March, i893. ANIMAL TEMPERATURE. 215 



tions now increased, numerous speculations as to the source of animal 

 heat arose. It must be connected with the movement of the blood, 

 for how otherwise could be explained the cooling of a corpse or of a 

 limb deprived of its circulation ? The heat might arise in the blood 

 alone or have its origin in the heart, and only be distributed by the 

 blood stream. 



It was well known that heat arose during fermentation and by 

 the contact of acid and base ; animal heat was, therefore, considered 

 to arise by some similar process or processes taking place in the 

 blood. The former opinion was held by Wilhs, the anatomist, who, 

 in his treatise " de Ascensione Sanguinis," written about the year 

 1670, gives the theory that there is in the blood a combustion which 

 depends upon the fermentation excited by the combination of different 

 chemical substances. 



Friction was another well-known source of heat, and was the 

 explanation given for animal heat by the celebrated Dutch physio- 

 logist, Boerhaave, who lived in the early part of the eighteenth 

 century. He explained the " vital " heat as due to the friction of the 

 particles of blood in the vessels. 



A much more correct opinion had already been formed by 

 Magin in 1674. This physiologist, after his experiments in the 

 constitution of air and its relation to the heat of combustion, extended 

 the analogy of combustion to animal heat. He held that the function 

 of the lungs was not to cool the blood, but to enable that fluid to 

 absorb the nitro-aerial spirit (oxygen) of the air, and so generate heat. 



Black, in 1757, discovered that carbonic acid gas, which is formed 

 by the burning of fuel, is also given off by the lungs ; he thus showed 

 a strict connection between the processes of combustion and respira- 

 tion ; he supported the theory that the heat arising from the union 

 of oxygen and carbon was the real " vital " heat. 



A few years later Lavoisier introduced a theory exactly similar 

 to Black's, but how far it was dependent on or independent of Black's 

 work is not known. 



A serious objection was quickly raised to Black and Lavoisier's 

 theory ; it was shown that if the heat were produced in the lungs, the 

 temperature of that organ would be incompatible with its life. To 

 remove this objection, Crawford in 1781 proposed his ingenious theory, 

 which he had based on experiments upon the specific heats of venous 

 and arterial blood. His experiments appeared to show that the 

 specific heat of arterial was considerably greater than that of venous 

 blood; the combination, therefore, of the blood with the oxygen of 

 the air in the lungs gave rise to the liberation of only a small amount 

 of heat in the lungs; the blood here was arterial. When, however, 

 the blood reached the systemic capillaries, it became venous, and the 

 latent heat was rendered sensible. This was a very "materialistic" 

 explanation of " vital" heat, and was, in certain respects, a distinct 

 advance towards truth. 



