THE CENTRAL NFRVOUS SYSTEM. 361 
sulcus rhinalis posterior (or sulcus postrhinalis) (4). 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 (c). 
The marginal gyrus (1) is dorsad of the splenial sulcus, pass- 
ing onto the lateral surface of the hemisphere, where it is 
Fic. 146.—DIAGRAM OF THE SULCI AND GYRI ON THE MEDIAL SURFACE OF 
. THE HEMISPHERE. 
a, sulcus splenialis: 4, sulcus marginalis; ¢, sulcus cruciatus; @, sulcus falcialis; 
e, sulcus supracallosalis; 7, sulcus rhinalis posterior; g, hippocampal sulcus; 4, 
corpus callosum. 1, 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 (4) occurs frequently between the sulcus splenialis 
and the dorsal margin of the hemisphere. Cranioventrad ot 
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 (2). 
Immediately dorsad of the corpus callosum (/), separating 
its dorsal surface from a part of the hemisphere, is the 
