MUSCULAR ACTIVITY 105 



esting experiment which stands as one of the classical experiments 

 in Physiology. The proof rests upon the fact that the decomposi- 

 tion of proteins gives rise to compounds which contain nitrogen 

 and which are discharged from the Body, except for a negligible 

 residue, by way of this kidneys. If the urine is collected and its 

 nitrogen content determined, the amount of protein decomposition 

 that has occurred in the Body can be calculated (see table, p. 11). 

 Two German Physiologists, Fick and Wislicenus, determined 

 their average daily loss of nitrogen over a period of several days of 

 relative inactivity and then engaged in a day of exceptionally 

 vigorous muscular exercise. The form of activity chosen was 

 mountain climbing; the mountain ascended was the Faulhorn in 

 the Alps, 1,956 meters (6,000 feet) high. In spite of the very 

 great increase in the amount of muscular energy manifested, and 

 the consequent great increase in the amount of fuel consumed, the 

 total loss of nitrogen from the Body was virtually the same as on 

 the previous days of inactivity. This experiment, which has been 

 repeated and confirmed many times, shows that proteins are not 

 ordinarily used by the muscles as fuel. Therefore the other energy 

 yielding foods must serve. (See also Chap. XXX). 



While fats are excellent fuel foods, and are, in all likelihood, used 

 by the muscles when brought to them by the blood, there is abun- 

 dant evidence that the muscles can get along without fats provided 

 they have enough fuel in the form of sugar. This evidence is 

 found in the experience of grazing animals which may never after 

 weaning have a particle of fat in their food and which, nevertheless, 

 are able to use their muscles to the very best advantage. When 

 dextrose or fats are burned the products of the oxidation are car- 

 bon dioxid and water. The reaction in the case of dextrose is 

 represented by the equation C 6 Hi 2 O 6 +6 O 2 =6 CO 2 +6 H 2 O. 

 That one or the other of the fuel substances mentioned above is ox- 

 idized during muscular activity is shown by an increase in the amount 

 of carbon dioxid produced in the Body and breathed out from the 

 lungs. By a comparatively simple device the amount of carbon 

 dioxid breathed out per minute can be determined, and there is 

 invariably a pronounced increase during and immediately follow- 

 ing muscular exercise. If the exercise is sharp the increase may be 

 7 or 8 fold. There is, of course, an equivalent increase in water 

 production, but the water so produced merely adds itself to the 



