THE NERVOUS SYSTEM 



253 



digit or digits is wanting. The arrangement of the columns or 

 tracts of fibers in the spinal cord is very similar in all the orders. 

 The anterior or direct pyramidal tract, however, is partially or 

 wholly absent in most orders below the Primates. It is best 

 developed in man, although in a number of cases it has been 

 found entirely wanting in the human. The number of the 

 cranial nerves is always twelve and their distribution is very 

 similar in all forms investigated. 



The structure of the brain in the Ornithodelphia and Didel- 

 phia differs considerably from that of the Monodelphia. In the 

 two former subclasses the corpus callosum and fornix are very 



Fig. 122. — Photograph of the Human Brain from the Lateral Aspect. 



Two-fifths natural size. 



rudimentary, but the anterior commissure piercing the corpora 

 striata is unusually large. The fibers, which in the Monodel- 

 phia arise from the cells of the hippocampus, and extend craniad 

 to form the fornix, cross transversely to the opposite hippocam- 

 pus 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, 



