148 ANIMAL PHYSIOLOGY. 



from the occupants; and in large nails the ventilation should 

 be placed at a variety of levels. 



112. Internal Temperature. The temperature of the in- 

 terior of the body is very constant, its healthy variation being 

 limited to a range of probably two degrees. In this respect 

 warm blooded animals differ from the cold blooded ; for the 

 evolution of heat in cold blooded animals being only sufficient 

 to warm them a very few degrees above the surrounding 

 medium, the temperature varies with that of the medium. 

 In all instances the heat evolved is the result of chemical 

 action; and the processes in warm blooded animals being more 

 active, a greater amount of heat is evolved, while, reciprocally, 

 a certain temperature is required for the carrying on. of these 

 processes by the more delicately constituted elements of 

 texture. 



The highest temperature is found in birds, in some of which 

 it reaches 1 1 F. or more. The temperature of the human 

 blood varies from 100 to 102; and it has been already 

 mentioned (p. 125) that the blood is the source of warmth 

 to the tissues. It has not exactly the same temperature 

 throughout the circulation, but exhibits definite, though 

 slight, differences in different parts. It was many years ago 

 observed that the blood in the jugular vein of an animal was 

 slightly cooler than .that in the corresponding artery (John 

 Davy), and it was judged, naturally, that the blood was 

 warmed as it became arterialized in the lungs. The observa- 

 tion was correct, and yet, curiously enough, the conclusion 

 was wrong ; for in later observations thermometers have been 

 introduced by Bernard, not merely into the jugular vein, but 

 into the heart itself, and into the renal and hepatic veins, 

 with an interesting result which explains the matter differ- 

 ently. The warmest blood in the body is that which has been 

 subjected to processes of change in the kidneys and liver ; 

 the coldest blood is that which returns from the surface of 

 the body, where heat is constantly lost by radiation; the blood 

 entering the heart from the inferior vena cava, containing 

 what comes from the liver and kidneys, is warmer than that 

 which returns by the jugular vein from the head; the 

 mingled contents of the right side of the heart have an inter- 

 mediate temperature ; this blood then passes into the lungs, 



