BY G ELLIOT SMITH. 647 



human cerebrum, says*: — "I believe that it [vordere Bogenfurche] 

 is transitory and that it is gradually obliterated during the time 

 that the corpus callosum assumes shape." In Proto- and Meta- 

 theria the anterior commissure is the cerebral commissure par 

 excellence, being the sole connection between all parts of the 

 hemispheres, excluding the hippocampi only. It is of interest to 

 note in this connection that the anterior commissure persists in 

 Eutheria in just that part of the mantle related to the descending 

 horn of the ventricle in which the hippocampus remains, while in 

 the region from which the hippocampus has disappeared the 

 anterior commissure is supplanted by the fibres of the proper 

 corpus callosum. In the temporal lobe of Placentalia the alveus 

 fibres (the root fibres of the hippocampal commissure) present the 

 same relationship to the terminal fibres of the anterior commissure 

 as they do to the same commissure in the whole range of the 

 extensive hippocampal region of Non-placentalia. And, as the 

 hippocampal region in the latter is co-extensive with the lateral 

 ventricle, so the anterior commissure is co-extensive with the 

 lateral ventricle, i.e., supplies, in these forms, the whole of the 

 cortex, excluding the hippocampus only. 



The important question, "What is the significance of the 

 corpus callosum 1" must now be considered. Is it a new com- 

 missure to connect cortical areas hitherto unconnected or not 

 present in lower mammals; or, on the other hand, is it merely a 

 new path for fibres which possess representatives in the brains of 

 the Meta- and Prototheria 1 From a consideration of the facts 

 before us, it seems possible that the corpus callosum appears in 

 response to the demand for a shorter connecting route between 

 the rapidly developing dorsal portions of the cortex cerebri. That 

 such factors are at work, the Marsupials themselves seem to afibrd 

 evidence. In Perameles the anterior commissure passes around 

 the corpus striatum as an external capsule to reach the various 

 regions of the mantle. In Phalanyista, a bundle of anterior 

 commissure fibres proceeds to the cortex vid the internal capsule, 



* Cunningham Memoirs of Roy. Irish Acad. No, vii. 1892, p, 5. 



