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ESSENTIALS OF PHYSIOLOGY. 



and these substances are essential for the building up of the body 

 proteins. In one experiment on a dog, it was found that nitrogenous 

 equilibrium could be maintained when five-sixths of the protein of the 

 diet was replaced by gelatin. In other experiments on animals, it has 

 been found possible to maintain nitrogenous equilibrium for a time on a 

 diet of gelatin to which tyrosine and tryptophane were added. But gelatin 

 alone cannot supply all the amino-acids necessary for the maintenance 

 of animal life. 



BEVERAGES. 



Tea and coffee owe their fragrance to aromatic substances and their 

 stimulating properties to the presence of caffeine or trimethyldi- 

 oxypurine. Cocoa contains about 30 per cent, of fat and 20 per cent, 

 of protein, and is therefore a food. It has also stimulating properties 

 owing to the presence of theobromine, or dimethyldioxypurine. The 

 methylpurines contained in these beverages do not give rise to uric 

 acid in the body, but are excreted unchanged. 



Alcohol undergoes oxidation in the body to a limited extent, and to 

 that extent it acts as a food. Its value as a food is, however, counter- 

 balanced by its action as a poison. If taken in any quantity, it inter- 

 feres first with the higher mechanism for inhibition, later it disturbs 

 the mechanism for muscular co-ordination, and finally it paralyses the 

 whole nervous system. The continued use of alcohol, moreover, leads 

 to degenerative changes in the tissues and organs of the body, and in 

 that way it shortens life. 



THE CONSTRUCTION OF A DIETARY. 



The amount of protein, carbohydrate, fat, salts, and water required 

 daily being known, a dietary can be constructed with the aid of a 

 table showing the composition of food-stuffs, such as that given below. 



