Ventricles.—The cavity of the neural tube constitutes the adult ventricles, which form a continuous median series extending from the canal of the spinal cord up to the level of the cerebral hemispheres; at that level the central cavity bifurcates into a branch for each hemisphere of the cerebrum (Figs. 17 and 18). Thus is formed the lateral ventricle in the cerebral hemi sphere and, below the cerebral hemispheres, the median series of cavities comprises the third ventricle in the inter-brain, the cerebral aqueduct in the mid-brain, and the fourth ventricle in the rhomb encephal on.
IValls.—The walls of these simple embryonic cavities undergo wonderful development and specialization; ultimately they produce all the multiform and complicated structures of the adult human brain.
Superior View.—The superior surface of the brain is markedly convex (Figs. 19 and 22). It is elliptical in outline, the major axis (15-17 cm.) being contained in the median line; the greatest transverse axis (14 cm.) is situated a little behind the middle and runs between the points which, when the brain is in the skull, underlie the tubera parietalia. This surface is closely adapted to the interior of the calvaria. Only the great con voluted hemispheres of the cerebrum are visible from the superior viewpoint. The two hemispheres are separated by a deep, median cleft, called the longitudinal fissure of the cere brum (fissura longitudinalis cerebri) from which the falx cerebri has been removed.
Posterior View.—When the brain is viewed from behind, three great structures and two transverse fissures are visible (Fig. 20): First, the occipital end of the cerebral hemispheres with their irregular gyri and sulci; second, the transversely laminated cerebellum, lying below the cerebrum and separated from it by the transverse fissure of the cerebrum (fissura transversa cerebri); and third, the inferior extremity of a relatively small median structure, the medulla oblongata. The cerebellum is especially characterized by its parallel crescentic sulci, which give it a stratified appearance. It shows a partial subdivision into lateral hemispheres produced by a posterior median de pression, called the posterior cerebellar notch, and by a longitudinal groove on its inferior surface, called the vallecula cerebelli. The vallecula is fitted over the posterior surface of the medulla. The cerebellum is, therefore, separated from the medulla ob longata by a sharply curved, rainbow-shaped fissure. That
fissure is the transverse fissure of the cerebellum (fissura transverse cerebelli) which, as already pointed out, is bridged over by the arachnoid and contains the cisterna cerebello medullaris.
Inferior View.—The base of the brain presents three areas, situated in three successive levels, which correspond in location and extent to the great fossx in the base of the cranium (Figs. 21 and 33). The anterior area, situated in the anterior cranial fossa, occupies the highest level; the middle area is intermediate in position; it occupies the middle fossa and together with the anterior area comprises all of the base of the cerebrum which is visible in the complete brain; and, the posterior area, which is but the base of the rhombencephalon, is situated at the lowest level in the posterior fossa of the cranium.
The anterior area of the base of the brain is divided into lateral halves by the longitudinal fissure of the cerebrum, and separated from the middle area by the fossa and fissura cerebri lateralis. The frontal lobe of the cerebral hemisphere, on either side of the longitudinal fissure, makes up nearly all this area. The inferior surface of the frontal lobe is concave and is adapted to the convex orbital plate of the frontal bone; its medial border is most prominent and presents, near the longitudinal fissure, an elongated gray mass, the olfactory bulb (if it has not been torn off) and a white strand, the olfactory tract. Running back ward from the bulb, parallel with the longitudinal fissure of the cerebrum to the fossa cerebri lateralis, the olfactory tract is seen to bifurcate into two distinct striae, a medial and a lateral.
The middle area of the inferior surface of the brain is promi uent laterally where it is formed by the temporal lobes of the cerebrum. It is depressed in its median portion and thus adapted to the hypophyseal region of the cranial floor. This median hypophyseal region extends from the end of the longi tudinal fissure, in front, backward to a great white, transversely eminence, called the pons; it contains several important Structures, viz., the bases pedunculi; posterior perforated sub mammillary bodies; tuber cinereum and stem of the infundibulum; optic chiasma, tracts and nerves; lamina cinerea terminalis; and the anterior perforated substance.