734 THE POPULAR SCIENCE MONTHLY, 



present variations sometimes of more than one hundred per cent. Such esti- 

 mates are usually made in view of some dynamic theory, and they are based 

 upon physiological data which are necessarily uncertain and subject to wide and 

 frequent variations. No approximate estimate, even, can be made of the actual 

 amount of heat produced within the living organism, except, perhaps, during a 

 condition of nearly absolute muscular repose. The only way in which tbis could 

 be done would be to deduct the force used in muscular work, circulation, respi- 

 ration, and the nutritive processes, from the heat- value or force-value of the food. 

 These elements of the question being uncertain, an accurate estimate of the heat 

 produced becomes impossible, as, at the best, the only definite quantity in the 

 problem is the total heat-value or force- value of food. 



"(c.) To compare an amount of muscular work actually performed with the 

 estimated force-value of food, apart from the impossibility of arriving at an 

 accurate estimate of the amount of food consumed in circulation, respiration, 

 the nutritive processes, and the production of heat, which is a necessary element 

 in the problem, the work actually performed in walking a certain distance must 

 be reduced to foot-pounds or foot-tons. The formula for this is so uncertain that 

 no such reduction can be made which can be assumed to be even approximatively 

 correct. 



"II. The method of calculating the possible amount of force of which the 

 body is capable, by using as the sole basis for this calculation the force-value 

 of food, must be abandoned until the various necessary elements of the problem 

 can be made sufficiently accurate to accord with the results of experiments upon 

 the living body. Until that time arrives, physiologists should rely upon the 

 positive results obtained by experiments rather than upon calculations made 

 from uncertain data and under the influence of special theories. In case of fatal 

 disagreement between any theory and definite experimental facts, the theory 

 must be abandoned, provided the facts be incontestable. 



" III. Experiments show that the estimated force-value of food, after deduct- 

 ing the estimated force used in circulation, respiration, the nutritive processes, 

 and in the production of heat, will sometimes account for a small fraction only 

 of muscular work actually performed, this work being reduced to foot-tons by 

 the uncertain process to which I have already alluded. The errors in these 

 calculations are manifestly so considerable that the calculated results seem to be 

 of little value, while the experimental fact that a certain amount of work has 

 been accomplished must remain. 



" IV. It must be admitted that, under ordinary and normal conditions of 

 diet and muscular exercise, the weight of the body being uniform, the ingress 

 and egress of matter necessarily balance each other. If this balance be disturbed 

 by diminishing the supply of food below the requirements of the system for its 

 nutrition and for muscular work, the body necessarily loses weight, a certain 

 portion of its constituent parts being consumed and not repaired. If the balance 

 be disturbed by increasing the muscular work to the maximum of endurance and 

 beyond the possibility of complete repair by food, the body loses weight. The 

 probable source of muscular power may be most easily and satisfactorily studied 

 by disturbing the balance between consumption and repair by increasing the 

 work. In this, it is rational to assume that the processes of physiological wear 

 of the tissues are not modified in kind, but simply in degree of activity. 



"V. Experiments show that excessive and prolonged muscular exercise may 

 increase the waste or wear of certain of the constituents of the body to such a 

 degree that this wear is not repaired by food. Under these conditions, there is 

 an increased discharge of nitrogen, particularly in the urine. This waste of 



