228 ELEMENTS OF MAMMALIAN ANATOMY. 



oped in man, although in a number of cases it has been found 

 entirely wanting in the human. The number of the cranial 



FIG. 113. PHOTOGRAPH OF THE HUMAN BRAIN FROM THE LATERAL 

 ASPECT. Two fifths natural size. 



nerves is always twelve and their distribution is very similar 

 in all forms investigated. 



The structure of the brain in the Ornithodelphia and 

 Didelphia differs considerably from that of the Mono- 

 delphia. In the two former subclasses the corpus callosum 

 and fornix are very rudimentary, but the anterior commis- 

 sure piercing the corpora striata is unusually large. The 

 fibers, which in the Monodelphia arise from the cells of the 

 hippocampus, and extend craniad to form the fornix, cross 

 transversely to the opposite hippocampus in the two lower 

 subclasses. 



In all higher mammals the cerebrum is greatly convoluted, 

 but in the lower ones the convolutions are few or almost 

 absent, as in Ornithorhynchus. The above-mentioned 

 features show that the ornithodelphian brain presents a 



