THE CONSERVATION OF ENERGY IN THE ANIMAL BODY. 261 
method for its determination at frequent intervals has been de- 
scribed by Benedict & Snell.* While it is true that the rectal 
temperature is not necessarily the average of that of the whole body, 
we may probably assume with safety that the variations of the two 
will substantially correspond and therefore that the error introduced 
by the use of the former will be insignificant. 
The question of possible chemical and physical changes in the 
make-up of the tissues has already been considered in the preceding 
chapter, where it was pointed out that their effect is in all proba- 
bility negligible in experiments of any considerable duration. 
Earty Exprriments.}—From a slightly different point of view 
the question under consideration may be stated as that of the source 
of animal heat. Is the energy given off by the animal in this form 
(in the absence of external muscular work) equivalent to the heat 
produced by the oxidation of the same materials outside the body? 
In this form the question could scarcely fail to attract attention as 
soon as man began to observe and reflect upon the phenomena of 
nature. 
The ancients regarded the “animal heat” or “vital heat” as 
“innate” and having its source in the heart. In more recent times 
it was attributed in a vague way to chemical action, and later was 
also explained as resulting from mechanical action and in particular 
from the pulsation of the blood in the blood-vessels. Our real 
knowledge of the subject, however, dates from the discovery of 
oxygen and from those researches by Lavoisier and others which 
established the true nature of combustion and laid the foundations 
of modern chemistry. 
Black { discovered that carbon dioxide was produced in animals 
by a process of combustion, and Lavoisier,§ along with his more 
purely chemical researches, studied the question of animal heat and 
advanced the hypothesis that respiration consists essentially of a 
slow oxidation of the carbon and hydrogen of the food by the oxygen 
of the air, and that this slow combustion is the source of the animal 
heat. 
* Arch. ges Physiol., 90, 33. 
{This paragraph follows substantially the historical introduction to 
Rubner’s paper, “ Die Quelle der thierischen Wirme.”' cited below. 
t Lectures on Chemistry, edited by Robison, Edinburgh, 1803 
§ Hist. Acad. Roy. d. Sci., Paris, 1780, 355. 
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