MAMMALS. 99 



inence joined to the shoulder-blade (scapula), while in many 

 the collar-bone (clavicle) also is lacking. The feet have 

 typically five toes, but not infrequently this number is re- 

 duced by a disappearance of the outer digits, the reduction 

 reaching its extreme in the cow, which has but two, and 

 the horse, which walks upon the tip of its middle toe. 



FIG. 37. Brain of Dog. (After Weidersheim.) II-XII, the cranial nerves 

 (see page 94). 



The most marked characteristic of the nervous system is 

 the great relative increase in size of the cerebrum, and, to 

 a less extent, of the cerebellum; the optic lobes and the 

 medulla, so prominent in the lower forms, being over- 

 shadowed by these parts. The cerebrum is the seat of in- 

 telligence, and this increase in size is correlated with the 

 higher mental powers of the mammals. Microscopic study 

 of the brain has shown that this organ is composed of two 

 different portions, called, according to their colors, white and 

 gray, and that the gray matter is the true brain substance, 

 while the white is composed of nerve-cords to transmit 

 nerve impulses. The gray matter is on the outside of the 

 cerebrum, hence the larger the brain the more surface it 

 has, and consequently the more gray matter it can have. 

 In the higher mammals the amount of surface of the cere- 

 brum is greatly increased by folds or convolutions, and the 



