428 MAMMALIA— PHYSIOLOGY 



Claws or Nails. — Claws or similar structures, nails or hoofs, are 

 found in all the mammals except the Cetacea. They are thickenings 

 of the epidermis. In the cat there are five claws on each fore foot 

 and four on the hind foot. The dermis forms a crescentic fold at 

 the root of each claw. 



Hair. — All mammals are born with a thin covering of very fine 

 hair, lanugo. It can be noted on a child from the day of birth to 

 two or three days of age, when it disappears. It is replaced by a 

 protective covering of more noticeable hair, as the child grows older. 

 The cat has long coarse hairs on each side the nose called vibrissae^ 

 which function as tactile organs. In the marine mammals, although 

 fetal hair appears, the majority have lost their hair, except for a 

 few bristles around the mouth. The fur seals retain a thick under 

 coat of fur — " seal skin." In the white whale and the narwhal, even 

 the fetus has lost its hair. Those mammals without hair develop 

 a thick layer oijat under the skin. The skin is richly supplied with 

 tactile and temperature perceiving corpuscles. 



Perspiration. — The body is continually throwing off perspiration 

 which we term insensible perspiration. Excessive amounts appear- 

 ing as droplets are termed sensible perspiration. In man, the in- 

 sensible perspiration is estimated to be from one to two quarts in 

 twenty-four hours. Increased exercise, pilocarpin, strychnin, 

 nicotine, nausea or mental excitement will increase perspiration 

 markedly. 



In the dog and the cow with perspiration limited by the nature of 

 their skins, we find that the muzzle shows evidence of considerable 

 evaporation. In the horse, it is estimated that about fourteen 

 pounds of insensible perspiration are extruded in a day, through the 

 skin, of course. 



Digestive System. Mouth. Teeth. — Mammals have two sets 

 of teeth, the deciduous or milk teeth, and the permanent teeth. 

 The whalebone whale has a fringe of baleen hanging down from the 

 upper jaw and acting as a strainer for the small fish and Crustacea 

 eaten by these monsters of the deep. The duck-mole {Ornithorhyn- 

 chus) has its teeth composed of thick strong plates. 



Salivary Glands. — The mammals possess parotid, submaxillary 

 and sublingual glands. These glands are serous, mucous and mixed, 

 but in some animals the submaxillary is mucous and the sublingual 

 is mixed, while in others the reverse is the process. Salivary glands 

 are lacking in the Cetacea. The vampire bat has a buccal gland 



