THE NERVOUS SYSTEM. 241 







with five digits. In those having a less number of digits 

 the nerve branch corresponding to the lacking 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 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 the same 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 commissure piercing the corpora striata is un- 

 usually large. The fibers, which in the Monodelphia 

 arise from the cells of the hippocampus, and extend 

 cephalad to form the fornix, cross transversely to the 

 opposite hippocampus in the two lower subclasses. 



In all higher mammals the cerebrum is greatly con- 

 voluted, 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 striking similarity to the brains of 

 reptiles and birds. The brains of Rodentia possess but 

 few convolutions, while the brain of man is the most 

 highly convoluted. 



The size of the brain varies widely. As a rule, the 

 larger the brain in proportion to the size of the animal, 

 the greater is its intelligence. However, in man this 

 statement does not hold true, as an individual with a 

 small brain may be much more capable mentally than 

 one with a large brain. Mental power in man seems 

 to depend upon the development of the cells and fibers 

 of the brain. 



