31 G YEUTEIiltATE ANIMALS. 



greut [jroportiDiiiitj il-v^'l(>;jiiK-iit of tlie brain, as compare, 1 with 

 the sj)iiial cord. 



In the higher Mammals also, tlie two halves (hemispbeie.s) of the 

 greater brain {i/eivbrmn.) are connected together by a great band or 

 bridge of nervous tissue, constituting wliat is known as tlie corpm 

 caHosiim. This structure is not a co}ispicuous feature in the two 

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

 perfection in the Mammals ; anil tlie only sense which can ever be 

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

 always present, though they m.iy be rudimentary ; and in those 

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

 eyes are wanting, but tliat the skin jiasses unbrokeuly over the 

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

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

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

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



The skill is invariably furnished over a gi'eater or less part of its 

 siu'face with the eiiidermic ajipendages known as hairs, whicli differ 

 from feathers chiefly in not sjilitting np as they are produced. In 

 the Scaly Ant-eater {Mania), the haii'S ai-e aggregated togetlier so as 

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

 animals, many of the liairs 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 

 univers.d presence of hair on .some part or other of the integument 

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

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

 however, such as the Wiiale.s, have a few bristles in the neighbour- 

 hood of the mouth even wlien aibilt. And the Doljihins, which are 

 totally hairless when grown up, exhiliit tufts of hair upon the 

 muzzle before they are born. 



With the exce])tion of the singular Monotremes, which are now 

 known to be o\iiiaiou.s, all Mannnals produce their young alive, and 

 the young animal is always born in a heljiless 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 main- 

 ■iiiarii aliiiah, which are present in both sexes, but are nornifdly un- 

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

 vary a go:>d ileal 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 ni])])le. In the Mono- 

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

 skin of the abdomen, .and not by distinct ni]iplea. In ordinary 

 -Mannnals llir milk is obtained I ly voluntary suction on the part of 



