lies in the
depression between the superior colliculi. It is placed beneath the splenium of
the corpus callosum, but is separated from this by the tela chorioidea of the
third ventricle, the lower layer of which envelops it. It measures about
The posterior commissure is a rounded
band of white fibers crossing the middle line on the dorsal aspect of the upper
end of the cerebral aqueduct. Its fibers acquire their medullary sheaths early,
but their connections have not been definitely determined. Most of them have
their origin in a nucleus, the nucleus of the posterior commissure (nucleus
of Darkschewitsch), which lies in the central gray substance of the upper
end of the cerebral aqueduct, in front of the nucleus of the oculomotor nerve.
Some are probably derived from the posterior part of the thalamus and from the
superior colliculus, while others are believed to be continued downward into
the medial longitudinal fasciculus.
The Hypothalamus includes the subthalamic
tegmental region and the structures forming the greater part of the floor
of the third ventricle, viz., the corpora mammillaria, tuber cinereum,
infundibulum, hypophysis, and optic chiasma.
25-26
Structures of the hypothalamus
The subthalamic tegmental region consists of
the upward continuation of the tegmentum; it lies on the ventro-lateral aspect
of the thalamus and separates it from the fibers of the internal capsule. The
red nucleus and the substantia nigra are prolonged into its lower part; in
front it is continuous with the substantia innominata of Meynert, medially with
the gray substance of the floor of the third ventricle.
It consists from above downward of three
strata: (1) stratum dorsale, directly applied to the under surface of
the thalamus and consisting of fine longitudinal fibers; (2) zona incerta,
a continuation forward of the formatio reticularis of the tegmentum; and (3)
the corpus subthalamicum (nucleus of Luys), a brownish mass
presenting a lenticular shape on transverse section, and situated on the dorsal
aspect of the fibers of the base of the cerebral peduncle; it is encapsuled by
a lamina of nerve fibers and contains numerous medium-sized nerve cells, the
connections of which are as yet not fully determined.
The corpora mammillaria (corpus
albicantia) are two round white masses, each about the size of a small pea,
placed side by side below the gray substance of the floor of the third
ventricle in front of the posterior perforated substance. They consist of white
substance externally and of gray substance internally, the cells of the latter
forming two nuclei, a medial of smaller and a lateral of larger
cells. The white substance is mainly formed by the fibers of the columns of the
fornix, which descend to the base of the brain and end partly in the corpora
mammillaria. From the cells of the gray substance of each mammillary body two
fasciculi arise: one, the thalamomammillary fasciculus (bundle of
Vicq d’Azyr), passes upward into the anterior nucleus of the thalamus; the
other is directed downward into the tegmentum. Afferent fibers are believed to
reach the corpus mammillare from the medial lemniscus and from the tegmentum.
Median sagittal section of brain. The
relations of the pia mater are indicated by the red color.
The tuber cinereum is a hollow
eminence of gray substance situated between the corpora mammillaria behind, and
the optic chiasma in front. Laterally it is continuous with the anterior
perforated substances and anteriorly with a thin lamina, the lamina
terminalis. From the under surface of the tuber cinereum a hollow conical
process, the infundibulum, projects downward and forward and is attached
to the posterior lobe of the hypophysis.
In the lateral part of the tuber cinereum
is a nucleus of nerve cells, the basal optic nucleus of Meynert, while
close to the cavity of the third ventricle are three additional nuclei. Between
the tuber cinereum and the corpora mammillaria a small elevation, with a
corresponding depression in the third ventricle, is sometimes seen. Retzius has
named it the eminentia saccularis, and regards it as a representative of
the saccus vasculosus found in this situation in some of the lower vertebrates.
The hypophysis (pituitary body)
(721) is a reddish-gray, somewhat oval mass, measuring about
The hypophysis cerebri, in position.
Shown in sagittal section.
Optic Chiasma (chiasma opticum; optic commissure).—The optic chiasma is a flattened, somewhat quadrilateral band of
fibers, situated at the junction of the floor and anterior wall of the third
ventricle. Most of its fibers have their origins in the retina, and reach the chiasma
through the optic nerves, which are continuous with its antero-lateral angles.
In the chiasma, they undergo a partial decussation (722); the fibers from the nasal half of the retina decussate and enter the
optic tract of the opposite side, while the fibers from the temporal half of
the retina do not undergo decussation, but pass back into the optic tract of
the same side. Occupying the posterior part of the commissure, however, is a
strand of fibers, the commissure of Gudden, which is not derived from
the optic nerves; it forms a connecting link between the medial geniculate
bodies.
Optic Tracts.—The
optic tracts are continued backward and lateralward from the postero-lateral
angles of the optic chiasma. Each passes between the anterior perforated
substance and the tuber cinereum, and, winding around the ventrolateral aspect
of the cerebral peduncle, divides into a medial and a lateral root. The former
comprises the fibers of Gudden’s commissure. The lateral root consists mainly
of afferent fibers which arise in the retina and undergo partial decussation in
the optic chiasma, as described; but it also contains a few fine efferent
fibers which have their origins in the brain and their terminations in the
retina. When traced backward, the afferent fibers of the lateral root are found
to end in the lateral geniculate body and pulvinar of the thalamus, and in the
superior colliculus; and these three structures constitute the lower visual
centers. Fibers arise from the nerve cells in these centers and pass
through the occipital part of the internal capsule, under the name of the optic
radiations, to the cortex of the occipital lobe of the cerebrum, where the higher
or cortical visual center is situated. Some of the fibers of the optic
radiations take an opposite course, arising from the cells of the occipital
cortex and passing to the lower visual centers. Some fibers are detached from
the optic tract, and pass through the cerebral peduncle to the nucleus of the
oculomotor nerve. These may be regarded as the afferent branches for the
Sphincter pupillæ and Ciliaris muscles. Other fibers have been described
as reaching the cerebellum through the superior peduncle; while others, again,
are lost in the pons.
The Third Ventricle (ventriculus tertius) is a median
cleft between the two thalami. Behind, it communicates with the fourth
ventricle through the cerebral aqueduct, and in front with the lateral
ventricles through the interventricular foramen. Somewhat triangular in shape,
with the apex directed backward, it has a roof, a floor, an anterior
and a posterior boundary and a pair of lateral walls.
Scheme showing central connections of
the optic nerves and optic tracts
The roof is formed by a layer of epithelium,
which stretches between the upper edges of the lateral walls of the cavity and
is continuous with the epithelial lining of the ventricle. It is covered by and
adherent to a fold of pia mater, named the tela chorioidea of the third
ventricle, from the under surface of which a pair of vascular fringed
processes, the choroid plexuses of the third ventricle, project
downward, one on either side of the middle line, and invaginate the epithelial
roof into the ventricular cavity.
The floor slopes downward and
forward and is formed mainly by the structures which constitute the
hypothalamus: from before backward these are: the optic chiasma, the tuber
cinereum and infundibulum, and the corpora mammillaria. Behind the last, the
floor is formed by the interpeduncular fossa and the tegmenta of the cerebral
peduncles. The ventricle is prolonged downward as a funnel-shaped recess, the recessus
infundibuli, into the infundibulum, and to the apex of the latter the
hypophysis is attached.
The anterior boundary is
constituted below by the lamina terminalis, a thin layer of gray
substance stretching from the upper surface of the optic chiasma to the rostrum
of the corpus callosum; above by the columns of the fornix and the anterior
commissure. At the junction of the floor and anterior wall, immediately above
the optic chiasma, the ventricle presents a small angular recess or
diverticulum, the optic recess. Between the columns of the fornix, and
above the anterior commissure, is a second recess termed the vulva. At
the junction of the roof and anterior wall of the ventricle, and situated
between the thalami behind and the columns of the fornix in front, is the interventricular
foramen (foramen of Monro) through which the third communicates with
the lateral ventricles.
The posterior boundary is
constituted by the pineal body, the posterior commissure and the cerebral
aqueduct. A small recess, the recessus pinealis, projects into the stalk
of the pineal body, while in front of and above the pineal body is a second
recess, the recessus suprapinealis, consisting of a diverticulum of the
epithelium which forms the ventricular roof.
Each lateral wall consists of an
upper portion formed by the medial surface of the anterior two-thirds of the
thalamus, and a lower consisting of an upward continuation of the gray
substance of the ventricular floor. These two parts correspond to the alar and
basal laminæ respectively of the lateral wall of the fore-brain vesicle
and are separated from each other by a furrow, the sulcus of Monro,
which extends from the interventricular foramen to the cerebral aqueduct (pages
741 and 742). The lateral wall is limited above by the tænia thalami. The
columns of the fornix curve downward in front of the interventricular foramen,
and then run in the lateral walls of the ventricle, where, at first, they form
distinct prominences, but subsequently are lost to sight. The lateral walls are
joined to each other across the cavity of the ventricle by a band of gray
matter, the
The telencephalon includes the cerebral hemispheres with their cavities,
the lateral ventricles.
Each cerebral hemisphere may be divided into three fundamental parts:
the pallium, the rhinencephalon, and basal nuclei, also fornix and corpus
callosum.
Cortex of the Cerebrum
A hemisphere is covered by cortex and has inferior, dorsolateral and
medial surfaces. The anterior end of the hemisphere is named the frontal pole; the posterior, the occipital pole; and the anterior end of
the temporal lobe, the temporal pole.
The right and left hemispheres are separated medially by a deep cleft, named
the longitudinal cerebral fissure.
The surfaces of the hemispheres are molded into a number of irregular
eminencies, named gyri or
convolutions, and separated by furrows termed fissures and sulci. The
hemispheres consist of 5 lobes: frontal, perietal, occipital, temporal and insula
(located in depth of lateral sulcus).
On the dorsolateral surface of the hemisphere can be finding the Central sulcus [Rolandi] that separates
frontal and parietal lobes. The Lateral
sulcus [Sylvii] that separates temporal lobe from the frontal and parietal
lobes. Parietooccipital sulcus
passes between parietal and occipital lobes on the medial surface.
There are some sulci on the dorsolateral surface of the frontal lobe: precentral sulcus,
superior frontal sulcus, and inferior frontal sulcus. They separate: precentral
gyrus, superior frontal gyrus, middle frontal gyrus, and inferior frontal
gyrus. The inferior frontal gyrus is divided into opercular, triangular and
orbital parts by anterior and ascending rami.
The lateral surface of the parietal
lobe is cleft by a well-marked furrow, the intraparietal sulcus and the
postcentral sulcus. There are postcentral gyrus, superior parietal lobule and
inferior parietal lobule. The last contains the supramarginal and angular gyri.
The temporal lobe is divided
into superior, middle, and inferior gyri by the superior and middle temporal
sulci. Three or four gyri will be seen springing from the depth of the hinder
end of the lateral sulcus, these are named the transverse temporal gyri (Heschl).
The occipital lobe is small
and pyramidal in shape. It is traversed by the transverse occipital sulci that
border occipital gyri.
On the insula (it is located
in depth of lateral sulcus) they distinguish circular and central sulci which
border longi and breve gyri.
There are some sulci on the medial surface of the hemisphere:
sulcus corporis callosi, cingulate sulcus, hippocampal sulcus, parietooccipital
and calcarine sulcus. They separte corporus callosum from gyrus cinguli,
superior frontal gyri, gyrus parahippocampalis, dentate gyrus, paracentral
lobule (of Bets), precuneus and cuneus. Gyrus cinguli and gyrus
parahippocampalis with isthmus form fornicate gyrus that it is the central part
of the rhinencephalon.
On the inferior surface of
the hemisphere can be finding: collateral sulcus, occipitotemporal sulcus,
rhinal sulcus and orbital sulcus. They separate the lateral occipitotemporal
gyrus, medial occipitotemporal gyrus, lingual gyrus, gyrus rectus and orbital
gyri.
Cortical
Fields
We distinguish some specific cortical fields that
control motor, sensory, language and others functions. They can be divided into
more general motor area, sensory area and also some specific sensory centres.
Motor area involved
with the control of voluntary muscles. It is located
in precentral gyrus and
paracentral lobule (motor homunculus).
Centre of conjugate deviaton
of the eyes to the opposite side is located in
posterior part of the middle frontal gyrus.
Sensory area
responsible for cutaneous and muscular sensations (temperature, pain and
tactile), located in postcentral gyrus and paracentral lobule (sensory homunculus).
Centre of stereognosia is located in
superior parietal lobule closely to intraparietal sulcus. It
responsible for the body
schema: the right can be differentiated from the left.
Auditory centre located in the transverse
temporal gyri (Heschl). Function is interpretation of auditory sensations.
Visual cortical centre situated in calcarine sulcus. Function of the
occipital lobe is connscious perception of vision.
Centre of praxia is located
supramarginal gyrus.
Smell and tasting centre located in part of limbic system – uncus (#
Language areas:
motor -
1. Motor
speech area (Broca's centre). Damage in
the region of the lower frontal convolution produces motor aphasia.
2. Writing
(graphic) area located in middle frontal gyrus.
sensory -
3. Auditory
language centre (Wernicke`s area) located in superior temporal gyrus. It
responsible for understanding of spoken language.
4. Reading
centre situated in angular gyrus (responsible
also for reading, writing, counting and calculating).
The
telencephalon includes: (1) the cerebral hemispheres with their
cavities, the lateral ventricles; and (2) the pars optica hypothalami
and the anterior portion of the third ventricle (already described under the
diencephalon). As previously stated (see page 744), each cerebral hemisphere
may be divided into three fundamental parts, viz., the rhinencephalon, the
corpus striatum, and the neopallium. The rhinencephalon, associated with the
sense of smell, is the oldest part of the telencephalon, and forms almost the
whole of the hemisphere in some of the lower animals, e. g., fishes,
amphibians, and reptiles. In man it is rudimentary, whereas the neopallium
undergoes great development and forms the chief part of the hemisphere.
The Cerebral Hemispheres.—The
cerebral hemispheres constitute the largest part of the brain, and, when viewed
together from above, assume the form of an ovoid mass broader behind than in
front, the greatest transverse diameter corresponding with a line connecting
the two parietal eminences. The hemispheres are separated medially by a deep cleft,
named the longitudinal cerebral fissure, and each possesses a central
cavity, the lateral ventricle.
The Longitudinal Cerebral Fissure (fissura
cerebri longitudinalis; great longitudinal fissure) contains a
sickle-shaped process of dura mater, the falx cerebri. It front and
behind, the fissure extends from the upper to the under surfaces of the
hemispheres and completely separates them, but its middle portion separates
them for only about one-half of their vertical extent; for at this part they
are connected across the middle line by a great central white commissure, the corpus
callosum.
In a median sagittal section the cut
corpus callosum presents the appearance of a broad, arched band. Its thick
posterior end, termed the splenium, overlaps the mid-brain, but is
separated from it by the tela chorioidea of the third ventricle and the pineal
body. Its anterior curved end, termed the genu, gradually tapers into a
thinner portion, the rostrum, which is continued downward and backward
in front of the anterior commissure to join the lamina terminalis. Arching
backward from immediately behind the anterior commissure to the under surface
of the splenium is a second white band named the fornix: between this
and the corpus callosum are the laminæ and cavity of the septum
pellucidum.
Surfaces of the Cerebral Hemispheres.—Each hemisphere presents three surfaces: lateral, medial, and inferior.
The lateral surface is convex in
adaptation to the concavity of the corresponding half of the vault of the
cranium. The medial surface is flat and vertical, and is separated from
that of the opposite hemisphere by the great longitudinal fissure and the falx
cerebri. The inferior surface is of an irregular form, and may be
divided into three areas: anterior, middle, and posterior. The anterior area,
formed by the orbital surface of the frontal lobe, is concave, and rests on the
roof of the orbit and nose; the middle area is convex, and consists of the
under surface of the temporal lobe: it is adapted to the corresponding half of
the middle cranial fossa. The posterior area is concave, directed medialward as
well as downward, and is named the tentorial surface, since it rests
upon the tentorium cerebelli, which intervenes between it and the upper surface
of the cerebellum.
These three surfaces are separated from
each other by the following borders: (a) supero-medial,
between the lateral and medial surfaces; (b) infero-lateral,
between the lateral and inferior surfaces; the anterior part of this border
separating the lateral from the orbital surface, is known as the superciliary
border; (c) medial occipital, separating the medial and
tentorial surfaces; and (d) medial orbital, separating the
orbital from the medial surface. The anterior end of the hemisphere is named
the frontal pole; the posterior, the occipital pole; and the
anterior end of the temporal lobe, the temporal pole. About
The surfaces of the hemispheres are
moulded into a number of irregular eminences, named gyri or convolutions,
and separated by furrows termed fissures and sulci. The furrows
are of two kinds, complete and incomplete. The former appear
early in fetal life, are few in number, and are produced by infoldings of the
entire thickness of the brain wall, and give rise to corresponding elevations
in the interior of the ventricle. They comprise the hippocampal fissure, and
parts of the calcarine and collateral fissures. The incomplete furrows are very
numerous, and only indent the subjacent white substance, without producing any
corresponding elevations in the ventricular cavity.
The gyri and their intervening fissures
and the sulci are fairly constant in their arrangement; at the same time they
vary within certain limits, not only in different individuals, but on the two
hemispheres of the same brain. The convoluted condition of the surface permits
of a great increase of the gray matter without the sacrifice of much additional
space. The number and extent of the gyri, as well as the depth of the
intervening furrows, appear to bear a direct relation to the intellectual
powers of the individual.
Certain of the fissures and sulci are
utilized for the purpose of dividing the hemisphere into lobes, and are
therefore termed interlobular; included under this category are the
lateral cerebral, parietoöccipital, calcarine, and collateral fissures,
the central and cingulate sulci, and the sulcus circularis.
Lateral surface of left cerebral
hemisphere, viewed from the side
The Lateral Cerebral Fissure (fissura
cerebri lateralis [Sylvii]; fissure of Sylvius) is a
well-marked cleft on the inferior and lateral surfaces of the hemisphere, and
consists of a short stem which divides into three rami. The stem is
situated on the base of the brain, and commences in a depression at the lateral
angle of the anterior perforated substance. From this point it extends between the
anterior part of the temporal lobe and the orbital surface of the frontal lobe,
and reaches the lateral surface of the hemisphere. Here it divides into three
rami: an anterior horizontal, an anterior ascending, and a posterior. The anterior
horizontal ramus passes foward for about
The Central Sulcus (sulcus
centralis [Rolandi]; fissure of Rolando; central fissure)is
situated about the middle of the lateral surface of the hemisphere, and begins
in or near the longitudinal cerebral fissure, a little behind its mid-point. It
runs sinuously downward and forward, and ends a little above the posterior
ramus of the lateral fissure, and about
The Parietoöccipital Fissure (fissura
parietoöccipitalis).—Only a small part of this fissure is seen on
the lateral surface of the hemisphere, its chief part being on the medial
surface.
The lateral part of the
parietoöccipital fissure is situated about
The Sulcus Circularis (circuminsular
fissure) is on the lower and lateral surfaces of the hemisphere: it
surrounds the insula and separates it from the frontal, parietal, and temporal
lobes.
Lobes of the Hemispheres.—By
means of these fissures and sulci, assisted by certain arbitrary lines, each
hemisphere is divided into the following lobes: the frontal, the parietal,
the temporal, the occipital, the limbic, and the insula.
Frontal Lobe (lobus frontalis).—On the lateral surface of the hemisphere this lobe extends from the
frontal pole to the central sulcus, the latter separating it from the parietal
lobe. Below, it is limited by the posterior ramus of the lateral fissure, which
intervenes between it and the central lobe. On the medial surface, it is
separated from the cingulate gyrus by the cingulate sulcus; and on the inferior
surface, it is bounded behind by the stem of the lateral fissure.
Principal fissures and lobes of the
cerebrum viewed laterally
The lateral surface of the frontal
lobe is tranversed by three sulci which divide it into four gyri: the sulci are
named the precentral, and the superior and inferior frontal; the gyri are the
anterior central, and the superior, middle, and inferior frontal. The precentral
sulcus runs parallel to the central sulcus, and is usually divided into an
upper and a lower part; between it and the central sulcus is the anterior
central gyrus. From the precentral sulcus, the superior and inferior
frontal sulci run forward and downward, and divide the remainder of the
lateral surface of the lobe into three parallel gyri, named, respectively the superior,
middle, and inferior frontal gyri.
The anterior central gyrus (gyrus
centralis anterior; ascending frontal convolution; precentral gyre) is
bounded in front by the precentral sulcus, behind by the central sulcus; it
extends from the supero-medial border of the hemisphere to the posterior ramus
of the lateral fissure.
The superior frontal gyrus (gyrus
frontalis superior; superfrontal gyre) is situated above the superior
frontal sulcus and is continued on to the medial surface of the hemisphere. The
portion on the lateral surface of the hemisphere is usually more or less
completely subdivided into an upper and a lower part by an antero-posterior
sulcus, the paramedial sulcus, which, however, is frequently interrupted
by bridging gyri.
Supero-Lateral
surface of the forebrain cortex
The middle frontal
gyrus (gyrus frontalis medius; medifrontal gyre), between the
superior and inferior frontal sulci, is continuous with the anterior orbital
gyrus on the inferior surface of the hemisphere; it is frequently subdivided
into two by a horizontal sulcus, the medial frontal sulcus of
Eberstaller, which ends anteriorly in a wide bifurcation.
The inferior frontal gyrus (gyrus
frontalis inferior; subfrontal gyre) lies below the inferior frontal
sulcus, and extends forward from the lower part of the precentral sulcus; it is
continuous with the lateral and posterior orbital gyri on the under surface of
the lobe. It is subdivided by the anterior horizontal and ascending rami of the
lateral fissure into three parts, viz., (1) the orbital part, below the
anterior horizontal ramus of the fissure; (2) the triangular part (cap
of Broca), between the ascending and horizontal rami; and (3) the basilar
part, behind the anterior ascending ramus. The left inferior frontal gyrus
is, as a rule, more highly developed than the right, and is named the gyrus
of Broca, from the fact that Broca described it as the center for
articulate speech.
The inferior or orbital surface
of the frontal lobe is concave, and rests on the orbital plate of the frontal
bone. It is divided into four orbital gyri by a well-marked H-shaped orbital
sulcus. These are named, from their position, the medial, anterior,
lateral, and posterior orbital gyri. The medial orbital gyrus
presents a well-marked antero-posterior sulcus, the olfactory sulcus,
for the olfactory tract; the portion medial to this is named the straight
gyrus, and is continuous with the superior frontal gyrus on the medial
surface.
Temporal Lobe (lobus temporalis).—The temporal lobe presents superior, lateral, and inferior
surfaces.
The superior surface forms the
lower limit of the lateral fissure and overlaps the insula. On opening out the
lateral fissure, three or four gyri will be seen springing from the depth of
the hinder end of the fissure, and running obliquely forward and outward on the
posterior part of the upper surface of the superior temporal gyrus; these are
named the transverse temporal gyri (Heschl).
The lateral surface is bounded above by the posterior ramus of the
lateral fissure, and by the imaginary line continued backward from it; below,
it is limited by the infero-lateral border of the hemisphere. It is divided
into superior, middle, and inferior gyri by the superior and middle temporal
sulci. The superior temporal sulcus runs from before backward across the
temporal lobe, some little distance below, but parallel with, the posterior
ramus of the lateral fissure; and hence it is often termed the parallel
sulcus. The middle temporal sulcus takes the same direction as the
superior, but is situated at a lower level, and is usually subdivided into two
or more parts. The superior temporal gyrus lies between the posterior
ramus of the lateral fissure and the superior temporal sulcus, and is
continuous behind with the supramarginal and angular gyri. The middle
temporal gyrus is placed between the superior and middle temporal sulci,
and is joined posteriorly with the angular gyrus. The inferior temporal
gyrus is placed below the middle temporal sulcus, and is connected behind
with the inferior occipital gyrus; it also extends around the infero-lateral
border on to the inferior surface of the temporal lobe, where it is limited by
the inferior sulcus.
Section of brain showing upper surface
of temporal lobe.
The inferior surface is concave,
and is continuous posteriorly with the tentorial surface of the occipital lobe.
It is traversed by the inferior temporal sulcus, which extends from near
the occipital pole behind, to within a short distance of the temporal pole in
front, but is frequently subdivided by bridging gyri. Lateral to this fissure
is the narrow tentorial part of the inferior temporal gyrus, and medial to it
the fusiform gyrus, which extends from the occipital to the temporal
pole; this gyrus is limited medially by the collateral fissure, which separates
it from the lingual gyrus behind and from the hippocampal gyrus in front.
The Insula (
The insula of the left side, exposed by
removing the opercula.
The term
limbic lobe was introduced by Broca, and under it he included the cingulate and
hippocampal gyri, which together arch around the corpus callosum and the
hippocampal fissure. These he separated on the morphological ground that they
are well-developed in animals possessing a keen sense of smell (osmatic
animals), such as the dog and fox. They were thus regarded as a part of the
rhinencephalon, but it is now recognized that they belong to the neopallium;
the cingulate gyrus is therefore sometimes described as a part of the frontal
lobe, and the hippocampal as a part of the temporal lobe.
On the medial surface of the brain students should
find:
·
Cerebral
hemispheres communicate each other by corpus callosum. Corpus callosum anteriorly carries a genu that passes into rostrum. Last continue as a lamina rostralis and lamina terminalis. Back part of the
corpus callosum called splemium
Lateral surface of left cerebral
hemisphere, viewed from above.
·
·
Above the corpus callosum there is fornix cerebri posteriorly passes into crura fornicis, anteriorly continue as a
columna fornicis
·
Lamina
septi pellucidi is tightened between corpus callosum and columna
fornicis
Thalamus positioned
under fornix
·
Backward from the thalamus there is quadrigeminal plate with two
upper and two lower hillocks, the superior and inferior colliculi
Medial surface of left cerebral hemisphere.
The medial part of the
parietoöccipital fissure runs downward and forward as a deep cleft on the
medial surface of the hemisphere, and joins the calcarine fissure below and behind
the posterior end of the corpus callosum. In most cases it contains a submerged
gyrus.
The Calcarine Fissure (fissura
calcarina) is on the medial surface of the hemisphere. It begins near the
occipital pole in two converging rami, and runs forward to a point a little
below the splenium of the corpus callosum, where it is joined at an acute angle
by the medial part of the parietoöccipital fissure. The anterior part of
this fissure gives rise to the prominence of the calcar avis in the
posterior cornu of the lateral ventricle.
The Cingulate Sulcus (sulcus
cinguli; callosomarginal fissure) is on the medial surface of the
hemisphere; it begins below the anterior end of the corpus callosum and runs
upward and forward nearly parallel to the rostrum of this body and, curving in
front of the genu, is continued backward above the corpus callosum, and finally
ascends to the supero-medial border of the hemisphere a short distance behind
the upper end of the central sulcus. It separates the superior frontal from the
cingulate gyrus.
The Collateral Fissure (fissura
collateralis) is on the tentorial surface of the hemisphere and extends
from near the occipital pole to within a short distance of the temporal pole.
Behind, it lies below and lateral to the calcarine fissure, from which it is
separated by the lingual gyrus; in front, it is situated between the
hippocampal gyrus and the anterior part of the fusiform gyrus.
·
Cerebral
aqueduct [Silvii] passes under mesencephalic tectum. It
communicates front with the 3d
ventricle, back with the 4th
ventricle
·
The 4th ventricle is bordered upper and lower by the superior medullary velum and
inferior medullary velum
·
The floor of the IVth ventricle,
the rhomboid fossa, occupies the
dorsal surface of the medulla oblongata and the pons
·
Cerebellum
consists of two hemispheres which connected by vermis
Cerebral
hemispheres are separated by median
longitudinal fissura that passes to the corpus callosum. Cerebrum is
separated from the cerebellum by transverse
fissura.
The medial surface of the frontal
lobe is occupied by the medial part of the superior frontal gyrus (marginal
gyrus) (727). It lies between the cingulate sulcus and the supero-medial margin of
the hemisphere. The posterior part of this gyrus is sometimes marked off by a
vertical sulcus, and is distinguished as the paracentral lobule, because
it is continuous with the anterior and posterior central gyri.
Parietal Lobe (lobus parietalis).—The parietal lobe is separated from the frontal lobe by the central
sulcus, but its boundaries below and behind are not so definite. Posteriorly,
it is limited by the parietoöccipital fissure, and by a line carried
across the hemisphere from the end of this fissure toward the preoccipital
notch. Below, it is separated from the temporal lobe by the posterior ramus of
the lateral fissure, and by a line carried backward from it to meet the line
passing downward to the preoccipital notch.
Orbital surface of left frontal lobe.
The lateral surface of the parietal
lobe (726) is cleft by a well-marked furrow, the intraparietal sulcus of
Turner, which consists of an oblique and a horizontal portion. The oblique part
is named the postcentral sulcus, and commences below, about midway
between the lower end of the central sulcus and the upturned end of the lateral
fissure. It runs upward and backward, parallel to the central sulcus, and is
sometimes divided into an upper and a lower ramus. It forms the
hinder limit of the posterior central gyrus.
From about the middle of the postcentral
sulcus, or from the upper end of its inferior ramus, the horizontal portion
of the intraparietal sulcus is carried backward and slightly upward on the
parietal lobe, and is prolonged, under the name of the occipital ramus,
on to the occipital lobe, where it divides into two parts, which form nearly a
right angle with the main stem and constitute the transverse occipital
sulcus. The part of the parietal lobe above the horizontal portion of the
intraparietal sulcus is named the superior parietal lobule; the part
below, the inferior parietal lobule.
The posterior central gyrus (gyrus
centralis posterior; ascending parietal convolution; postcentral gyre)
extends from the longitudinal fissure above to the posterior ramus of the
lateral fissure below. It lies parallel with the anterior central gyrus, with
which it is connected below, and also, sometimes, above, the central sulcus.
The superior parietal lobule (lobulus
parietalis superior) is bounded in front by the upper part of the
postcentral sulcus, but is usually connected with the posterior central gyrus
above the end of the sulcus; behind it is the lateral part of the
parietoöccipital fissure, around the end of which it is joined to the
occipital lobe by a curved gyrus, the arcus parietoöccipitalis;
below, it is separated from the inferior parietal lobule by the horizontal
portion of the intraparietal sulcus.
The inferior parietal lobule (lobulus
parietalis inferior; subparietal district or lobule) lies below the
horizontal portion of the intraparietal sulcus, and behind the lower part of
the postcentral sulcus. It is divided from before backward into two gyri. One,
the supramarginal, arches over the upturned end of the lateral fissure;
it is continuous in front with the postcentral gyrus, and behind with the
superior temporal gyrus. The second, the angular, arches over the
posterior end of the superior temporal sulcus, behind which it is continuous
with the middle temporal gyrus.
The medial surface of the parietal
lobe is bounded behind by the medial part of the parietoöccipital fissure;
in front, by the posterior end of the cingulate sulcus; and below, it is
separated from the cingulate gyrus by the subparietal sulcus. It is of
small size, and consists of a square-shaped convolution, which is termed the precuneus
or quadrate lobe.
Occipital Lobe (lobus occipitalis).—The occipital lobe is small and pyramidal in shape; it presents three
surfaces: lateral, medial, and tentorial.
The lateral surface is limited in
front by the lateral part of the parietoöccipital fissure, and by a line carried
from the end of this fissure to the preoccipital notch; it is traversed by the
transverse occipital and the lateral occipital sulci. The transverse
occipital sulcus is continuous with the posterior end of the occipital
ramus of the intraparietal sulcus, and runs across the upper part of the lobe,
a short distance behind the parietoöccipital fissure. The lateral
occipital sulcus extends from behind forward, and divides the lateral
surface of the occipital lobe into a superior and an inferior gyrus,
which are continuous in front with the parietal and temporal lobes.
The medial surface of the occipital
lobe is bounded in front by the medial part of the parietoöccipital
fissure, and is traversed by the calcarine fissure, which subdivides it into
the cuneus and the lingual gyrus. The cuneus is a wedge-shaped area between the
calcarine fissure and the medial part of the parietoöccipital fissure. The
lingual gyrus lies between the calcarine fissure and the posterior part
of the collateral fissure; behind, it reaches the occipital pole; in front, it
is continued on to the tentorial surface of the temporal lobe, and joins the
hippocampal gyrus.
The tentorial surface of the
occipital lobe is limited in front by an imaginary transverse line through the
preoccipital notch, and consists of the posterior part of the fusiform gyrus
(occipitotemporal convolution) and the lower part of the lingual gyrus,
which are separated from each other by the posterior segment of the collateral
fissure.
The cingulate gyrus (gyrus cinguli; callosal
convolution) is an arch-shaped convolution, lying in close relation to the
superficial surface of the corpus callosum, from which it is separated by a
slit-like fissure, the callosal fissure. It commences below the rostrum
of the corpus callosum, curves around in front of the genu, extends along the
upper surface of the body, and finally turns downward behind the splenium,
where it is connected by a narrow isthmus with the hippocampal gyrus. It
is separated from the medial part of the superior frontal gyrus by the
cingulate sulcus, and from the precuneus by the subparietal sulcus.
The hippocampal gyrus (gyrus
hippocampi) is bounded above by the hippocampal fissure, and below by the
anterior part of the collateral fissure. Behind, it is continuous superiorly,
through the isthmus, with the cingulate gyrus and inferiorly with the lingual
gyrus. Running in the substance of the cingulate and hippocampal gyri, and
connecting them together, is a tract of arched fibers, named the cingulum
(page 843). The anterior extremity of the hippocampal gyrus is recurved in the
form of a hook (uncus), which is separated from the apex of the temporal
lobe by a slight fissure, the incisura temporalis. Although
superficially continuous with the hippocampal gyrus, the uncus forms
morphologically a part of the rhinencephalon.
Scheme of rhinencephalon.
The Hippocampal sulcus begins
immediately behind the splenium of the corpus callosum, and runs forward
between the hippocampal and dentate gyri to end in the uncus. It is a complete
fissure (page 819), and gives rise to the prominence of the hippocampus in the
inferior cornu of the lateral ventricle.
Prepared by
Reminetskyy B.Y.