CHAPTER VI 



THE EXCRETORY SYSTEM 



WE have considered the circulatory, the respiratory, 

 and the digestive systems, with the various chemic 

 substances that are required to maintain the life, growth, 

 and activity of the body. These substances, entering 

 the current of the blood, are carried to all organs and tis- 

 sues, and are incessantly combining with the chemic sub- 

 stances of which these tissues are composed. These com- 

 binations are not dependent upon chance; each tissue has 

 a special affinity for the chemic substance in the blood 

 that it requires for its own growth and special form of 

 activity ; for example, the secretory cell of the liver selects 

 substances from which it can elaborate bile and glycogen; 

 the muscle-fiber assimilates those that will promote the 

 changes upon which the power of contractility depends. 



We know that the proteid compounds contain the most 

 essential elements for the formation of all forms of tissue, 

 and that phosphate of lime is a necessary factor in the 

 hardening of bone, but we are utterly ignorant of the proc- 

 ess by which each tissue element is enabled to select the 

 particular substance it needs and to reject that which it 

 does not require. 



Metabolism. Our bodies are masses of changing 

 atoms, some of which, we may say, are on the "up grade," 

 to construct the various tissues, and some on the "down 

 grade," to form the waste matters that are the final prod- 

 ucts of the tissues' activity. These changes, which are 

 going on incessantly while life lasts, are described under 

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