I7 8 ELEMENTS OF MAMMALIAN ANATOMY. 



portions of the gyri marginalis and suprasylvius constitute 

 a portion of the occipital lobe. These two gyri are sepa- 

 rated by the lateral sulcus. The suprasylvian sulcus ex- 

 tends between the gyrus suprasylvius and the gyrus ecto- 

 sylvius. The splenial sulcus (Fig. 95) separates the gyrus 

 marginalis from the gyrus fornicatus on the medial aspect 

 of the parietal and occipital lobes. There are no definite 

 sulci or fissures separating the parietal, occipital, and tem- 

 poral lobes. The postrhinal fissure, extending caudad from 

 the fissure of Sylvius, divides the temporal lobe into two 

 portions on the ventral aspect. 



The ventral surface (Fig. 93) of the brain also presents 

 important features which should be noted by the student 

 before investigating the internal structure. The anterior 

 pyramids, two indistinctly differentiated 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 transverse fibers appearing just caudad of the pons 

 Varolii and laterad of the anterior pyramids is the corpus 

 trapezoideum. 



The pons Varolii is itself composed of a band of trans- 

 verse 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. 93), 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 pituitary fossa of the skull and is usually torn 

 off in removing the brain. Caudad of the hypophysis are 



