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 
Fic. 113. PHotocraPH 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 
