502 THE RESPIRATORY EXCHANGE 



traction. Numerous experiments made upon men and animals 

 have shown that work does not increase the discharge of nitro- 

 genous substances in the urine ; even more numerous observations 

 have proved that the respiratory exchange is immediately and 

 markedly increased by exercise, and within limits varies as the 

 work done. The conclusion is, therefore, drawn that muscular 

 energy is obtained at the expense of non-nitrogenous substances. 

 The demand for this energy can be met by food consisting largely 

 of carbohydrates and fats ; proteid, it is true, can be used and 

 often with great advantage, for it is easily digested. The nitro- 

 genous excretion under ordinary conditions varies with and in 

 the same direction as the nitrogenous intake, and the balance 

 of evidence tends to show that proteid food is entirely broken 

 down during digestion ; the non-nitrogenous products of this 

 decomposition can be retained and formed into fat and carbo- 

 hydrate, and in the latter form may be again linked to the proteid 

 of the tissues. 



It is impossible to adequately discuss here the sources of 

 muscular energy, but it may not be amiss to point out that the 

 great increase in the output of carbon dioxide during the per- 

 formance of muscular work shows the necessity of an adequate 

 supply of food. Men and horses work best when they are well 

 fed, and feed best when they are well worked. Work creates a 

 craving for food, and thus assists digestion and absorption. Here 

 is a natural stimulus to the appetite, more potent than any drug ; 

 it is known to some, but many will not be cured of their ailments 

 by muscular work ; they seek some miracle-working drug, or the 

 waters of a fashionable health-resort where they are unconsciously 

 made to take exercise and lead a more natural life. 



These facts are of practical importance, for they show how 

 far-reaching are the effects of exercise, and how necessary a rational 

 training is for soldiers, athletes, and horses. Experience is indeed 

 the result of a series of experiments made upon a large number of 

 individuals, and its teaching cannot be safely ignored. The chief 

 danger lies in dogmatic and rigid systems ; the wide range of 

 physiological variations is known, is even well expressed by the 

 saying, " Ae man's meat's anither man's poison," but it is 

 frequently forgotten. Some men have as great a craving for 

 regulating their own and other people's food and exercise as 

 theologians have for uniformity in belief. Vegetarians, flesh- 



