Chemical Changes in Animal Organism. 283 



The diseases of beri-beri and scurvy are also stated to be 

 due to some deficiency in the diet. 



Closely connected with the experiments on insufficient 

 diets are those dealing with the foods essential to growing 

 animals. A diet which may suffice for the bodily needs 

 of the adult is not necessarily adequate for those of the 

 adolescent individual, which requires, apparently, certain 

 specific materials for tissue growth. It has been found, 

 for example, that young rats will die long before they 

 reach maturity if their sole diet is composed of a pure fat 

 such as lard, with a carbohydrate such as starch, and a 

 pure protein such as caseinogen, together with the neces- 

 sary inorganic salts. Hopkins has shown that something 

 contained' in the milk, after separation of the proteins 

 and the greater part of the fat, when added to the above 

 diet will make good the deficiency, even when added in 

 very small quantity. Osborne and Mendel have quite 

 recently shown that an essential substance soluble in ether 

 can be obtained from butter, and that cod-liver oil also 

 contains the same or a similar substance which must be 

 added to a pure fat-carbohydrate-protein diet to maintain 

 normal growth. The actual nature of the substance has 

 not yet been ascertained. 



These experiments have been mentioned in this place, 

 although they do not entail necessarily the complete in- 

 vestigation of the total metabolism of an animal. The 

 determinations of the rate of growth alone on varied diets 

 yield sufficiently striking results. 



Examples have been already given of the value of the 

 determinations of the gaseous products (the " respiratory 

 exchanges"). (See p. 276.) 



A few words are now necessary as to the value of 

 urine examinations. In the cruder metabolism experi- 

 ments, it is sufficient to determine the total intake and 



