652 MANUAL OF ZOOLOGY. 



the inferior surface of the body, and their ducts in the great 

 majority of cases open collectively upon a common elevation 

 the "teat" or "nipple." In the Monotremata, however, 

 there are no nipples, the ducts of the mammary glands open- 

 ing either into a pouch of the integument (Echidna] or upon a 

 flat surface (Ornithorhynchiis). 



The young Mammal is nourished for a longer or shorter 

 time by the milk secreted by the mammary glands of the 

 mother. In ordinary cases the milk is obtained by voluntary 

 suction on the part of the young animal ; but in the Marsupials 

 the young are at first unable to suck for themselves, and the 

 milk is forced out of the gland by the contractions of a special 

 muscle. 



The nervous system of Mammals is chiefly remarkable for 

 the great proportionate development of the cerebral mass as 

 compared with the size of the spinal cord. In the higher 

 Mammals, again, the hemispheres of the cerebrum are much 

 more largely developed proportionately than thfe 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 trans- 

 verse commissure the pans Varolii ; and the hemispheres 

 of the cerebrum are connected by a great commissure the 

 corpus callosum which is, however, of small size in the lower 

 Mammalia. 



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. 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 eye- 

 lids 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 dermis, but they do not split up in 

 the process of development as feathers do. In the Mam's 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. 

 The only apparent exception to the universal presence of 



