SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 399 



torius medialis. Caudally the commissure is imbedded between 

 the primordium and the hippocampus proper and dorsally the 

 cells of the indusium are occasionally found intermingled with 

 the bundles of the corpus callosum (fig. 47) so as to establish 

 a continuity between the indusium and the primordium. In 

 front of the commissure there is a broad continuity of these 

 bodies as already described. It thus appears that the corpus 

 callosum and hippocampal commissure are imbedded in a con- 

 tinuous mass of gray matter which occupies the thickened lamina 

 supraneuroporica and extends into either hemisphere as the hip- 

 pocampal formation. At the point m where the tela chorioidea 

 is attached to the caudal surface of the hippocampal commissure 

 is a prominent module of cells to which Elliot Smith ('97 c) has 

 called attention in Nyctophilus. This is the primary upper bor- 

 der of the lamina supraneuroporica and the cells belong to the 

 indusium verum as defined by Professor Smith ('97 e). It might 

 be called nodulus marginalis. 



In the rat there is a large corpus callosum with well developed 

 genu and splenium. • The hippocampal commissure is a plate 

 of fibers broad dorso-ventrally and standing nearly vertically 

 beneath the body of the corpus callosum, a little nearer to the 

 genu. At its dorsal border the plate of fibers bends caudad 

 beneath the corpus callosum and becomes continuous with the 

 thin edge of the splenium (fig. 89). This form of the commis- 

 sures is apparently due to the dorsal end of the large hippo- 

 campus which fills the angle between the hippocampal commis- 

 sure and the splenium, the hippocampal flexure of Elliot Smith. 

 The pillars of the fornix are large and rise in front of the hippo- 

 campal commissure relatively free from mingling with it (fig. 64). 

 Just beneath the corpus callosum they turn latero-caudad in the 

 fimbria. Between the fornix columns and the hippocampal 

 commissure, and to some extent mingled with the latter, is the 

 small-celled nucleus already described in the bat. Beneath the 

 rostral part of the corpus callosum the septum pellucidum is 

 filled with large cells as in the bat. Here in the rat these cells 

 become more compactly arranged near the median line and are 

 directly continuous with the indusium around the genu as de- 

 scribed above from transverse sections. Rostral from the genu 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 23, NO. 5 



