HEAT AXD LIFE. 403 



Several experimenters have corrected or perfected some points, but 

 the general doctrine has not been shaken by the recognition of its 

 secondary and very subtle difficulties, several of which still puzzle 

 physiologists. It is, indeed, undeniable that the greater part of the 

 reactions which occur in the system, with the production of heat, do 

 bring out, as a result, the exhalation of watery vapor and carbonic 

 acid from the lungs ; but these two gases cannot arise from a direct 

 combustion of hydrogen and carbon, because the system does not 

 contain such substances in a free state. They represent really only 

 the close of a succession of transformations, often distinct from com- 

 bustions, properly so called. On the other hand, these are not the only 

 residue of the chemical operations performed in the vital furnace. 

 Besides the water and carbonic acid thrown off by animals in breath- 

 ing, which are like the smoke of this elaboration of nutrition, they 

 excrete by other channels certain principles which are, as it were, tbe 

 6Corice. Now, these principles of disassimilation, among which should 

 be noted urea, uric acid, creatine, cholesterine, etc., could not be 

 results of pure combustion, and they denote that the circulating cur- 

 rent is the seat of extremely manifold reactions, the laws of which we 

 are only beginning to gain a glimpse of. 



The latest advances of chemistry allow us, indeed, to follow the 

 linked sequence of the gradual transformations of nutritive substances 

 into the cycle of vital operations. It is well, at the outset, to fix ex- 

 actly the seat of these phenomena. They take place in all the points 

 of the system traversed by the capillary vessels. The glands, the 

 muscles, the viscera, in brief, all the organs, are in a state of constant 

 burning thev are every instant receiving oxvaren, which brings about 

 alterations of various kinds in the depth of their substance. In a 

 word, every organ breathes at all its points at once, and breathes in 

 its special way. Certain physiologists of the present day are wrong 

 in localizing the phenomena of breathing in the capillary vessels. 

 They are merely the channels of transfer for oxygen, which, by exos- 

 mosis, penetrates their thin walls, and then effects, by direct contact 

 with the smallest particles of the organized mass, the chemical action 

 which keeps up the fire of life. It is easy to prove this by placing 

 any tissue, lately detached from the body, in an oxygenated medium. 

 We remark in this case an escape of carbonic acid, together with a 

 development of heat, and this possibility of breathing outside the 

 system proves clearly that such act can be accurately compared, as 

 Lavoisier thought, to the combustion of any substance. The only dif- 

 ference is with regard to intensity. While a candle or a bit of wood 

 burns rapidly, with a flame, the combustible materials of organic pulp 

 unite with oxygen in a more slow and quiet manner, less violently 

 and manifestly. 



The blood, which flows and reflows incessantly in the most slender 

 vessels of our bodies, and charges itself full with oxygen' every time 



