Closer examination of the base of the brain leads further to the location of the exits of the individual cerebral nerves from the brain, concerning which the following table may afford explanation. The exits of these nerves from the skull are also noted.
Let us now examine a median sagittal section through the brain. In the first place, we recognize the brain-mass belonging to the hemisphere, with its fissures and convolutions, and, further, the corpus callosum, the large commissure connecting the two cerebral hemispheres. The middle part of the bridge is the body (truncus corporis callosi) ; behind, the commissure thickens to form the spknium ; while in front, it bends sharply downward and forms the knee, genu corporis callosi, that tapers into the beak like rostrum corporis callosi. The latter is prolonged as a short thin white lamella, the lamina rostralls, which is continuous with the attenuated lamina terminalis that extends to the front surface of the chiasma opticum. Behind the corpus callosum, covered by the hinder part of the hemisphere, lies the cerebellum ; the deep fissura transversa cerebri is plainly seen separating the hemisphere and cerebellum.
Let us examine the parts of the brain lying beneath the corpus callosum. Closely attached to the under surface of the latter, a lamella of white matter extends forward from the place where the splenium joins the body or trunk of the corpus callosum. The structure gradually leaves the corpus callosum, arches downward with forwardly directed curve until close behind the lamina rostralis, and then sinks deeply into the brain-sub stance, just behind a transversely cut white bundle of fibres, the anterior commissure or commissura anterior. This white lamella belongs to the fornix. Between the fornix, on the one hand, and the truncus, genu, rostrum and lamina rostralis of the corpus callosum, on the other, extends a thin white sheet, the septum pellucidum. Beneath the fornix and the hind part of the corpus callosum is situated the thalamus, between whose fore-end and the descending fornix lies an opening, the foramen interventriculare or foramen of Monro. At the posterior end of the thalamus, beneath the splenium corporis callosi,
lies the pineal body, the corpus pineale. The cleft, which penetrates the pineal body in front, is called the recessus pinealis. Immediately beneath is found the cross-section of the commissura posterior cerebri, with which are joined, proceeding backward, the lamina quadrigemina, the velum medullare anterius and the cerebellum. On the median surface of the thalamus, behind the foramen interventriculare, lies the cross-section of the middle commissure or massa intermedia, by means of which the opposed surfaces of the two thalami are connected.
The sulcus hOothalamicus (Monroi) is a furrow that extends backward from the foramen interventriculare, beneath the massa intermedia, towards the commissura posterior and separates the region of the thalamus from the more dependent hypothalamus. On examining this region more closely, we note again parts that have been mentioned in connection with the base of the brain ; in front the lamina terminalis that joins the anterior surface of the chiasma opticum, the recessus ofiticus between the lamina and the chiasma and behind the latter, the recessus infundibuli, the infundibulum with the hypoph ysis, the tuber cinereum, the corpus mamillare, and the substantia fierforata posterior, forming the floor of the fossa interpeduncularis (Tarini).
Continuing backward, the cerebral peduncle, the pons and the medulla oblongata are seen in cross-section. The sulcus hypothalamicus, running backward from the foramen interventriculare, opens into the aquaeduclus cerebri, or aqueduct of Sylvius, which extends beneath the quadrigeminal plate and joins the fourth ventricle that under lies the cerebellum (Figs. i6, z7, and x8).