THE CENTRAL NFRl/OUS SYSTEM. 



361 



sulcus rhinalis posterior (or sulcus postrhinalis) (K). This 

 extends caudad onto that portion of the hemisphere which faces 

 the cerebellum. It forms the lateral boundary of a large elon- 

 gated oval lobe lying at the side of the ventral floor of the 

 midbrain and 'tween-brain. This lobe has been called tractus 

 postrhinalis or lobus pyriformis (Fig. 145, 10; Fig. 138, /). 



In addition to the sulci and gyri above mentioned, any 

 given specimen will usually show a number of small inconstant 

 sulci and gyri in various regions ; these inconstant structures 

 will not be here described. 



On the medial surface of the hemisphere (Fig. 146), the 

 following arrangement of sulci and gyri seems to be typical. 

 Some distance from the dorsal margin a long sulcus runs 

 parallel with the margin; this is the sulcus splenialis (a). 

 The marginal gyrus (i) is dorsad of the splenial sulcus, pass- 

 ing onto the lateral surface of the hemisphere, where it is 



FIG. 146. DIAGRAM OF THE SULCI AND GYRI ON THE MEDIAL SURFACE OF 



THE HEMISPHERE. 



a, sulcus splenialis: t>, sulcus marginalis; f. sulcus cruciatus; d, sulcus falcialis; 

 <?, sulcus supracallosalis ; f, sulcus rhinalis posterior; g, hippocampal sulcus; A, 

 corpus callosum. I, gyrus marginalis; 2, gyrus fornicatus; 3, gyrus compositus 

 posterior. 



bounded by the lateral sulcus. The splenial sulcus extends 

 onto the caudal surface of the hemisphere. A shallow sulcus 

 marginalis (^) occurs frequently between the sulcus splenialis 

 and the dorsal margin of the hemisphere. Cranioventrad 01 

 the cranial end of the splenial sulcus is that portion of the 

 sulcus cruciatus (c) that lies on the medial surface of the 

 hemisphere. Ventrad of this is a short shallow furrow which 

 has been called the sulcus falcialis (d}. 



Immediately dorsad of the corpus callosum (//), separating 

 its dorsal surface from a part of the hemisphere, is the 



