212 ELEMENTS OF MAMMALIAN ANATOMY 



The ventral surface (Fi^. 104) of the brain also presents impor- 

 tant features which should be noted by the student before 

 investigating the internal structure. The anterior pyramids, 

 two indistinctly difTerentiated bundles of fibers, occupy the 

 middle of the ventral region of the medulla, and craniad of the 

 pons help form the peduncles of the cerebrum. Laterad of 

 each pyramid is the olivary projection. A broad band of trans- 

 verse fibers appearing just caudad of the pons Varolii and lateral 

 of the anterior pyramids is the corpus trapezoideiim. 



The pons Varolii is itself composed of a band of transverse 

 fibers which on either side forms the middle peduncle of the 

 cerebellum. The fibers originate either in cells of the medulla 

 or those of the cerebellum. 



The crura cerebri, or peduncles of the cerebrum, the ventral 

 portions of which are continued as the anterior pyramids (Fig. 

 104), are seen just craniad of the pons. Their fibers unite the 

 cerebrum to the rest of the brain and the spinal cord. In the 

 space between the crura and the optic chiasm is a prominent 

 projection, the terminal nodular portion of which is the pituitary 

 body or hypophysis. It occupies the hypophyseal fossa of the 

 skull and is usually torn off in removing the brain. Caudad of 

 the hypophysis are two small rounded white bodies, the corpora 

 mamillaria. The tuber cinereum, a slightly elevated mass 

 of gray matter behind the optic chiasm, bears on its surface 

 the funnel-shaped stalk, the infundibulum, to which the 

 hypophysis is attached. If the two latter parts are removed, 

 there is seen a small elongated aperture through the tuber 

 cinereum into the third ventricle (Figs. 104 and 106). 



The optic commissure, or optic chiasm, is the commissure 

 formed by the crossing of the optic nerves just craniad of the 

 tuber cinereum. The prolongation of the optic nerves dorsad 

 from the optic commissure forms the optic tracts, partly covered 

 by the temporal lobes. On either side of the median fissure 

 just craniad of the optic chiasm is a somewhat triangular area 

 known as the anterior perforated space because of the numerous 

 vessels that enter the brain in this region. Laterally this space 



