144 THE BODY AT WORK 



stitute its " nitrogenous moiety." The hypothesis that the 

 metabolic capacity of the body is limited to analytical pro- 

 cesses justified the supposition that, when more fat is laid on 

 than the food contains, the balance comes from proteid sub- 

 stances, which split into nitrogenous and fatty moieties. It 

 has been shown, however, that an animal during fattening 

 may put on more fat than is contained as such in the food, or 

 obtainable from its diet, even though all the atoms of carbon 

 and hydrogen in its proteid food were devoted to its forma- 

 tion. The balance must come from carbohydrates. Perhaps 

 a still more striking illustration of constructive capacity is 

 the power of making glycerin. If a dog receive fatty acids in 

 its diet, it accumulates normal fats. The glycerin which, united 

 with fatty acids, constitutes the fat, was not contained in its 

 food. Starch and sugar are sources of fat. As yet there is no 

 evidence that fat can be converted into sugar. 



The chemistry of the nitrogen-containing compounds appears 

 to present more difficult problems. Plants build up proteins. 

 Is the animal's relation to these substances limited to their 

 disintegration ? Do proteins inevitably descend from step to 

 step until they reach urea ? There are reasons for think- 

 ing that, even when dealing with nitrogenous substances, 

 the metabolic power of the body is not exclusively ana- 

 lytical. The liver can make urea from ammonia-salts, such 

 as lactate, or even carbonate, of ammonia substances more 

 stable, and therefore in the chemical sense simpler, than urea. 

 This is an indication, though a faint one, that the body has a 

 constructive capacity, a power of producing more complex 

 from simpler substances, even in the case of nitrogenous 

 compounds. Beef -tea, mutton broth, meat-extracts have long 

 been regarded as foods of value when the power of assimilation 

 is low. Chemists point out that the nitrogenous substances 

 which these decoctions contain are so near the bottom of the 

 ladder that the energy set free by their further oxidation to urea 

 is scarcely worth consideration. They admit that their ready 

 availability renders them useful as restoratives, but they deny 

 them the status of foods, on the assumption that their further 

 progress must be downward. As was stated when the conversion 

 of peptones into leucin and tyrosin was described, evidence is be- 

 ginning to accumulate which shows that within certain limits, at 



