THE MECHANISM OF LIFE 555 



compensatory source of muscle-energy while carbohydrates and 

 fats were lacking in the food, Pfliiger ('92) carried on a series 

 of experiments with mixed food. These led to the important 

 result that with a food composed of proteid, carbohydrates, 

 and fats, the quantity of the two latter substances that is 

 destroyed in metabolism depends wholly upon the fact whether 

 much or little proteid is fed. " In general, the quantity of carbohy- 

 drate and fat that undergoes destruction is smaller, the greater the 

 income of proteid." The undestroyed carbohydrate and fat are 

 changed into body-fat and accumulate as reserve-material, while, 

 as is well known, the introduced proteid, however much it may be, 

 is destroyed even to an excessively small fraction. It may, there- 

 fore, be said : " the need of food is satisfied first by proteid." Pro- 

 teid is the " primitive food," carbohydrate and fat are simply a 

 " compensatory food " employed during a lack of proteid. 



Although in accord with this it is established beyond doubt that 

 muscular work is made possible primarily by the decomposition of 

 proteid, the equally undeniable fact, that the excretion of nitrogen 

 in the urine does not appear to be correspondingly augmented by 

 the most intense muscular activity, must excite surprise. In this 

 connection another experiment of Pfliiger deserves attention. 

 Pfliiger found that with pure proteid food and with an equal 

 quantity of food during rest and during labour the excretion of 

 nitrogen is increased by muscular activity very inconsiderably, 

 and under certain circumstances not at all. Nevertheless, all the 

 working-power must be derived from the decomposition of 

 proteid alone, since no carbohydrate and fat are fed. With an 

 excess of proteid such a remarkable phenomenon would be directly 

 comprehensible, if we were to bear in mind that even during rest 

 all proteid introduced into the body is decomposed ; for 

 if, as has been shown, the energy of muscular work is derived 

 from the proteid decomposed, it might be concluded that an 

 amount of proteid equal to that consumed during activity 

 has been saved elsewhere in the body. This would be 

 supported by the fact that all proteid eaten beyond a certain 

 quantity is a luxus consumption, and is, therefore, available when- 

 ever needed. But if it be borne in mind that, as Voit ('60, '66) 

 has shown, in hunger the excretion of nitrogen in the urine of the 

 dog is increased by labour in the treadmill either not at all or 

 only inconsiderably, this conclusion cannot be drawn, and the above 

 explanation of the non-increase of nitrogen excretion does not 

 suffice. 



There still remains one possibility, which Pfliiger has only 

 touched upon, namely, that during labour a transformation of 

 proteid takes place in the muscle without the nitrogen of the 

 transformed proteid appearing in the urine. 



This idea, to which we are forced by the facts, although directly 



