136 METABOLISM AND NUTRITION 



proteid had first to be organized in order to be used by the cells, the same would 

 of necessity be true of the nonnitrogenous foodstuffs, of alcohol, etc.; before 

 they could be destroyed they would have to become integral constituents of the 

 living substance. We cannot get away from this consequence; but what direct 

 proof have we for it? 



How much simpler is the other view that the organized tissues do not them- 

 selves break down their molecules not being destroyed during activity, but 

 are relatively stable substances which perform their duties at the expense of the 

 combustible stuffs present in the lymph; that the tissues draw upon the lymph 

 for whatever they require ; that in all probability they take up these stuffs (pro- 

 teid, fat, carbohydrate, etc.), into their own mass, not, however, organizing them 

 into their own substance, but destroying them sooner or later, according to the 

 intensity of their vital activities, as so much fuel. 



In so regarding the living substance as relatively stable, we do not mean 

 to say that, longer periods of time considered, it may not be destroyed 

 and be restored again. Indeed, it will be found expressly stated under the 

 appropriate sections, that certain organized structures, like the blood corpus- 

 cles, epidermal cells of the skin and its appendages, epithelia of the intestine, 

 etc., are all the time breaking down and being lost, and it is more than 

 probable that the other tissues exhibit the same phenomena. 



There is another question, the satisfactory solution of which is of the 

 greatest importance for this conception of metabolism, namely, Why is it that 

 with sufficient nonnitrogenous foodstuffs to cover the calorific requirements 

 of the body, it cannot entirely dispense with proteid in the food ? 



Of course this is partly due to the fact that the living substance is being 

 destroyed to a certain extent, and needs proteid for its restitution. More 

 than this, proteid is used up in the formation of the digestive fluids, in the 

 secretion of milk, etc. Just how great is the quantity necessary to cover these 

 absolute requirements of the body, cannot be stated at this time; but it is 

 considerably less than the quantity which appears to be necessary to maintain 

 the body in a satisfactory state of nutrition. 



Voit answered the question before us by simply saying that the tissues have 

 need of a certain amount of proteid for their own maintenance. But this answer 

 is only another way of stating the facts to be explained. The following appears 

 to throw some light on the question. 



The lymph is the medium in which the cells and the tissues live. It con- 

 tains proteid as one of its necessary constituents. But when proteid is present, 

 it is destroyed by the tissues with the greatest avidity. In starvation the pro- 

 teid of the lymph, therefore, is gradually used up, so that the latter would become 

 unsuitable as a medium for the tissues, if they did not themselves give up some 

 of their proteid to the lymph. This proteid is in its turn destroyed, and a new 

 moiety from the tissues takes its place. Thus it goes on continually; and our 

 question, why proteid is destroyed in the body not only in starvation but even 

 when the supply of nonnitrogenous food is as great as possible, is to be answered 

 through this continual need of a lymph with the same peculiar constituents all 

 the time, and through the peculiar preference of the cells for proteid before all 

 the other organic foodstuffs. Such an explanation does not require us to sup- 

 pose that the tissues or the organized molecules themselves must break down 

 with every manifestation of life. From the same point of view we can explain 

 also the phenomena attending deprivation of salt (cf. page 131). 



