The special significance of these data, as bearing upon the topic under discussion, is that apparently all three of the subjects were drawing in a measure upon their body material. As stated by Atwater and Sherman, Pilkington lost per day 5.1 grams of nitrogen; that is to say, the total nitrogen excreted exceeded the total nitrogen of the food by 5.1 grams per day, corresponding to 33 grams of proteid, which must have been drawn from the supply in the body. If we assume that lean flesh contains 25 per cent of proteid, this would mean about 4-3/4 ounces per day. The other two subjects, Miller and Albert, lost from the body per day 8.6 grams and 7.1 grams respectively of nitrogen, which would imply a loss of about 54 grams and 44 grams of body proteid respectively, or 8 ounces and 6-1/4 ounces of lean flesh per day. It is evident, therefore, that none of the three subjects consumed sufficient food to avoid loss of body proteid, under the existing conditions of muscular activity. Indeed, it may be noted in Miller’s case that the average fuel value of the food per day was 4770 calories, while the average expenditure of energy per day was 4820 calories. We should naturally expect, however, that any small deficiency in fuel value would be made good by a call upon body fat. “Why the body should use its own substance under such circumstances is a question which at present cannot be satisfactorily answered. The fact that such was the case, each of the contestants who finished the race consuming during the period body protein equivalent to 2 or 3 pounds of lean flesh, and that no injury resulted therefrom, would seem to indicate that these men had stores of protein which could be metabolized to aid in meeting the demands put upon the body by the severe exertion, without robbing any of the working parts, and at the same time relieving the system of a part of the labor of digestion. Possibly, the ability to carry such a store of available protein is one of the factors which make for physical endurance.”[40] This possibility we shall have occasion to discuss in another connection. At present, the facts presented are to be accepted as accentuating the general law that the energy exchange of the body, everything else being equal, is increased proportionally to increase in the extent of external muscular activity. It may be noted that Albert, who did considerably less work than Miller, showed a much larger exchange of energy than the latter athlete. This, however, is to be connected with the fact that his fuel intake was 1300 calories larger per day than Miller’s; in other words, the conditions were not equal. This fact also calls to mind the observations of Schnyder,[41] who, studying the relationship between muscular activity and the production of carbon dioxide, maintained that the quantity of this excretory product formed depends less upon the amount of work accomplished than upon the intensity of the exertion; efficiency in