868 PHYSIOLOGY 



disintegrative processes in the tissues, and should be diminished when 

 the nutritive changes in these tissues, especially in the muscles, are 

 reduced to a minimum. The end-products of tissue metabolism there- 

 fore should be increased under the following conditions : 



(1) Increased motor activity involving increased wear and tear of 

 the muscular tissues. 



(2) In fevers, especially in those where there is severe toxaemia and 

 rapid wasting of the muscles of the body. 



On the other hand, it should be diminished where the activity of 

 the muscular tissue is reduced to a minimum, as under the influence 

 of sleep or soporifics, or where the bulk of the muscular tissue is 

 reduced as well as its activity, as in cases of widespread muscular 

 atrophy and paralysis. 



The excretion of creatinine has been investigated under these 

 various conditions by Van Hoogenhuyze and Verploegh, and their 

 results fully bear out the view expressed above as to the intimate 

 relation of creatinine with the tissue metabolism of protein. 



During protein starvation the uric acid output, though diminished, 

 does not show a change which is at all proportional to that shown by 

 the urea. This substance also might therefore represent an end- 

 product of tissue metabolism. Since, however, uric acid is an out- 

 come of the metabolism of a special group of bodies, the nucleins and 

 purine bases, we shall have to devote a complete section to its con- 

 sideration. 



Although the urea is diminished in protein starvation, it still 

 remains the most abundant nitrogenous constituent of the urine. We 

 are therefore not justified in excluding this substance from the pro- 

 ducts of tissue metabolism. Creatine, for example, may undergo 

 complete oxidation in the body, so that during protein starvation a 

 certain proportion of the urea may be derived in this way. We shall 

 see later that uric acid may also undergo further oxidation with the 

 formation of urea. It is possible, however, that even during complete 

 protein starvation some of the urea which is turned out may be the 

 expression of a utilisation of protein through deamination for the 

 energy needs of the body. The active cells are bathed everywhere 

 with a tissue fluid in which protems form a preponderating constituent, 

 and it is possible that, even in the times of greatest protein need, these 

 cells utilise the proteins of their surrounding medium, though in a 

 reduced degree, for the production of energy. In this case the active 

 cell would initiate the utilisation by throwing off that part of the 

 protein molecule, namely, NH 2 , which is useless to the cell as a source 

 of energy, so that deamination would be carried out in the working 

 tissues, and not., as in the rapid formation of urea after a heavy meal, 

 in the tissues of the intestine and liver. 



