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tissue with the lamina terminalis on the antero-superior aspect of the anterior commissure (Fig. 558).

The inferior surface of the corpus callosum, on each side of the median plane, is coated with ependyma (Fig. 564, p. 635), and forms the roof of the anterior horn and the central part of the lateral ventricle. In the median plane, however, it is attached to subjacent parts, viz., to the septum pellucidum in front and directly or indirectly (Fig. 564) to the body of the fornix behind (Fig. 558, p. 630).

The transverse fibres of the corpus callosum, as they enter the white medullary centre of the cerebral hemisphere, separate from each other so as to reach most parts of the cerebral cortex. These diverging fibres are termed the radiatio corporis

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Transverse fibres of corpus callosum

Occipital part of radiation of corpus callosum,

Splenium

Stria longitudinalis medialis

FIG. 559. THE CORPUS CALLOSUM, exposed from above and the right half dissected,
to show the course taken by its fibres.

The lateral longitudinal stria (which lies near the cingulum) is not shown.

callosi, and they intersect those which form the corona radiata or, in other words, the fibres which extend between the internal capsule and the cerebral cortex (Figs. 570, p. 640, and 576, p. 649). The more anterior of the fibres which compose the genu of the corpus callosum sweep forwards in a series of curves into the anterior frontal region of the hemisphere. A large part of the splenium, forming a solid bundle termed the occipital part of the radiation of the corpus callosum (O.T. forceps major), bends suddenly and abruptly backwards into the occipital lobe (Fig. 559). Fibres from the body and superior part of the splenium, curving round the lateral ventricle, form a very definite stratum, called the tapetum. This is a thin layer in the medullary centre of the hemisphere, which constitutes the immediate roof and lateral wall of the posterior horn and the lateral wall of the posterior part of the inferior horn of the lateral ventricle

In frontal sections through the occipital and posterior temporal regions the tapetum stands out very distinctly (Fig. 559, p. 631; see also Figs. 565, p. 636, and 567, p. 638).

Septum Pellucidum.-The septum pellucidum is a thin vertical partition which intervenes between the two lateral ventricles. It is triangular in shape, and posteriorly it is prolonged backwards for a variable distance between the body of the corpus callosum and the fornix, to both of which it is attached. In front it occupies the gap behind the genu of the corpus callosum, whilst below, in the narrow interval between the posterior edge of the rostrum of the corpus callosum and the fornix, it is prolonged downwards in the paraterminal body towards the base of the brain. The septum pellucidum is composed of two thin laminæ in apposition with each other in the median plane (Fig. 562; Fig. 564, p. 635).

Cavum Septi Pellucidi.-This name is applied to the median cleft between the

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Piriform area
Thalamus (cut surface)
Rhinal fissure
Cauda fascia dentata
Hippocampus

Crus fornicis
Gyrus paradentatus
Fimbria

FIG. 560. THE MEDIAL ASPECT OF THE RIGHT HALF OF THE BRAIN EXPOSED BY A MEDIAN SAGITTAL SECTION.

two laminae of the septum pellucidum. It varies greatly in size in different brains.

VENTRICULUS LATERALIS.

The cavity in the interior of the cerebral hemisphere is called the lateral ventricle. It is lined throughout by ependyma continuous with the ependymal lining of the third ventricle. In some places the walls of the cavity are in apposition, whilst in other localities spaces of varying capacity, and containing cerebro-spinal fluid, are left between the bounding walls.

The lateral ventricle communicates with the third ventricle of the brain by means of a small foramen, just large enough to admit a crow-quill, which is termed the foramen interventriculare. This aperture is placed in front of the anterior end of the thalamus and behind the column of the fornix.

The highly-irregular shape of the lateral ventricle can be best understood by the

study of a cast of its interior (Figs. 561 and 545, p. 618). It is usual to describe it as being composed of a body and three horns, viz. an anterior, a posterior, and an inferior horn. The cornu anterius is that part of the cavity which lies in front of the interventricular foramen. The body or pars centralis is the portion of the ventricle which extends from the interventricular foramen to the splenium of the corpus callosum. At this point the posterior and inferior horns diverge from the posterior part of the body. The cornu posterius curves backwards and medially into the occipital lobe. It is very variable in its length and capacity: the chief reason for this variability is that adhesions between the walls of this part of the ventricle are of common occurrence. The cornu inferius proceeds with a bold sweep round the posterior end of the thalamus, and then tunnels in a forward and medial direction through the temporal lobe

Corpus callosum

Lateral ventricle

Column of fornix

Chorioid plexus_

Foramen interventriculare

Septum pellucidum

Nucleus lentiformis Claustrum

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FIG. 562.-FRONTAL SECTION THROUGH THE CEREBRAL HEMISPHERES so as to cut through the anterior horns of the lateral ventricles, through which the central part of the ventricles, the columns of the fornix, and the interventricular gated

which

foramina can be seen.

towards the temporal pole.

The early foetal lateral ventricle is very

capacious and presents an arched or semilunar form. It is composed of parts which correspond to the anterior horn, the central part and the inferior horn, and there is little or no demarcation between them. The posterior horn is a later production. It comes into existence as a diverticulum or elon

pouch, grows

the roof. Into this secondarily formed caudal extension of the chorioid lamina the invagination that commenced in the roof of the foramen interventriculare extends until it reaches the inferior extremity of the deep cleft separating the cerebral hemisphere from the thalamus (Fig. 548, A). Below this point the thalamus remains in uninterrupted continuity with the floor of the cerebral hemisphere (Cs), which is becoming thickened to form the corpus striatum.

At a very early stage in the development of the embryo, long before there is any sign of the hemisphere vesicles, the ectoderm upon each side of the anterior neuro

pore (see p. 500) becomes thickened to form the area olfactoria (see Fig. 440, D, p. 501). Certain of the epithelial cells in this area become converted into bipolar sensory cells, -Foramen interventriculare. which become specially adapted

to be affected by certain kinds of air-borne chemical stimuli that awaken a consciousness of smell. These cells always remain in situ in the olfactory epithelium, just as the most primitive sensory cells do in Hydra (Fig. 439, p. 497). But other nerve - cells seem to be derived from the area olfactoria which do not remain in the parent epithelium,

but become attached to the adjoining part of the neural tube. These cells form the olfactory ganglion, which acts as the receptive organ for the impressions brought into it by the processes of the sensory cells in the olfactory epithelium; and the

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ISPHERE

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A

Fissura chorioidea.

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Floor plate.

FIG. 549.-DIAGRAM OF A TRANSVERSE SECTION THROUGH A
FOETAL BRAIN TO SHOW THE INVAGINATION OF THE ROOF
THROUGH EACH INTERVENTRICULAR FORAMEN.

-Pallium.

-Ventriculus lateralis.
Lamina chorioidea
ventriculi lateralis.

- Corpus striatum.

-Third ventricle.

Lamina chorioidea
ventriculi tertii.

PINEAL DIVERTICULUM
METATHALAMUS

MESENCEPHALON

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OLFACTORY BULB

LAMINA
TERMINALIS

THALAMUS

OPTIC N.

P.O.H.

P.M.H.

OPTIC
RECESS CHIASMA

HYPOPHYSEAL DIVERTICULUM

P.M.H. Pars mamillaris hypothalami.
P.O. H. Pars optica hypothalami.

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M. Mamillary region.

F. Frontal lobe.
P. Parietal lobe.

RHINENCEPHALO

FIG. 550.-Two DRAWINGS BY HIS, ILLUSTRATING THE DevelopmeNT OF THE HUMAN BRAIN.

A, Median section through a foetal human brain in the third month of development.

B, Schema showing the directions in which the cerebral hemisphere expands during its growth

O. Occipital lobe.
T. Temporal lobe.

area of the neural tube to which it becomes attached is destined to become part of the cerebral hemisphere. At the end of the first month this portion of the hemisphere becomes drawn out as a hollow protrusion, the distal end of which is coated with a layer of olfactory ganglion and is known as the bulbus olfactorius; the rest forms a peduncle. In the course of its subsequent development in the human brain (though not in those of most mammals) the cavity in the bulb and peduncle becomes completely obliterated. The peduncle becomes so greatly elongated and attenuated that, to the unaided eye, it appears to be wholly formed

of white nerve-fibres passing to and fro between the olfactory bulb and the hemisphere; hence it is called the tractus olfactorius.

The cerebral hemisphere first appears in the form of a slight bulging upon each side of the fore-brain, but it soon assumes large dimensions. At first it grows forwards and upwards (Fig. 550), and a distinct cleft, the floor of which is the roof-plate and lamina terminalis, appears between the two hemispheres: this is known as the fissura longitudinalis cerebri. The separation of the two cerebral vesicles by the longitudinal fissure begins at the end of the first month. This fissure becomes occupied by mesodermic tissue, which later on forms the falx cerebri. The cerebral hemisphere, in its further growth, is carried progressively backwards over the posterior parts of the developing brain. At the end of the third month it has covered the thalamus. A month later it reaches the corpora quadrigemina, and by the seventh month it has not only covered these, but also the entire upper surface of the cerebellum.

In the earlier stages of its development the cerebral hemisphere is a thin-walled vesicle with a relatively large cavity, which represents the primitive condition of the lateral ventricle. At first the vesicle is bean-shaped and the cavity is curved. As development proceeds the posterior portion of the hemisphere grows backwards over the cerebellum in the shape of a hollow protrusion, and a distinct occipital lobe enclosing the posterior horn of the lateral ventricle is the result. This developmental stage begins about the fourth month.

THE CONNEXIONS OF THE OLFACTORY NERVES.

The olfactory nerves are the axons of the spindle-shaped bipolar cells situated in the olfactory mucous membrane (Fig. 551). These axons collect in the submucous layer to form small bundles, which enter the cranial cavity through the foramina in the lamina cribrosa of the ethmoid bone. They at once enter the inferior surface of the bulbus olfactorius, and each fibre breaks up into a tuft of terminal filaments. Towards these tufts dendrites proceed from large mitral cells placed in a deeper plane within the bulb, and each dendrite also breaks up into numerous terminal branches intertwined with those of the olfactory nerves. In this way are formed a large number of globular bodies, each consisting of the arborescent terminations of a mitral dendrite and of certain olfactory nervefibres. These are the olfactory glomeruli of the bulb. Each mitral cell gives off several dendrites and one axon. Only one dendrite enters into the formation of a glomerulus, but several nerve-fibres may be connected with such a body. It thus happens that, through its dendrite, a mitral cell may stand in connexion with several olfactory nerve-fibres. The axon of the mitral cell passes upwards to the white matter of the bulb, enters this, and, bending backwards, is conducted through the tract towards the FIG. 551.-DIAGRAM OF THE MINUTE cerebral cortex.

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STRUCTURE OF THE OLFACTORY BULB.

MITRAL CELL

LAYER or

GLOMERULI

LAMINA
CRIBROSA

OLFACTORY MUCOUS

In JUN MEMBRANE

The olfactory bulb is a small, flattened, elliptical mass of gray substance placed upon the upper surface of the lamina cribrosa of the ethmoid. Its posterior extremity is attached to the rest of the cerebral hemisphere by the long tractus olfactorius (Fig. 476), a prismatic band of white substance placed in a furrow (sulcus olfactorius) on the under surface of the frontal region of the cerebral hemisphere. A short distance in front of the optic chiasma each olfactory tract becomes inserted into the hemisphere (Fig. 552). The swollen pyramidal-shaped

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