832 ANIMAL HEAT. 
variation of the temperature of the body, in the rise of temperature 
observed after exercise, and during residence in tropical climates. 
The regulation of temperature, therefore, embraces two processes — 
regulation by varying loss of heat, regulation by varying production of 
heat. 
The regulation of heat production. — In considering the regulation 
of heat production, it is necessary to trace out briefly the various 
discoveries which have established, as a fact, that animal heat is due 
to combustion within the tissues. 
Historical account of facts and theories upon the sources of 
animal heat. 1 — The ancients considered animal heat to be beyond the reach 
of physical and chemical laws. They could assign no cause for it, and there- 
fore looked upon it as some innate quality, something essentially "vital." 
This "vital" heat was supposed to be concentrated in the heart (Plato, Aris- 
totle, Galen), and to be distributed to the body by the blood in the veins. It 
was prevented from accumulating by respiration, the chief f miction of which 
was to cool and temper the blood. 
As knowledge in physical and chemical processes increased, attempts were 
made to give a rational explanation of animal heat. 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. Willis, 2 about the year 1670, put forward the 
theory that there is in the blood a combustion which depends upon the fer- 
mentation excited by the combination of different chemical substances. Fric- 
tion was another well-known source of heat, and was the explanation given by 
Boerhaave; 3 he considered that animal heat was due to the friction of the 
blood corpuscles in the vessels. Stephen Hales 4 adopted this theory, and gave 
certain experiments, which he thought supported it. 
A much more correct opinion had already been formed in 1674 by Alayow, 
who, after his experiments on 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 gas (oxygen) of the air, and so generate 
heat. 
Later research has shown that the heat of living things is not due to any 
mystical so-called " vital " force, but to the processes of combustion, which 
form one of the most important phenomena of life. The different steps by 
which this knowledge has been attained are found in the discovery of Black, 
that carbon dioxide was produced in animals by a process of combustion ; in 
the work of Lavoisier " and Crawford, 8 who showed that the heat of an animal 
might be accounted for by the processes of combustion ; in the researches of 
Dulong 9 and Despretz, 10 whose results, when critically examined and explained 
by Liebig, 11 formed an important support for the law of the conservation of 
energy. 
1 Accounts of the old theories will be found iu C. Bostock, "Essay on Respiration " ; 
"An Elementary System of Physiology," 2nd edition, 1828, vol. ii. p. 243 ; Gavarret, 
" De la chaleur produite par les etres vivants," Paris, 1855; and " Les phenomenes phy- 
siques de la vie," Paris, 1869; Lorain, "De la temperature da corps humain," Paris, 
1S77, vol. i. p. 39 ; Rubner, Ztschr. f. Biol., Miinchen, 1893-94, Bd. xxx. S. 73. 
- "De Accensione Sanguinis." s " Aphor. cum Notis Sweiten," pp. 382, 675. 
4 "Statical Essays," 2nd edition, 1733, vol. ii. p. 90. 
3 "Tractatus Quinque," Oxonii, 1674. 
,; " Lectures on Chemistry," edited by Robison, Edinburgh, 1S03. 
7 Hist. Acad. ray. d. sc, Paris, 1777. 
8 "De calore Animali," 1779; "Experiments and Observations on Animal Heat," 
1788. 
9 Ann. de chim. etphys., Paris, 1843, Ser. 3, tome i. p. 440. 
10 Ibid., 1824, Ser. 2, tome xxvi. p. 337. u "Thierchemie," S. 28. 
