NUTRITION AND THE CHANGES IN THE TISSUES. 367 



in his furnace to heat the building. According to this view 

 the proteids or albumens of the blood are looked upon as 

 the source of the new tissues, while the remaining albu- 

 mens not so needed, and the fats and carbohydrates are 

 used for direct oxidation purposes only. 



Acknowledging that we are not yet able to answer 

 definitely these intricate questions, there seems a good 

 deal of probability that this view is not entirely correct. 

 Repeated and careful experiments seem to lead to the con- 

 clusion that under normal circumstances all of the foods are 

 first built into living tissue and then oxidized. The marked 

 exception to this is in the case of excessive proteids taken 

 into the body, which since they cannot be stored and must 

 be eliminated, must be burned in the liver in the manner 

 indicated in the preceding chapter. Here, of course, is a 

 clear instance of a food directly oxidized by the living cells 

 of the liver without ever becoming an integral part of that 

 tissue. But it would hardly be right to look upon this ex- 

 cess of proteid as a food. Its very disintegration argues 

 that it was not intended as a food but was eliminated as an 

 injurious ingredient. The normal amount of proteids, how- 

 ever, of the blood, as well as the sugars and the fats, and 

 of course the oxygen, must all be looked upon as tissue 

 formers. But with the exception of certain special tissues in 

 the body, such as the supporting tissues, all are essentially 

 albuminous in their nature, and it seems at first difficult to 

 understand how the fats and the sugars, which are not at 

 all albuminous, containing no nitrogen whatever, could 

 possibly be built into living tissues which are albuminous, 

 that is, contain nitrogen. 



In the case of the proteids this difficulty is absent. We 

 must imagine a peculiar constructive chemical process 

 going on in the living cell by means of which the proteid 

 from the lymph, possibly with some salts, is combined with 

 the oxygen and built up into some highly complex sub- 

 stance which we call protoplasm. Just as in the manu- 

 facture of gunpowder the charcoal and the oxygen contained 



