THE RHINENCEPHALON 271 



is a thin band of fibers, running along the medial surface of the hippocampus 

 and joining with the alveus to form the floor of the inferior horn of the lateral 

 ventricle (Figs. 201, 204, 209). The fimbria increases in volume as it is traced 

 toward the splenium of the corpus callosum, to the under surface of which it 

 becomes applied, where, together with its fellow of the opposite side, it forms 

 the fornix. 



The fornix, which is represented diagrammatically in Fig. 203, is an arched 

 fiber tract, consisting of two symmetric lateral halves, which are separate at 

 either extremity, but joined together beneath the corpus callosum. This 

 medially placed portion is known as the body of the fornix. From its caudal 

 extremity the fimbria diverge, and one of them runs along the medial aspect of 

 each hippocampus. In man the hippocampus does not reach the under surface 



Column of fornix 



Body of fornix 



- Hippocampal commissure 



| 



Cms of fornix 



Fimbria of hippocampus 



Fig. 203. Diagram of the fornix. 



of the corpus callosum, and the part of the fimbria which joins the body of the 

 fornix, being unaccompanied by hippocampus, is known as the cms fornicis. 

 Rostrally the fornix is continued as two arched pillars, the columnce fornicis, 

 to the mammillary bodies. 



The body of the fornix is triangular, with its apex directed rostrally. It con- 

 sists in large part of two longitudinal bundles of fibers, representing the con- 

 tinuation of the fimbriae, widely separated at the base of the triangle, but closely 

 approximated at the apex, whence they are continued as the columnae fornicis. 

 At the point where these longitudinal bundles diverge toward the base of the 

 triangle they are united by transverse fibers which join together the two hippo- 

 campi by way of the fimbriae. These fibers constitute the hippocampal com- 

 missure. This part of the fornix, because of its resemblance to a harp, was 

 formerly known as the psalterium (Fig. 184). The hippocampal commissure 



