CHARACTERS OF MAMMALIA. 549 



more largely developed proportionately than the remaining 

 parts of the brain. The brain of the Mammals is chiefly dis- 

 tinguished from that of the lower Vertebrata by the fact that the 

 two hemispheres of the cerebellum are united by a transverse 

 commissure the pom Varolii and the hemispheres of the 

 brain are connected by a great commissure the corpus cal- 

 losum which is, however, of small size in the lower Mam- 

 malia. 



The senses, as a rule, attain great perfection in the Mam- 

 mals ; and the only sense which appears to be ever entirely 

 wanting is that of vision. In one of the most familiar instances 

 of this last-mentioned fact namely, in the Mole it has 

 recently been shown that it is only in the adult that vision is 

 lost, but that the organs of sight are well developed in the 

 young. The sclerotic coat of the eye is never supported by a 

 ring of bony plates as in Birds and many Reptiles. As a rule, 

 in addition to the upper and lower eyelids there is a third 

 perpendicular lid the membrana nictitans but this is wanting 

 or quite rudimentary in Man and in the Monkeys. 



An external ear or concha for collecting the vibrations of 

 sound is usually present, but is wanting in the Cetacea, many of 

 the Seals, and in some other cases. 



The integument is furnished over a greater or less portion 

 of its surface with the epidermic appendages known as " hairs." 

 These are developed, much as feathers are, upon little emi- 

 nences or papillae of the derma, but they do not split up in 

 the process of development as feathers do. In the Manis or 

 Scaly Ant-eater the epidermic appendages are in the form of 

 horny scales, and not uncommonly they are developed into 

 long spines, as in the Echidna, Porcupine, and Hedgehog. 

 In the Armadillos, again, the integument has the power of 

 developing plates of bone over a greater or less extent of its 

 surface. The only apparent exception to the universal presence 

 of hairs in some part or other of the skin of all Mammals is 

 constituted by the Cetacea, some of which are without hairs in 

 the adult state. Some, however, of these (such as the Whales) 

 possess a few bristles in the neighbourhood of the mouth even 

 when fully grown. And the Dolphins, which are totally hair- 

 less when adult, exhibit tufts of hair on the muzzle in the 

 foetal state. 



