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occupies the fossa hypophyseos in the floor of the cranium. It is composed of two lobes a large anterior lobe and a smaller posterior lobe, which are closely applied
the one to the other. The infundibulum, which extends downwards from the tuber cinereum, is attached to the posterior lobe.
The infundibulum and posterior lobe of the hypophysis are developed in the form of a hollow diverticulum, which grows downwards from the floor of that part of the embryonic brain which afterwards forms the third ventricle. The original cavity of this diverticulum becomes obliterated, except in the superior part of the infundibulum. In structure, the posterior lobe of the hypophysis shows little trace of its origin from the wall of the brain-tube. It is chiefly composed of connective tissue and blood-vessels, with branched cells scattered throughout it,
The anterior lobe has quite a different origin, and may be regarded
as the functional part of the hypophysis. It is derived from a tubular diverticulum which grows upwards from the primitive buccal cavity or stomodæum. Its connexion with the latter (canalis craniopharyngeus) is in the course of time cut off, and the diverticulum becomes encased within the cranial cavity in intimate association with the cerebral portion of the organ. Structurally, it consists of tubules or alveoli, lined with epithelial cells and surrounded by capillary vessels. Its structure is in some respects not unlike that of the parathyreoid bodies. In the disease known as acromegaly, the hypophysis is usually greatly enlarged.
Lamina Terminalis.-This is a thin, delicate lamina which may be seen on the basal aspect of the brain, stretching from the upper aspect of the optic chiasma in an upward direction to become connected with the anterior end of the corpus callosum.
Anterior Commissure of the Cerebrum.-In the anterior part of the cleft between the two thalami, and immediately in front. of the columns of the fornix, a round bundle of fibres crosses the median plane. This is the anterior commissure.
Subarachnoid tissue in cisterna basalis Infundibulum
Posterior part of subarachnoid space
Basilar part of the occipital bone
Ventriculus Tertius. — The third ventricle is the narrow cleft which separates the two thalami. Its depth rapidly increases from behind forwards, and it may be said to extend from the pineal body behind to the lamina terminalis in front. Its floor is formed by the tuber cinereum and the corpora mamillaria: the gray matter of the substantia perforata posterior, and the tegmenta of the cerebral peduncles may also be looked upon as forming part of the floor (Figs. 542 and 543). It is interesting to note that the central gray
FIG. 543.-MEDIAN SECTION THROUGH THE HYPOPHYSEAL
matter which surrounds the aqueduct is directly continuous with the gray matter of the substantia perforata posterior and tuber cinereum, and in this way it comes to the surface in the base of the brain. The optic chiasma crosses the floor in front and marks the place where the latter becomes continuous with the anterior wall of the cavity. The anterior wall of the third ventricle is formed by the lamina terminalis, which extends upwards from the optic chiasma. The anterior commissure, as it crosses from one side to the other, projects into the ventricle, but, of course, it is excluded from the cavity by the ventricular epithelial lining. It may be taken as indicating the place where the roof joins the anterior wall. The roof of the third ventricle is formed by a thin epithelial layer which stretches across the median plane from one tænia thalami to the other, and is part of the thin epithelial lining of the cavity. Applied to the superior surface of the epithelial roof
FIG. 544. THE PARTS OF THE BRAIN CUT THROUGH IN A MEDIAN SAGITTAL SECTION.
is the fold of pia mater, termed the tela chorioidea, and the roof is invaginated into the cavity along its whole length by two delicate chorioid plexuses, which hang down from the under surface of this fold. When the pia mater is removed the thin epithelial roof is torn away with it, leaving only the lines of attachment in the shape of the tænia thalami (Fig. 549).
The side wall of the third ventricle is formed for the greater part of its extent by the medial surface of the thalamus, covered by a thick layer of central gray matter continuous with the central gray matter of the mesencephalon. A little in front of the middle of the ventricle the cavity is often crossed by the massa intermedia, which connects the thalami one with the other, and in front of this the columna fornicis is seen curving downwards and backwards in the side wall. At first the bulging which it forms is distinctly prominent, but it gradually subsides as the strand, on its way to end in the corpus mamillare, becomes more and more sunk in the gray matter on the side of the ventricle.
The third ventricle communicates with both of the lateral ventricles, and also with the fourth ventricle. The aquæductus cerebri, the narrow channel which tunnels the mesencephalon, brings it into communication with the fourth ventricle. The opening of this aqueduct is placed at the posterior part of the floor of the third ventricle, immediately below the posterior commissure. The foramina interventricularia bring it into communication with the lateral ventricles. These apertures are placed at the upper and anterior parts of the side walls, and lead laterally and slightly upwards between the most prominent parts of the columns of the fornix and the anterior tubercles of the thalami. They are just large enough to admit a crow-quill, and through these passages the epithelial lining of the three ventricles becomes continuous. From the foramen a distinct groove on
BODY OF LATERAL VENTRICLE
FIG. 545.-PROFILE VIEW OF A CAST OF THE VENTRICLES OF THE BRAIN (from Retzius).
R.SP. Recessus suprapinealis.
A.S. Aquæductus cerebri.
F.M. Foramen interventriculare.
the side wall of the ventricle leads backwards towards the mouth of the aqueduct. It is termed the sulcus hypothalamicus, and is of interest, inasmuch as it is considered by His to represent in the adult brain the furrow which divides the side wall of the embryonic brain-tube into an alar and a basal lamina.
The outline of the third ventricle, when viewed from the side in a median section through the brain (Fig. 544), or as it is exhibited in a plaster cast of the ventricular system of the brain (Fig. 545), is seen to be very irregular. It presents several diverticula or recesses. Thus, in the anterior part of the floor there is a funnel-shaped pit or recess, leading down through the tuber cinereum into the infundibulum of the hypophysis. Another recess, the recessus opticus, leads forwards immediately in front of this, above the optic chiasma. Posteriorly two diverticula are present. One, the recessus pinealis, passes backwards above the posterior commissure and the mouth of the aquæductus cerebri for a short distance into the stalk of the pineal body. The second is placed above this and is carried backwards for a greater distance. It is a diverticulum of the epithelial roof, and, therefore, is difficult to demonstrate. It is termed the recessus suprapinealis.
CEREBRAL CONNEXIONS OF THE OPTIC TRACT.
At the optic
The optic nerve is connected with the hypothalamus. chiasma the optic nerves of the two sides are joined together and a partial decussation of fibres takes place. The fibres which arise in the medial half of each retina cross the median plane and join the optic tract of the opposite side. The optic tract proceeds backwards round the cerebral peduncle, and in the neighbourhood of the geniculate bodies appears to divide into two roots, viz., a lateral and a medial (Fig. 546), but only the former is really part of the tract.
Commissure of Gudden.-The so-called medial root disappears under cover of
FIG. 546. THE VENTRAL ASPECT OF PART OF THE PROSENCEPHALON, SHOWING THE RIGHT OPTIC TRACT. The mesencephalon has been cut across. Olfactory area, dull yellow; optic fibres, blue; motor fibres,
red; acoustic fibres, bright yellow.
the medial geniculate body and a large proportion of its fibres arise or end in this nuclear body. The medial root, although it is composed of fibres which are intermingled with those of the optic tract, has nothing to do with the optic nerve. These fibres, when traced forwards, cross the median plane in the posterior angle of the optic chiasma and are carried backwards alongside the opposite optic tract. The fibres constitute a bond of union, called the commissure of Gudden, between the medial geniculate body of one side and the colliculus inferior of the other (Fig. 547).
The Optic Tract.-The optic tract is composed of fibres which come-(1) from the lateral half of the retina of its own side; and (2) from the medial half of the retina of the opposite side, which have crossed the median plane in the optic chiasma. But in addition to the afferent retinal fibres there are a certain number
of efferent fibres in the optic tract-fibres which take their origin in the brain and end in the retina. These are distinguished from the afferent retinal fibres by their exceeding fineness.
The fibres of the optic tract end in the superior colliculus, in the lateral geniculate body, and in the pulvinar of the thalamus (Fig. 546). The fibres to the superior colliculus reach it through the superior brachium (p. 586), and for the most part sink into its substance to end in terminal arborisations around its cells. The corpus geniculatum laterale receives the largest contribution of fibres from the optic tract. These partly sink into its interior and partly spread out over its surface. The former enter into the construction of the curved lamella of white matter which traverse this nuclear mass, and to a large extent end in the gray matter which intervenes between these lamellæ. The deep fibres which are not exhausted in this way proceed onwards through the lateral geniculate body and enter the pulvinar. Of the superficial fibres which spread over the surface of the lateral geniculate body some dip into its substance and end there, but the majority are carried over it and enter the stratum zonale of the pulvinar. Most of the fibres
CORP. GEN. MED.
CORP. GEN. LAT.
of the optic tract, which end in the pulvinar, therefore reach their destination by passing either over or through the lateral geniculate body.
Cortical Connexions of the Optic Path. The superior colliculus, the lateral geniculate body, and the pulvinar constitute the lower visual centres or terminal nuclei of the optic tract. The higher visual centre is placed in the cortex of the occipital region of the cerebral hemisphere, and the connexions between this and the lower centres are established by a large strand of fibres which runs in the central white matter of the posterior part of the cerebral hemisphere, and which constitutes the optic radiation. The optic radiation is composed both of corticipetal and corticifugal fibres. The former arise as the axons of the cells in the lateral geniculate body and the pulvinar, around which the retinal fibres end, and they terminate in the cortex of the occipital lobe. The corticifugal fibres take origin in the cortex of the occipital lobe and end in the pulvinar and superior quadrigeminal body (Ferrier and Turner). Thus constituted, the optic radiation forms a conspicuous strand (Figs. 546, p. 619; 552, p. 624; 567, p. 638), which, reaching the
FIG. 547.—DIAGRAM OF THE CENTRAL CONNEXIONS retrolenticular part of the internal capsule,
OF THE OPTIC NERVE AND OPTIC TRACT.
sweeps backwards into the occipital lobe of the cerebral hemisphere on the lateral side of the posterior horn of the lateral ventricle. Its connexions will be studied more fully at a later stage.
THE PARTS DERIVED FROM THE TELENCEPHALON.
The cerebral hemispheres form the largest part of the fully developed brain. When viewed from above they form an ovoid mass, the broadest end of which is directed backwards, and the longest transverse diameter of which will be found in