MAMMALIA [CH. 



is called the papilla of the hair, it obviously corresponds to the 

 knob of dermis in the base of the feather, and so a hair might be 

 compared to a feather consisting only of the shaft and sunk in a 

 very deep and narrow pit of the skin (Fig. 314). In a few cases 

 hairs may be aggregated so as to form overlapping scales, and 

 practically all Mammals have nails or claws on the fingers and toes 

 which resemble essentially the horny reptilian scale. 



There is one respect in which Mammals and Birds agree with 

 each other and differ from all other kinds of animals, and this is 

 that their body temperature is considerably higher than that of their 

 usual surroundings and is capable of varying with safety to the 

 extent of only a few degrees. This condition of a constant tempera- 

 ture is known as the homoiothermal (so-called "warm-blooded") 

 condition and differs strikingly from the poikilothermal (so-called 

 " cold-blooded ") one of other animals, in which the body tempera- 

 ture varies with that of the surroundings and is usually only one or 

 two degrees above the latter. The temperature of a Bird or Mammal 

 is maintained constant by regulation both of the loss of heat by 

 radiation at the surface and of the manufacture of heat by tissue 

 oxidation. 



Perspiration or sweat is also characteristic of Mammals. This 

 consists of a fluid secreted by certain cells of the epidermis which 

 remain soft and are not converted into horn like most of the outer 

 cells. The cells which manufacture the perspiration are arranged 

 to form long tubes called sweat-glands, which penetrate far below 

 the epidermis into the dermis underneath (Fig. 314). The pro- 

 duction of sweat is a factor in the regulation of the body temperature 

 and by it also certain excreta leave the body. The fluid poured out 

 carries off a certain amount of heat and by its evaporation cools the 

 skin. Besides the sweat-glands there are other tubes which are 

 invaginations of the epidermis and consist of a special kind of 

 celL These tubes, sebaceous glands, open into the hair follicles. 

 They secrete the fatty substance or sebum which gives the natural 

 gloss to the hair (Fig. 314). 



Mammals, as we have seen, feed their young after they are born 

 by suckling them, that is providing them with milk. This milk is 

 a peculiar fluid produced by the mammary glands, consisting of 

 epidermal tubes crowded together over certain areas of the ventral 

 surface. They open at certain spots, raised above the general level, 

 which constitute the nipple or teat. It has been recently shown 

 that the mammary glands are simply enlarged and modified sebaceous 



