&n the Influence of the Brain , &c. 391 
and respect, but without trespassing on these feelings, I may 
be allowed to say, that it does not appear to me that any of 
the theories hitherto proposed afford a very satisfactory expla- 
nation of the source of animal heat. 
Where so many and such various chemical processes are 
going on, as in the living body, are we justified in selecting 
any one of these for the purpose of explaining the production 
of heat ? 
To the original theory of Dr. Black, there is this unan- 
swerable objection, that the temperature of the lungs is not 
greater than that of the rest of the system. To this objection, 
the ingenious and beautiful theory of Dr. Crawford is not 
open ; but still it is founded on the same basis with that of 
Dr. Black, “ the conversion of oxygen into carbonic acid in 
the lungs,” and hence it appears to be difficult to reconcile 
either of them with the results of the experiments which have 
been related. 
It may perhaps be urged, that as in these experiments the 
secretions had nearly, if not entirely ceased, it is probable that 
the other changes, which take place in the capillary vessels 
had ceased also, and that although the action of the air on the 
blood might have been the same as under ordinary circum- 
stances, there might not have been the same alteration in the 
specific heat of this fluid, as it flowed from the arteries into 
the veins. But, on this supposition, if the theory of Dr. Craw- 
ford be admitted as correct, there must have been a gradual, 
but enormous accumulation of latent heat in the blood, which 
we cannot suppose to have taken place without its nature 
having been entirely altered. If the blood undergoes the 
usual change in the capillary system of the pulmonary, it is 
