Marcu, 1893. 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 Willis, 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-aérial 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 wasshown 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. 
