38 CONSIDERATIONS OF THE BRAIN OR ENCEPHALON. 



running downward toward the median line, there may be seen a 

 white striated band, a half-inch broad, called the basis pedunculi, 

 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 ante- 

 rior limbs are the optic nerves and its posterior, the optic tracts 

 (Fig. 21). The optic tract, when traced backward and out- 

 ward, 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 com- 

 monly called the interpeduncular space. You observe in it three 

 structures: (i) 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 mammil- 

 lary bodies (corpora mammillaria), and (3) a triangular, perforated 

 mass of dark gray substance, called the posterior perforated sub- 

 stance (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 hemis- 

 pheres, toward the corpus callosum. That is the lamina cinerea 

 terminalis. It bounds posteriorly the frontal part of the longit- 

 udinal 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 



