THE SOURCE OF MUSCULAR ENERGY. 381 



ment, the general fact remains the same, that the quantities of carbonic 

 acid and water eliminated during work are much greater than during 

 rest, in many cases the ratio being as high as two to one. It is also 

 found that the oxygen taken up, though increased during muscular 

 exercise, is not increased in proportion to the carbonic acid eliminated. 

 The result is, that the ratio of the volume of oxygen consumed to the 

 volume of carbonic acid eliminated, which is normally somewhat less 

 than unity, tends to approach unity during muscular work. It should 

 be here remarked that investigations dealing with total respired gases, 

 although doubtless in the main reliable, are not without certain de- 

 fects. If we could be certain that muscular exercise left all other 

 organic functions unaffected, we could safely attribute the observed 

 changes to the muscular contraction alone. But such is probably not 

 the case. The functions of organs are influenced by the activity of 

 others, and hence the changes noticed in products of elimination or in 

 the consumption of oxygen can not with safety be attributed solely to 

 the muscular work performed, as these substances are consumed or 

 produced by the combined activity of all the living tissues of the or- 

 ganism. Hence the value of the corroborative testimony of the other 

 methods of investigation noticed above. 



The influence of muscular exertion on the elimination of nitrogen 

 has also received much attention, inasmuch as the nitrogen eliminated 

 (mainly in the form of urea by the kidneys) may be taken as a measure 

 of the amount of nitrogenous food or tissue decomposed in the organ- 

 ism. The influence, then, of muscular exertion on the excretion of 

 nitrogen is of importance as showing also its influence on the decom- 

 position of albuminoids (foods or tissues). The results of the numer- 

 ous investigations on this subject have been somewhat at variance. 

 Many have found no material increase in the elimination of nitrogen 

 during muscular exertion ; others find a slight increase, but not suffi- 

 cient to indicate any immediate relation of the nitrogen eliminated to 

 the work performed. Passing over the work of earlier investigators, 

 we will consider briefly the results of some of the later investigators. 

 Voit was one of the first to make careful and exact experiments ex- 

 tending over a considerable period of time, and he determined that 

 the increase in elimination of nitrogen during muscular exertion is 

 very slight ; that it bears no constant relation to the work done, and is 

 more influenced by diet than by work. Fick and Wislicenus made an 

 ascent of the Faulhorn in the Alps, with the purpose of determining 

 the possibility or impossibility of albuminoids being the fuel-material 

 for muscular power. They estimated the mechanical work necessary 

 to raise their own bodies through the vertical distance to which they 

 ascended. They then calculated the amount of albuminoids necessary 

 to produce so much force by its combustion. They determined experi- 

 mentally the amount of nitrogen in their excreta during the period of 

 the ascent, and, having taken no nitrogenous food during that period, 



