MAMMALIA 283 



the leucocytes have a wandering existence among the 

 tissues and accumulate in lymphoid tissue and in lymphatic 

 glands. The thrombocytes, or blood platelets, are directly 

 concerned in the formation of fibrin, and thus occluding vessels 

 when punctured and injured. They form thromboses around 

 intruding objects which penetrate the vessels. The blood of 

 the mammal, like that of the bird, differs from the blood of 

 all other animals, however, in that it is maintained at a rela- 

 tively constant temperature. It is warm and the temperature 

 varies slightly, according to species. In man the temperature 

 is about 98-4 F. (37 C.), in the rabbit about 102 F. (39 C.), 

 in the horse 100 F. (37 '8 C.), in the cow 101 F. (38-3 C.), 

 in the sheep and the pig 103-104 F. (39-40 C.) ; in the case 

 of birds it is even higher, about 107-108' F. (42 C.). It is 

 evident, therefore, that mammals and birds differ funda- 

 mentally from other animals in possessing a mechanism for 

 regulating the gain and loss of heat. The temperature of the 

 lower animals is near to that of the temperature of their 

 surroundings and varies with it, and such are called poikilo- 

 thermic, or simply cold-blooded. Those animals which 

 maintain a relatively constant temperature independently 

 of their environment are called homoiothermic, or warm- 

 blooded. For one thing, the feathers and the hair contain 

 air next the skin, and the mammal and the bird carry around 

 them a layer of air which is warm and protected from undue 

 loss of heat. The heat, however, is mainly generated by an 

 increase in oxidation, and oxidation is promoted by the 

 increase of the heat. Thus the loss of the heat is prevented 

 by the presence of the body coverings, in the case of aquatic 

 animals by the great thickness of the fatty layer of the skin, 

 and the temperature maintained by the dilatation or con- 

 striction of the blood-vessels of the skin. Besides transitory 

 regulation according to circumstances of action, a general 

 change of this nature is an accompaniment of season, the skin 

 being more richly supplied with blood in the summer and less 

 so in the winter. A lowering of temperature means a slowing 

 down of metabolic processes, and in some animals leads to a 

 state of hibernation. 



The heart is developed by the approximation and fusion of 



