402 J. B. JOHNSTON 



CONSTITUTION 6f DORSAL COMMISSURE 



Symington ('93) and Elliot Smith ('97 d, '02, '03 b) have pre- 

 sented evidence that the marsupials do not possess a true corpus 

 callosum. The corpus callosum of mammals is defined as con- 

 sisting of fibers coming from undoubted neopallial areas, invad- 

 ing the alveus of the hippocampus and crossing to the opposite 

 hemisphere through the dorsal commissure. Elliot Smith ('03) 

 believes that while such a commissure does not exist in mar- 

 supials, the function of a corpus callosum is performed by fibers 

 which cross in the anterior commissure, reaching it by way of 

 the external capsule. The writer has at hand at the time of 

 writing only Smith's reply to Zuckerhandl (Smith ^03 b). As 

 this clearly defines the questions involved it will be of interest 

 to note some evidence bearing on the subject derived from the 

 brain of the opossum. In Weigert sections corresponding to the 

 one from the brain of Perameles beautifully figured by Elliot 

 Smith the writer can not doubt that fibers enter the alveus from 

 the medio-dorsal cortex far beyond the transitional area between 

 hippocampus and general cortex (fig. 37). Sagittal sections show 

 this more clearly Tfig. 39). Since Weigert sections are not con- 

 clusive on a point like this, an attempt was made to test the 

 presence of callosal fibers in the dorsal commissure by experi- 

 ment. The attempt in two operations to cut the dorsal com- 

 missure without injury to anything else failed, but in other 

 experiments light was thrown on this question. In one animal 

 an area of dorsal cortex (shown in fig. 40, a) was scraped out. 

 The animal when killed showed no infection and there is in the 

 sections no evidence of any injury being done to the hippocampus 

 or alveus. Preparations were made by the Marchi method and 

 showed the following results: internal capsule deeply degener- 

 ated; external capsule affected; anterior commissure not affected 

 at all; alveus and dorsal commissure contain many degenerated 

 fibers, traced from the lesion (fig. 40). 



In another animal a much larger area of dorsal cortex was 

 destroyed (fig. 40, a). Although the internal capsule was badly 

 degenerated and the external capsule likewise, no degeneration 



