THE SOURCE OF MUSCULAR POWER. 735 



tissue may be repaired if food can be assimilated in sufficient quantity, but in my 

 experiments it was not repaired. The most important question to determine 

 experimentally in this connection is with regard to the influence of excessive 

 and prolonged muscular exercise upon the excretion of nitrogen. It is shown 

 experimentally that such exercise always increases the excretion of nitrogen to 

 a very marked degree, under normal conditions of alimentation ; but the pro- 

 portionate quantity to the nitrogen of food is great when the nitrogen of food 

 remains the same as at rest, and is not so great, naturally, when the nitrogen of 

 food is increased. In the latter case, the excessive waste of the tissues is in part, 

 or it may be wholly, repaired by the increased quantity of food. Experiments 

 upon excessive exertion with a non-nitrogenous diet are made under conditions 

 of the system that are not physiological ; and the want of nitrogen in the food 

 in such observations satisfactorily accounts for the diminished excretion of 

 nitrogen. 



"VI. By systematic' exercise of the general muscular system or of particular 

 muscles, with proper intervals of repose for repair and growth, muscles may be 

 developed in size, hardness, power, and endurance. The only reasonable theory 

 that can be offered in explanation of this process is the following : While exer- 

 cise increases the activity of disassimilation of the muscular substance, a neces- 

 sary accompaniment of this is an increased activity in the circulation in the mus- 

 cles, for the purpose of removing the products of their physiological wear. This 

 increased activity of the circulation is attended with an increased activity of the 

 nutritive processes, provided the supply of nutriment be sufficient, and provided 

 also, that the exercise be succeeded by proper periods of rest. It is in this way 

 only that we can comprehend the process of development of muscles by training ; 

 the conditions in training being exercise, rest following the exercise, and appro- 

 priate alimentation, the food furnishing nitrogenized matters to supply the waste 

 of the nitrogenized parts of the tissues. This theory involves the idea that mus- 

 cular work consumes a certain part of the muscular substance, which is repaired 

 by food. The theory that the muscles simply transform the elements of food 

 into force directly, these elements not becoming at any time a part of the muscu- 

 lar substance, is not in accordance with the facts known with regard to training. 



" VII. All that is known with regard to the nutrition and disassimilation of 

 muscles during ordinary or extraordinary work teaches that such work is always 

 attended with destruction of muscular substance, which may not be completely 

 repaired by food, according to the amount of work performed and the quantity 

 and kind of alimentation. 



" VIII. In my experiments upon a man walking three hundred and seventeen 

 and one-half miles in five consecutive days, who at the beginning of the five 

 days had no superfluous fat, the loss of weight was actually 3.45 pounds, while 

 the total amount of nitrogen discharged from the body in excess of the nitrogen 

 of food taken for these five days, assuming that three parts of nitrogen repre- 

 sent one hundred parts of muscular substance, as has been shown by analysis to 

 be the fact, represented 3.037 pounds of muscular substance. This close corre- 

 spondence between the actual loss of weight and the loss that should have oc- 

 curred, as deduced from a calculation of the nitrogen discharged in excess of 

 the nitrogen of food, seems to show very clearly that, during these five days of 

 excessive muscular work, a certain amount of muscular substance was consumed 

 which had not been repaired, and that this loss could be calculated with reason- 

 able accuracy from the excess of nitrogen excreted. 



"IX. Finally, experiments upon the human subject show that the direct 

 source of muscular power is to be looked for in the muscular system itself. The 



