CHAP, v.] NUTRITION. 631 



in the metabolic stream, and their presence is in some way 

 essential to the various metabolic processes ; hence they need 

 to be always present in daily food. In what way it is that 

 they thus direct metabolism we do not know; we are aware 

 that the properties and reactions of various proteid substances 

 are closely dependent on the presence of certain salts ; but 

 beyond this we know very little. The inorganic salts are 

 those, the nutritive value of which has been chiefly studied by 

 experiment; but we have reason to believe that the organic 

 salts, or extractives, which are present in greater or less quan- 

 tity in all food of both vegetable and animal origin, are no less 

 essential to the proper metabolic activities of the body. The 

 undoubted connection of scurvy with the lack of fresh vegeta- 

 ble food, other conditions helping, may perhaps turn in part 

 on this, for the evidence that the disease is due to the defi- 

 ciency of potash alone is not conclusive. 



Lastly, water has an effect on metabolism, as shewn, among 

 other things, by the fact that when the water of a diet is 

 increased, the urea is increased to an extent beyond that which 

 can be explained by the increase of fluid increasing the facilities 

 of mere excretion. 



