EXCRETION". 341 



CHAPTER XVII. 



EXCRETION. 



WE have now come to the last division of the great nutritive 

 function, viz., the process of excretion. In order to understand fairly 

 the nature of this process we must remember that all the component 

 parts of a living organism are necessarily in a state of constant 

 change. It is one of the essential conditions of their existence and 

 activity that they should go through with this incessant transforma- 

 tion and renewal of their component substances. Every living 

 animal and vegetable, therefore, constantly absorbs certain materials 

 from the exterior, which are modified and assimilated by the pro- 

 cess of nutrition, and converted into the natural ingredients of the 

 organized tissues. But at the same time with this incessant growth 

 and supply, there goes on in the same tissues an equally incessant 

 process of waste and decomposition. For though the elements of 

 the food are absorbed by the tissues, and converted into musculine, 

 osteine, hasmatine and the like, they do not remain permanently in 

 this condition, but almost immediately begin to pass over, by a con- 

 tinuance of the alterative process, into new forms and combinations, 

 which are destined to be expelled from the body, as others continue 

 to be absorbed. Thus Spallanzani and Edwards found that every 

 organized tissue not only absorbs oxygen from the atmosphere 

 and fixes it in its own substance; but at the same time exhales 

 carbonic acid, which has been produced by internal metamorphosis. 

 This process, by which the ingredients of the organic tissues, al- 

 ready formed, are decomposed and converted into new substances, 

 is called the process of Destructive Assimilation. 



Accordingly we find that certain substances are constantly mak- 

 ing their appearance in the tissues and fluids of the body, which 

 did not exist there originally, and which have not been introduced 

 with the food, but which have been produced by the process of in- 

 ternal metamorphosis. These substances represent the waste, or 

 physiological detritus of the animal organism. They are the forms 



