RECENT LITERATURE. 



The Cerebral Commissures. 



The homologies of the commissural systems of the cerebrum are 

 confessedly perplexing, and we must welcome any attempt to remove 

 the existing obscurity. The chief occasion of ambiguity seems to be 

 the question of definition. It is especially the callosum which lacks 

 determinateness. 



In the higher mammals there is, of course, no difficulty in recog- 

 nizing that body, but the moment one searches for the familiar band 

 in marsupials or non-mammals the difficulty is apparent. Recent 

 comparative studies have thrown into greater prominence the hippo- 

 campal commissure and emphasized the close relation between it and 

 the callosum. 



Those who have recently contended for the existence of a callo- 

 sum in lower groups, among whom the writer is numbered, have done 

 so on the understanding that a mantle commissure, cephalad of, and 

 in the same horizontal plane as the hippocampal commissure must be 

 the homologue of the callosum. It is, of course, recognized by all 

 that the greater relative extent of the osmatic centres in the lower 

 animals renders mistake easy in locating homologous areas. 



Dr. A. Meyer has attempted to limit the entension of the word 

 callosum. 1 Meyer would reserve the name callosum " for the ana- 

 tomically well delimited commissural system of that region which 

 gives origin to the genuine corona radiata, the fibres of the internal 

 capsule." With this understanding the marsupials and, of course, all 

 lower vertebrates have no callosum. 



Naturally Osborn has to bear the brunt of the criticism, for he has 

 expressly said that the callosum is composed of a cephalic or frontal 

 portion which supplies the dorso-median part of the mantle and the 

 commissure of the Ammon's horn. Meyer lays great stress on the 

 absence of the inner capsule. Edinger indeed applies this name to 

 certain fibres passing through the median wall to the brain base. The 



^Zur Homologie der Fornixcommissur und des Septum lucidum bei den 

 Reptilien und Saeugern. Anat. Anzeiger, X, 15. 



