CHAPTER VI 



THE INTEKDEPEKDENCE OF OKGANS AND OF CELLS 

 INTERNAL SECRETIONS 



1. The products of cellular activity not necessarily harm- 

 ful. We have now learned that the active living cells of 

 the body are the seat of chemical changes which produce 

 new substances ; that the accumulation of these products of 

 activity often limits the working power of the cells in which 

 they are produced, and may even depress the activity of 

 other cells to which they are carried by the blood. In the 

 case of the skeletal muscles we have spoken of the carbon 

 dioxide, the sarcolactic acid, etc. as "waste products," mean- 

 ing thereby that they are incapable of serving as sources of 

 power for the work of the muscle; and this term, together 

 with the fact that they constitute one cause of fatigue, is apt 

 to mislead us into supposing that they can be of no further 

 use to the body or, even more, that they are necessarily 

 harmful and that their presence in the blood is objectionable. 



These conclusions, however, do not necessarily follow from 

 the facts. It does not even follow that a substance which 

 produces fatigue for that reason serves no useful purpose. 

 Most adults can recall times when because of long-continued 

 application to mental work or because of worry or other 

 nervous strain they have become overexcitable and restless 

 and have been unable to obtain the sleep of which the body 

 as a whole stands in need. At such times sleep is often 

 best secured by producing general fatigue through muscular 

 work. The waste products, by their very act of fatiguing 

 the overexcited nerve cells, may be of service to the body 



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