OF THE BRAIN, ETC. 33 



It must be owned, that this part of physiology 

 still presents an ample field for investigation. 



Of opinions sanctioned by the names of Black, 

 Laplace, Lavoisier, and Crawford, it is proper to 

 speak with caution 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 explanation 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 general theory of Dr. Black, there is 

 this unanswerable objection, that the tempera- 

 ture of the lungs is not greater than that of the 

 rest of the system. To this objection the in- 

 genious 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 circumstances, there might 



