In general, this dissection presents a median longitudinal depression, and two lateral masses, one on each side. In the middle line, between the lateral masses, is a longitudinal cleft, the third ventricle. The third ventricle is one and half an inch in length, extends vertically to the base of the brain, w here its floor is formed by the parts contained in the inter peduncular space, including the lamina, cinerea, optic commis sure and tuber cinereurn. Communicating with the ventricle below is a canal leading to the pituitary body, the infundi bulum. The pillars of the fornix descend in the anterior extremity of the third ventricle. By separating the anterior pillars of the foruix the middle portion of the anterior com missure is exposed, and is seen crossing the anterior wall of the ventricle in front of them. The anterior commissure of the third ventricle is directed outward and backward on each side to the extremity of the inferior ramus of the internal capsule of the hemisphere, its extremities terminating in the temporal lobes. It is supposed to be the commissure of the centres of smell and taste. The internal surfaces of the thalami form the lateral walls of the third ventricle. Crossing its middle, uniting the central part of one thalamus h that of the other, is a broad gray commissure, the mid dle or gray commissure of the third ventricle. The ventricle is pointed behind, and is continuous with a canal which passes backwards beneath another transverse band of white fibres, the posterior commissure of the third ventricle. The canal just mentioned leads downwards and backwards, beneath the posterior commissure and the commissure of the corpora quadrigemina and valve of Vieussens, to communicate with the fourth ventricle. This canal is called the aqueduct of Sylvius, or the iter e tertio ad quartem ventriculuni, or, for short, the iter. Extending from the anterior pillars of the fornix forward to the genu of the corpus callosum, and sep arating the lateral ventricles, is the septum lucidum.
Behind the posterior extremity of the ventricle, and above its posterior commisure, is a small globular body- some what elongated from before backwards, the pineal gland. Two delicate white tracts extend forward from the anterior extremity of the pineal gland, one on each side, upon the lateral margins of the third ventricle, they are the anterior pillars of the pineal gland and mark the division between the superior and internal surfaces of the thalamus. These tracts are lost in front of the anterior extremity of the thalamus, where they are in conjunction with the descending anterior pillars of the fornix. The anterior extremity of the pineal gland is attached to the upper surface of the posterior commissure of the third ventricle by a delicate membrane, whose lateral margins are thickened and ca,lled the posterior pillars of the pineal gland. The posterior naargin and sides of the pineal gland are free, and it rests above, int front of, and between the anterior tubercles of the corpora quadrigemina.
The corpora quadrigemina or optic tubercles are four small bodies placed above and between the cruri cerebri, and in the interval between the posterior extremities of the thalami optici. They are enclosed behind by a notch upon the anterior part of the cerebellum the incisura anterior. The corpora quadrigemina are in pairs, an anterior pair, the nates, and ft posterior pair, the testes. The tubercles of each pair are placed one on either side of the median line and are connect ed by a commissure.foirected obliquely outward and forward from the nates and the testcs, on either side, are two bands of white fibres, the brachia,anterius and posterius, the former being connected with the nates, and the latter with the testes. These tracts are the external margins of the horizontal layers of fibres which pass from the corpora quadrigemina, beneath the posterior extremity of the thalamus, to the internal capsule. They are also connected with the posterior extremities of the optic tract and geniculate bodies, the brachium anterior with the corpus geniculatum externum and the brachium posteri with the corpus geniculatum internum. Behind the corpora quad rigi mina, and covered by the cerebellum, are two broad and rounded cords, one on each side, which ascend from the Mins of the cerebellum and are the processes, or the superior peduncles of the cerebellum. These are united along their inner margins by a thin. translucent membrane, which is de pressed upon its upper surface, and is called the valve of Vieussens. The valve is pointed at its anterior extremity which projects upwards between the testes, and is elevated above the general surface. On each side of its anterior extrem ity are the apparent origins of the fourth nerves. The under surface of the valve and of the processes form the roof of the fourth ventricle. The superior vermiform process of the cerebellum rests upon the valve of Vieussens and the infer. ior vermiform process is beneath it in the fourth ventricle. Upon the outer surface of each processus at its anterior part, crossing it obliquely in an upward direction, is a band of fibres parallel to those of the crusta from which it is divided by the sharp vertrical furrow previously mentioned. This band of fibres terminates above in the testes and brachium pos terior, and below it is continuous with the fundamental root zone of the spinal cord.
At the upper extremity of the depression between the crusta aud the tract just mentioned is a tubercle, which is connected behind with the brachium posterius, and in front with the lower division of the optic tract, and is called the corpus geniculatum internum. External and above the corpus geniculatum internum, upon the outer division of the extrem ity of the optic tract, is an elevation, the corpus geniculatum esternum. These bodies are apparently the outer terminations of the brachia, but, as before stated, the major part of the fibers of the brachia passes beneath the thalamus and optic tract to the internal capsule.