THE PURIFICATION OF THE BLOOD, ETC. 39 



reasoning to the animal economy that has ever 

 been presented to the world," we cannot say 

 that we feel satisfied with it. In the volumi- 

 nous lungs of tortoises and other cold-blooded 

 animals, the same combustion goes on, but their 

 temperature is little more than that of the 

 surrounding medium in which they live. " It 

 is exceedingly probable," says Dr. Prout, " that 

 though the evolution of carbonic acid gas may 

 be one of the means possessed by the animal 

 economy for generating heat, there are yet 

 other means, the nature of which at present 

 is quite unknown." 



We have already stated that more oxygen is 

 absorbed into the system during respiration 

 than is to be accounted for by the amount of 

 carbonic acid expired, and we have intimated 

 that this unites with carbon received into the 

 blood during its circulation. It is in the capil- 

 lary arteries that this union or combustion 

 takes place, and that carbonic acid is formed. 

 Now there is not a tissue, nor a portion of the 

 body in which capillary arteries are not present 

 in inconceivable multitudes ; these arterial 

 capillary vessels merge into venous capillaries, 

 and 1 ransfer into them Ihe carbonic acid which 

 darkens the venous blood. From these venous 

 capillaries it travels to the right side of the 

 heart, and thence to the lungs, where a new 

 combination with oxygen takes place, and the 

 blood, losing its carbonic acid, returns reno- 

 vated to the left side of the heart. Hence the 

 capillaries of every part of the system are 



