Rhixexcephalox 99 



a very thin band of gray matter, the indusium {indusiiim griseum, gyrus 

 siipracallosus) , accompanied by a few white fibres {striae longitudinales 

 Lancisii), which runs along the dorsal surface of the corpus callosum under 

 the edge of the outermost layer of the cerebral cortex. Johnston figures a 

 groove, the fimbrio-dentate fissure, between the indusium and the medial 

 stria of Lancisi in the rat, but the present writer has been unable to 

 recognize this in his specimens. 



Fig. 5 



Transverse section of the left cerebral hemisphere of the frog, from 

 Herrick, 



C.S., corpus striatum; lob. p., lobus piriformis; nuc. ac, nucleus 

 accumbens septi; p. hip., primordium hippocampi; tr. ol. v. I., tractus 

 olfactorius ventro-lateralis. 



The indusium, or supracallosal hippocampus, Is less 

 reduced in the rat than in man, but even here it appears in 

 cross sections as a very small group of cells, which require a 

 special cell stain to demonstrate them clearly (Fig. 9, Pis. 

 XVIII. -XX.). In front of the corpus callosum, there is an 

 unusually large remnant of the hippocampus to be found in 

 the rat, where the typical hippocampal structure Is to be 

 seen, though without any involution (Figs. 8, 9). At the 

 genu of the corpus callosum, part of the hippocampal forma- 

 tion extends back ventral to it for a short distance, while the 

 rest is seen to curve round it and become continuous with the 

 indusium. The dorsal band of cells is accompanied by a 

 somewhat dispersed group of fine myelinated fibres, mostly 

 belonging to the olfacto-cortical tract, which form the striae 

 Lancisii. 



