270 THE NERVOUS SYSTEM 



fissure, that marks the line along which the hippocampus has been rolled into 

 the ventricle. 



The hippocampus and fascia dentata belong to the archipallium. In the 

 marsupials and monotremes this extends dorsally on the medial surface of the 

 hemisphere in a curve, which suggests that of the corpus callosum (Fig. 202). 

 In the higher mammals the presence of a massive corpus callosum seems to 

 inhibit the development of the adjacent part of the hippocampal formation, 

 which remains as the vestigial indusium griseum, or supracallosal gyrus. This 

 hippocampal rudiment is a thin layer of gray matter on the dorsal surface of the 

 corpus callosum, within which are found delicate strands of longitudinal fibers. 

 Two of these strands, placed close together on either side of the median plane, 



Cerebral cortex 



^. 



, Hippocampal fissure 



Hippocampus and fascia 

 dentata 



Chorioid fissure 

 - Thalamus 



Olfactory bulb . ^_^- , _, 



^ ^ " Pynform area 



Tuberculum olfactorium 



Rhinal fissure 



Fig. 202. Median view of the cerebral hemisphere of a monotreme Ornithorhynchus. (Elliot 



Smith.) 



are more conspicuous than the others, and are known as the medial longitudinal 

 stria. On either side, where the supracallosal gyrus bounds the sulcus of the 

 corpus callosum, there is a less distinct strand, the lateral longitudinal stria 

 (Figs. 174, 175). The hippocampal rudiment can be traced upon the medial 

 surface of the hemisphere from the region of the medial olfactory gyrus (or stria) 

 toward the rostrum of the corpus callosum, then around the dorsal surface of 

 that great commissure to the splenium, behind which' it becomes continuous 

 with the hippocampus proper, where this comes to the surface in the angle 

 between the fascia dentata and the hippocampal gyrus (Fig. 200 Elliot Smith, 

 1915). 



The Fornix. Within the hippocampus fibers arise which run through the 

 white coat on its ventricular surface, known as the alveus, into thefimbria. This 



