K.ATABOLISM. 4 23 



waste to be disposed of in a similar way — i. e., partly by 

 the lungs and partly by the kidneys. The whole process 

 may be likened to a current in a siphon : the shorter arm 

 is anabolism, the longer arm and the one which de- 

 termines the whole current is katabolism. 



5. We have said that respiration is a process of com- 

 bustion in which oxygen of the air unites with C and H 

 of the blood and produces C0 2 and H 2 0. Now exactly 

 the same takes place in the burning of oil. The material 

 is the same (C and H) ; the process is the same (union 

 with oxygen of the air) ; and the product, chemical and 

 physical, is the same — viz., C0 2 and H 2 and heat. 

 Moreover, it is certain that, estimating the whole force 

 produced in terms of heat, the amount of heat is the 

 same in the two cases. But in the case of the animal 

 body the heat is spread over a larger space and over a 

 longer time, and is therefore less intense. 



6. Another important question is, Where does the 

 combustion take place? The old view was that it takes 

 place in the lungs, and that this organ is the furnace of 

 the animal machine, that circulation brings the fuel and 

 respiration brings the air, and that the fuel is burned 

 at once then and there. It is now known that this is 

 not the fact. The blood brings the products of com- 

 bustion (C0 2 ) to the lungs to be eliminated. Respira- 

 tion brings a fresh supply of oxygen to be taken by the 

 blood to the place of combustion. This place of com- 

 bustion is in the capillaries in contact with the tissues. 

 Yet neither are the decomposing tissues burned at once, 

 but circulate in the blood and undergo changes there 

 before they are finally consumed into C0 2 . These 

 changes are very obscure and little understood. Some 

 of them are spoken of later. 



Thus the blood is a reservoir for many things. It is 

 a reservoir for oxygen, which circulates in it until used. 



