SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 403 



is seen in the anterior commissure. The dorsal commissure shows 

 degenerated fibers, but in this case there was possibility of direct 

 injury to the alveus. 



In a third animal in which a large lesion was made in the 

 thalamus and midbrain, there was incidental injury to the dorso- 

 medial angles of the hemispheres. On the left side this invaded 

 the hippocampus secondarily, largely destroying its dorsal por- 

 tion; on the right it affected only the dorsal cortex. The hippo- 

 campal commissure was of cjourse deeply affected and both sides 

 show degeneration in the internal capsule. The anterior com- 

 missure shows no degeneration. The character of the degener- 

 ation in the dorsal commissure differs as the lesion affects the 

 hippocampus or the general cortex. After hippocampal lesion 

 the degeneration is more profuse and the blackened droplets are 

 much coarser. This is perhaps to be explained on the suppo- 

 sition that fibers arising in the general cortex are more lightly 

 medullated. Attention has been called to the fact that the dor- 

 sal commissure contains bundles of lightly medullated and non- 

 medullated fibers. A similar explanation can scarcely be given 

 for the apparent absence of fibers from the dorsal cortex in the 

 anterior commissure. One receives the impression from Weigert 

 sections that the anterior commissure consists almost wholly of 

 medullated fibers. • 



It thus appears that in the opossum we have positive evidence 

 in Weigert and Marchi preparations for the presence of corpus 

 callosum fibers in the dorsal commissure and negative evidence 

 from Marchi preparations regarding fibers from the dorsal cortex 

 crossing in the anterior commissure. Further study of the dis- 

 tribution of the anterior commissure is under way. 



The facts above stated warrant a reinvestigation of the Aus- 

 tralian marsupials. The presence of true corpus callosum fibers 

 in marsupials and also in reptiles is to be expected in view of 

 the fact that in fishes, amphibians and reptiles sensory radia- 

 tions ascend from the thalamus to the telencephalon and of the 

 further fact that in selachians the telencephalic center for these 

 thalamic radiations is connected with its fellow by a commissure 

 which bears the morphological relations characteristic of the 



