CHAPTER VIII. 

 ANIMAL HEAT. 



FROM the earliest ages it must have been noticed that the 

 bodies of many animals, and particularly of men, are warmer 

 than the air and than most objects around them. The 

 ' vulgar opinion ' of Bacon's time, ' that fishes are the least 

 warm internally, and birds the most,' if it does not imply 

 a very extensive knowledge of animal temperature, at least 

 shows that the fundamental distinction of warm and cold- 

 blooded animals, which is to-day more accurately expressed 

 as the distinction between animals of constant temperature 

 (homoiothermal) and animals of variable temperature 

 (poikilothermal), had been grasped, and was even popularly 

 known. Since that time the accumulation of accurate 

 numerical results, and the advance of physical and physio- 

 logical doctrine, have given us definite ideas as to the rela- 

 tion of animal heat to the metabolic processes of the body. 

 It is impossible to understand the present position of the 

 subject without an elementary knowledge of the science of 

 heat. For this the student is referred to a text-book of 

 physics. All that can be done here is to preface the physio- 

 logical portion of the subject by a few remarks on the 

 physical methods and instruments employed : 



Temperature. Two bodies are at the same temperature if, when 

 placed in contact, no exchange of heat takes place between them. 

 They are at different temperatures if, on the whole, heat passes from 

 one to the other, and that body from which the heat passes is at the 

 higher temperature. It is known by experiment that if two bodies of 

 different temperature are placed in contact, heat will pass from one 

 to the other till they come to have the same temperature. If, then, 



