334 



NERVOUS SYSTEM OF VERTEBRATES. 



terminalis in front of the median ventricle is a small anterior 

 commissure connecting the corpora striata. This is largely made 

 up of olfactory tract fibers coming from the olfactory bulb. The 

 lamina terminalis is continued upward and greatly stretched 

 backward to join the splenium of the corpus callosum. In this 

 stretched portion of the lamina terminaHs runs a thin band of 

 transverse fibers constituting a commissure of the hippocampi, 

 the psalterium. The corpus callosum is a thick lamina of trans- 

 verse fibers belonging to the neopalHum, slightly arched. The 

 mesial wall beneath it is formed of the stretched and thinned 

 dorsal portion of the precommissural body and is kno\vn as the 

 septum pellucidum. 



The floor of the temporal horn of the lateral ventricle is formed 

 by the hippocampus which approaches the under side of the 

 splenium of the callosum. The liippocampus does not end here 

 but bends backward and curves over the splenial border of the 

 callosum to run forward upon the dorsal surface of the latter. 

 The portion of the hippocampus in this position consists of paired 

 rudiments of the hippocampal fold (indusium jalsum) in which 

 runs a strand of fibers belonging to the fornix system, the striae 

 Lancisii, and a median film of gray matter connecting the hip- 

 pocampal rudiments {indusium verum). This rudimentary hippo- 

 campus runs the whole length of the callosum in man, and in 

 some mammals continues forward to the olfactory peduncles, 

 forming the boundary line between the praecommissural area and 

 the neopalHum in the original position of the hippocampus in 

 monotremes and marsupials. From the chief part of the hippo- 

 campus in the temporal region fibers run forward over its surface 

 forming the fimbria. When the hippocampus bends back to 

 gain the upper surface of the callosum the fimbria continues 

 forward in the lamina terminalis, and is usually termed the body 

 of the fornix. The two bodies converge forw^ard and the triangular 

 space between them is bridged by the exchange of fibers which 

 form the psalterium. This is a true commissure of the hippocam- 

 pus and is homologous with the hippocampal commissure of rep- 

 tiles and monotremes. In the lamina terminalis the bodies of 

 the fornix bend do^^^lward in front of the foramina of Monro and 



