THE NERVOUS SYSTEM. 



189 



sylvius constitute a portion of the occipital lobe. These 

 two gyri are separated by the lateral sulcus. The supra- 

 sylvian sulcus extends between the gyrus suprasylvius 

 and the gyrus ectosylvius. The splenial sulcus (Fig. 92) 

 separates the gyrus marginalis 

 from the gyrus fornicatus on 

 the mesal aspect of the parie- 

 tal and occipital lobes. There 

 are no definite sulci or fissures 

 separating the parietal, occipi- 

 tal, and temporal lobes. The 

 postrhinal fissure, extending 

 caudad from the fissure of Syl- 

 vius, divides the temporal lobe 

 into two portions on the ven- 

 tral aspect. 



The ventral surface (Fig. 90) 

 of the brain also presents im- 

 portant features which should 

 be noted by the student before 

 investigating the internal 

 structure. The anterior pyra- 

 mids, two indistinctly differ- 

 entiated bundles of fibers, oc- 

 cupy the mesoventral region 

 of the medulla, and cephalad 

 of the pons help form the crura 

 cerebri. Laterad of each pyra- 

 mid 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 



FIG. 



89. DORSAL ASPECT OF 

 THE BRAIN. 



Gyrus marginalis; ac, white 

 matter of the cord; ad, gray 

 matter of the cord; b, gyrus 

 suprasylvius ; c, gyrus ectosyl- 

 vius ; cr, crucial fissure ; ce, lat- 

 eral lobe of cerebellum ; d, lat- 

 eral sulcus; e, suprasylvian 

 sulcus; Ig, great longitudinal 

 fissure; md, medulla oblon- 

 gata ; n, first spinal nerve ; ol, 

 olfactory lobe; p, posterior 

 pyramids; sp, spinal cord; v, 

 vermis of cerebellum. 



