Issuing from the under surface of the cerebral hemisphere and running downward toward the median line, there may be seen a white striated band, a half-inch broad, called the basis pe dunculi, which, on approximating its fellow in the median plane disappears into the pons. Anteriorly, the X-like optic chiasma (chiasma opticum) is easily identified near the longitudinal fissure; its anterior limbs are the optic nerves and its posterior, the optic tracts (Fig. 21). The optic tract, when traced back ward and outward, under the overhanging temporal lobe, is observed to cross the basis pedunculi at its point of emergence from the cerebral hemisphere. Thus the optic tract and the basis pedunculi form the lateral boundary of a diamond-shaped space extending from the optic chiasma, in front, backward to the pons. This is commonly called the interpeduncular space. You observe in it three structures: (1) A gray eminence just behind the optic chiasma called the tuber cinereum; (2) a pair of white, nipple-like bodies, an eighth of an inch in diameter, known as the white or mammillary bodies (corpora mammillaria), and (3) a triangular, perforated mass of dark gray substance, called the posterior perforated substance (substantia perforata posterior). In the normal condition, the infundibulum projects downward and forward from the center of the tuber cinereum and connects it with the hypophysis cerebri; but it is usually broken in removing the brain and the hypophysis left behind in the hypophyseal fossa.
If the optic chiasma be drawn slightly downward and back ward, a transverse and nearly vertical sheet of gray matter will be seen extending upward from it, between the cerebral hemi spheres, toward the corpus callosum. That is the lamina cinerea terminalis. It bounds posteriorly the frontal part of the longitudinal fissure of the cerebrum. Lateral to the optic chiasma and anterior to the optic tract, the gray substance is perforated by many vessels; it is called the anterior perforated substance (substantia perforata anterior) to distinguish it from a similar posterior region located between the bases pedunculi.
Posterior Area.----The posterior area of the base of the brain is formed by the pons, the cerebellum, and the medulla oblongata, which constitute the rhombencephalon (Fig. 21). The pons and medulla are median structures. They are separated by a well marked transverse groove, the ponto-medullary groove, contain ing the roots of the sixth, the seventh, the intermediate and the eighth cerebral nerves. The transverse strands of the pons traced lateralward are observed to form a large round bundle, called the brachium pontis, which extends into the hemisphere of the cerebellum on either side. Between those pontine strands, at the lateral border of the pons, there should be noticed the roots of the great trigeminal nerve. A sagittal line through this nerve at its attachment to the pons may be regarded as the boundary between the pons and the cerebellar hemisphere.
The hemispheres of the cerebellum form the lateral part of the posterior area; their stratified appearance is already familiar. Inferior to the pons is the medulla oblongata. The medulla is
about an inch long and three-quarters of an inch broad near the pons, but measures less than one-half inch in width at the lower end. It is partially divided into lateral halves by the anterior median fissure, which is deep, above, but is almost obliterated in the lower half of the medulla by the crossing of the lateral pyramidal tracts, the decussatio pyramidum. On either side of the anterior median fissure, the student should notice, in this order, the pyramid, the olive, and the restiform body. The pyramid (pyramis) bounds the anterior median fissure. It is an eighth of an inch in width, is most prominent near the pons and tapers off inferiorly because about 8o per cent. of its fibers cross over to the opposite side and sink backward in the medulla. It is bounded laterally by a slight longitudinal furrow, the anterior lateral sulcus (sulcus lateralis anterior) which contains the roots of the twelfth cerebral nerve, and separates the pyramid from the olive and from the flat surface of the lateral funiculus of the medulla. The olive (oliva) occupies the upper half of the lateral surface of the medulla; the lateral funiculus, the lower half. The olive is equal in breadth to the pyramid. It is quite prominent, is white in color and is elliptical in outline. The posterior lateral sulcus (sulcus lateralis posterior) separates it from the restiform body. The roots of the ninth, tenth and eleventh cerebral nerves, which are contained in that groove and the restiform body which lies beyond it, can be seen only by pressing aside the hemisphere of the cerebellum.
The Roots of the Cerebral (Cranial) Nerves (Fig. 21).—The cerebral nerves (nervi cerebrales) are numbered from before backward according to the order of their points of attachment to the brain surface. Those points of attachment are, for the motor roots, points of exit from the brain; and are points of entrance into the brain, for all the sensory roots. The genetic nucleus (nucleus originis), which is the real origin of a motor root, and the terminal nucleus (nucleus terminalis), which contains the real central termination of any sensory root, are imbedded within the brain substance and do not at present concern us.
i. The olfactory nerves (nervi olfactorii) are the first. They are the nerves of smell. They are composed of the peripheral olfactory neurones whose cell-bodies form the olfactory ganglion, located in the olfactory area of the nasal mucous membrane. The short dendrites protrude slightly from the mucous mem brane into the nasal cavity and end in a tuft of cilia; the vari cose axones, leaving the deep end of the cell-bodies, plunge into the tunica propria and ascend through the cribriform plate to the olfactory bulb, forming close plexuses on the way; they are non-medullated and are collected into 20 or 3o bundles, each of which is invested by a nucleated sheath like a neu rolemma. The fibers proceed some distance into the gray substance of the olfactory bulb, which constitutes the terminal nucleus of the first nerves, and there branch richly and end in relation with the mitral and brush-cells.