373 
686. To this doctrine of animal heat, we are a- 
ware that many objections have been made, and 
that some physiologists still seem disposed to ques- 
*ion its validity. But whether we consider the na- 
tural constitution of the air, the actual changes which 
it suffers in the lungs, and the extrication of caloric 
that accompanies these changes; or whether we attend 
to the increase of specific heat acquired by the blood, 
and the universal relation which animal temperature 
bears to the extent and perfection of the respiratory 
organs ; or whether, lastly, we contemplate the total 
insufficiency of every other known function or opera- 
tion to afford that permanent supply of heat which 
the exigencies of the animal system require; we are 
equally conducted to the conclusion, that the change 
induced on the air by respiration is the true, suffi- 
cient, and only original source of animal heat. If, 
indeed, it be not to afford that subtile matter which 
it yields by its decomposition in the lungs, we know 
no essential purpose which the air can be considered 
to serve; for it has, we trust, been established (621,), 
by an induction as strict and ample as the nature 
of the case demands, that, in respiration, no part of 
the ponderable matter of the air enters into the blood, 
but that the conversion of its oxygenous portion in- 
to an equal bulk of carbonic acid gas constitutes the 
only necessary change which the air experiences 
during the exercise of this animal function. 
