286 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



structive forces prevail and the struggle terminates in 

 death. 



3. Or, again, the living body may be compared to a 

 pool, with its inlet and outlet. The form remains the 

 same, but the matter is continually changing. The more 

 rapid the change, the quicker and fresher is the water. 



Waste Removal. — It is seen, then, that one necessity 

 arising from waste is food. But there is another and 

 far more urgent necessity — viz., the quick removal of 

 waste from the body. The reason is that the waste is 

 poisonous to the blood. Food-taking may be delayed for 

 a day or several days, or even perhaps for forty days ; 

 but waste removal, suspended for five to ten minutes, 

 destroys life. 



For the removal of waste there are two main pipes — 

 viz., the lungs and the kidneys. In the one case the re- 

 moval is by combustion, in the other by solution. In the 

 one the final product is gaseous, in the other liquid. 

 By far the largest amount — seven eighths of the whole* 

 — is removed by the lungs, and the urgency of this re- 

 moval is also the greatest. Stop the removal by the 

 lungs, and death occurs in five minutes; stop the elimi- 

 nation by the kidneys, and death occurs by blood 

 poisoning in about forty-eight hours. 



Thus, to summarize, there are going on continually 

 in the living body two opposite processes — the one gath- 

 ering and constructive, the other destructive and removing ; 

 the one ascensive from food to tissue, the other descensive 

 from tissue to waste removed. The one is called anabo- 

 lism, the other katabolism. The one is nutrition proper, 

 the other is decomposition and elimination. The latter — 

 i.e., katabolism — is that which is most closely connected 

 with life; it is that which starts the whole process, that 



* Berthelot, Rev. Sci., viii, 134, 1S97. 



