648 CEREBRAL COMMISSURES OF THE MAMMALIA, 



in addition to the external capsule. In Macro/ms, in order, 

 apparently, to meet the demands of the greatly increased mantle, 

 almost half of the anterior commissure fibres pass through the 

 internal capsule to reach the dorsal part of the mantle. It seems 

 as if this principle became extended in the placental mammal, 

 and to provide for the enormous mantle development of the 

 higher Mammalia, a shorter supraventricular route was established 

 in place of the circuitous path, which the fibres of the anterior 

 commissure would otherwise have had to take to reach such a 

 region, for instance, as the callosal convolution in man. 



Although from these considerations it seems as if the callosal 

 fibres were serially homologous with the fibres of the anterior 

 commissure, i.e., fibres separated from the anterior commissure to 

 meet the exigencies of a huge mantle-development, it must not 

 be forgotten that the parts of the cerebrum which the corpus 

 callosum connects, only develop late in the phylogenetic and 

 ontogenetic history of the individual. Thus Sir Wm. Turner* 

 states that the relative proportion of mantle to rhinencephalon 

 decreases as we descend the Mammalian series until in the lowest 

 mammals the pallium is almost as small as the rhinencephalon. 

 In Perameles this reduction of pallium goes still further, as, 

 instead of being as large as the rhinencephalon, the pallium forms 

 merely a small cap placed upon the rhinencephalon. In the 

 Eutheria a considerable portion of the mantle, viz., the temporal 

 lobe, is connected by the anterior commissure. If, then, from 

 the small cap of mantle found in FeraraeJes we subtract the portion 

 corresponding to the temporal mantle of higher forms, there will 

 be little, if any, mantle left to be connected by a corpus callosum. 

 In such higher Metatherians as the Wallaby the demand for a 

 corpus callosum becomes greater, and is met to some extent by 

 added fibres of the anterior commissure passing through the 

 internal capsule. If in such a highly developed brain as that of 

 the Wallaby, where the demand for a true corpus callosum is 

 evident, such a structure is wanting, it seems a priori highly 



* Journ. of Anat. Vol. xxs'. 



