344 HUMAN PHYSIOLOGY 



cent, of the whole body. (The presence of the bones 

 somewhat lowers the average.) These proportions 

 cannot be much altered by the severest drafts upon the 

 water of the organism. When it is reduced, even by 

 a little, the secretions are diminished and thirst becomes 

 imperious. It is plain from these facts that water is 

 necessary to growth; it is as essential to new tissue as 

 protein itself. 



Water is necessary to maintenance as well as to 

 growth because it must* make good the losses from the 

 body. These losses cannot be checked because the 

 urinary excretion must have water as its vehicle and 

 because much of the time water must be evaporated 

 from the skin and the breathing passages to remove an 

 excess of heat. The secretion through the skin falls 

 to a low level when there is no call for it, but there seems 

 to be no way to keep back the water that is taken by the 

 respired air. This is a secretion which is never sup- 

 pressed and, unlike most others, it is one which environ- 

 mental conditions directly determine, the nervous 

 system seeming not to regulate it at all. 



Mineral Matter. There is abundant evidence to 

 show that living protoplasm cannot exist without 

 mineral as well as organic constituents. It may be that 

 while the living state continues these mineral substances 

 salts of sodium, potassium, calcium, etc. are united 

 with the proteins in temporary combinations. In fact, 

 the materials which we call proteins as we handle them 

 in the laboratory are not often " ash-free." This means 

 that they leave mineral residues when they are burned. 

 Many experiments have proved that the thorough re- 

 moval of ash from the food given to animals impairs 

 its nutritive value. 



The need of a mineral ration is naturally more distinct 

 when growth is in progress than it is later. But it is 

 usually stated that it never entirely ceases. The ash 

 of milk has been laboriously analyzed and it has been 

 found to show a marvelous fitness for the purpose it 



