316 VERTEBRATE ANIMALS. 



great pi'oportion:itj (hvelopiueiit of the brain, as compared with 

 the spinal cord. 



In the higher Mammals, also, the two halves (hemispheres) of the 

 greater brain (cerebrum) are connected together by a great band or 

 bridge of nervous tissue, constituting what is known as the corpios 

 callosum. This structure is not a consjjicuous feature in the two 

 lowest orders of the Mammalia. The senses, as a rule, attain great 

 perfection in the Mammals ; and the oidy sense which can ever be 

 said to be entirely wanting is that of sight. Eyes, however, are 

 always present, though they may be rudimentary ; and in those 

 Mammals which are said to be " blind," it is not generally that the 

 eyes are wanting, but that the skin passes unbrokenly over the 

 eyeball. Even in these cases, however, it is not impossible that 

 there may be some perception of light through the skin. An exter- 

 nal ear for collecting sounds is usually present ; but it is wanting in 

 the Whales and Dolphins, and in some of the Seals. 



The skill is invariably fundshed over a greater or less pai-t of its 

 surface with the epidermic ajjpendages known as hairs, which differ 

 from feathers chiefly in not splitting up as they are produced. In 

 the Scaly Ant-eater (Jfaids), the hairs are aggregated together so as 

 to form horny scales ; and in the Hedgehog, Porcupine, and other 

 animals, many of the hairs are developed into long spines or 

 prickles. In the Armadillos, again, the skin is more or less covered 

 by an armour of bony plates. The only apparent exception to the 

 universal presence of hair on some part or other of the integument 

 of all Mammals is constituted by the true Cetaceans (Whales and 

 Dolphins), many of which are without hair when grown up. Some, 

 however, such as the Whales, have a few bristles in the neighbour- 

 hood of the mouth even when adult. And the Dolphins, which are 

 totally hairless when grown \\\>, exhibit tufts of hair upon the 

 muzzle before they are born. 



With the exception of the singular jSIonotremes, which are now 

 known to be oviparous, all Mammals produce their young alive, and 

 the young animal is always born in a helpless condition, and is 

 nourished for a longer or shorter time by means of the milk of the 

 mother. The milk is secreted by special organs, called the mam- 

 marij glanch, which are present in both sexes, but are normally un- 

 developed in the male. The number and position of the mammae 

 vary a good deal in different cases, but they are always placed on 

 the lower surface of the body, and their ducts almost always open 

 upon a special eminence, called the teat or nipple. In the Mono- 

 tremes, however, the mammary glands open by simjjle slits in the 

 skin of the abdomen, and not by distinct nipples. In ordinary 

 JMammals the milk is obtained by voluntary suction on the part of 



