366 SYSTEMATIC ZOOLOGY. 



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 

 intelligence, and this increase in size is correlated with 

 the higher mental powers of the mammals. Microscopic 

 study of the brain shows 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 cerebrum is greatly increased by folds 

 or convolutions, and the extent and complexity of these 

 convolutions correspond well with the intelligence of the 

 form. 



In the eyes the nictitating membrane or 'third eyelid' 

 of the birds is reduced to a small fold at the inner angle of 

 the eye (p. 303). Except in the whales, and some seals, 

 moles, etc., external ears are developed, while the internal 

 parts of the ear become considerably modified. Thus the 

 quadrate and one other bone pass in to the middle ear, 

 where they, together with a third bone (stapes), form a 

 chain to convey sound-waves to the sensory portions. 

 In the inner or sensory portion a spiral outgrowth, the 

 cochlea, occurs (fig. 112), and in this is a most wonderfully 

 intricate sensory apparatus the organ of Corti the func- 

 tions of which are as yet uncertain. 



The mouth is usually provided with fleshy lips, and all 

 mammals, except mono tremes, some edentates and whales, 



