175 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 
