540 H. F. Osbom 



There is no question that such is the distribution of the callosal 

 fibres. This supports the opinion previously expressed that the an- 

 terior commissure and corpus callosum are analogous dorsal and ven- 

 tral commissures supplying- respectively the ventro-lateral and dorso- 

 medial portions of the mantle. 



The evidence regarding the fornix in the Ophidia is not as yet 

 sufficiently clear to be conclusive, but I believe that further evidence 

 will soon be forthcoming to show that there are true fornix tracts 

 in the reptilian brain. 



Leaving out of view the inner olfactory tract, which, according 

 to our present knowledge, is not a commissure but an inner posterior 

 nerve root, the following analysis of the typical cerebral commis- 

 sures may now be offered: 



The anterior commissure consists of two distinct divisions; 

 a posterior, the pars temporalis supplying the temporal region 

 of the mantle; an anterior, this divides into the pars olfactoria 

 which supplies the olfactory lobes, and the pars frontalis which 

 supplies the ventro-lateral portion of the mantle. The corpus cal- 

 losum consists of an anterior, or frontal division, supplying the 

 dorso-medial portions of the mantle ; a posterior division, the com- 

 missura cornu ammonis, supplying the mantle area above the 

 cornu ammonis. 



According to our present knowledge, the history of the deve- 

 lopment of these commissures is as follows (compare the conclusions 

 of part I): Anterior commissure. In the Amphibia the main 

 portion of this commissure consists of the united pars frontalis and 

 olfactoria. In the Chelonia and Aves the pars olfactoria, as a di- 

 stinct tract, is wanting but the pars frontalis is represented by the 

 dorsal portion of the anterior commissure. In the Ophidia, both the 

 pars olfactoria and frontalis are well developed and lie ventral to 

 the pars temporalis. In the mammalia, the pars olfactoria is ge- 

 nerally proportional in size with the olfactory lobes. The pars fron- 

 talis is complemeutal in size with the corpus callosum. Thus it is 

 very large in the monotremes and marsupials and gradually dimi- 

 nishes in the higher mammals, with the increase of the corpus cal- 

 losum. 



The size of the pars temporalis is in relation to the backward 

 extension of the hemispheres, or growth of the temporal lobes. Thus 

 as the hemispheres overlap the thalamencephalon, or primary fore- 

 brain, more and more, the pars temporalis increases in size. It is 



