THE SOURCE OF MUSCULAR POWER. 733 



miles in seventy-five consecutive hours ; 3. A walk of four hundred 

 . and fifty miles in six consecutive days. The proportionate excretion 

 of nitrogen was estimated for these periods of exercise, and was also 

 calculated for eight days of rest. The daily average excretion of 

 nitrogen for the eight days of rest was 60.90 parts for every 100 parts 

 of nitrogen of food. The daily average excretion of nitrogen for the 

 eleven days of walking was 87.34 parts for every 100 parts of nitrogen 

 of food an increased proportion of 43.44 per cent. These experi- 

 ments, like those of Prof. Flint, show a very great increase in the pro- 

 portionate excretion of nitrogen produced by the excessive and pro- 

 longed muscular exertion. 



Dr. Pavy admits, as the result of his own experiments, the simple 

 fact that muscular exercise increases the proportionate excretion of 

 nitrogen, but he does not accept the view advanced by Prof. Flint, 

 that the muscular system, in exerting force, consumes its own sub- 

 stance, and that this substance is repaired by food. Dr. Pavy made 

 a series of calculations, in connection with his experiments, comparing 

 the force-value of the excess of nitrogen excreted during exercise over 

 the nitrogen excreted during rest with the work actually performed 

 in walking. He attempted to show that the force represented by this 

 excess of nitrogen excreted would not account for the work accom- 

 plished. These calculations of Dr. Pavy involve formulas for reducing 

 miles walked to foot-pounds, and estimates of the force exerted in 

 respiratory movements, the action of the heart, etc. Prof. Flint, in 

 his essay, gives an elaborate review of these calculations, and objects 

 to many of the formulas as necessarily inaccurate. It is impossible, in 

 a short abstract, to give a satisfactory account of Prof. Flint's argu- 

 ment upon these points. The following are the conclusions arrived at 

 by Prof. Flint, as the result of the various experiments which he has 

 discussed : 



" I. While the various elements of food burned in oxygen out of the body- 

 will produce a definite amount of heat which may be calculated as equivalent to 

 a definite number of foot-pounds of force, the application of this law to the 

 changes which food or certain of the constituents of the body undergo in the 

 living organism is uncertain and unsatisfactory, for the following reasons : 



" (a.) There is no proof that the elements of food undergo the same changes 

 in the living body as when burned in oxygen, or that definite amounts of heat 

 or force are necessarily manifested by their metamorphoses in such a way that 

 they can be accurately measured. 



" (b.) Assuming that the elements of food contain a definite amount of 

 locked-up force, to measure the part of this force which is expended in muscu- 

 lar work, it is indispensable to be able to estimate accurately the force used in 

 circulation, respiration, and the various nutritive processes, and to measure the 

 heat evolved which maintains the standard animal temperature and which com- 

 pensates the heat lost by evaporation from the general surface. It does not 

 seem that any accurate idea can be formed of the amount of force used in circu- 

 lation and respiration, and the estimates made by different observers of authority 



