490 HEALTH AND DISEASE 



by which such an animal as the horse is able to accomplish its wonderful 

 feats of locomotion, often under the disadvantage of having to drag a 

 weighty load or to carry a heavy rider, something must be oxidized. At 

 first sight it might reasonably be supposed that this something is the 

 substance of the muscle itself, and this was long supposed to be the true 

 explanation; but if this were the case, then nitrogen, which is an essential 

 constituent of muscle proteid, ought to appear in the excreta after exertion 

 in augmented quantity, either free or combined, as one of the products of 

 the disintegration of that tissue. Careful chemical research, however, 

 made both upon man and animals shows that exercise, even when violent 

 and prolonged, does not materially increase the discharge of urea from 

 the kidney, which is the chief channel by which nitrogen leaves the body. 

 Hence the conclusion has been arrived at, that just as locomotion is effected 

 in a steam-engine, with little wear-and-tear of the machine itself, the force 

 being derived from the latent energy stored up in the fuel, so in the case of 

 muscle the tissue itself is only the machine which utilizes the force set free 

 by the combustion of some organic substance within it in which carbon is 

 predominant. This substance, there is reason to believe, is glycogen, which 

 is present in muscle at rest, but disappears after exercise, a substance that 

 is in constant course of formation by the liver, and is stored up in the cells 

 of that organ until it is again used up either by long fasting or after pro- 

 longed muscular exertion, and which, lastly, is a compound containing much 

 carbon, whilst the oxygen and hydrogen in its composition are in the pro- 

 portions to form water. During exercise more blood traverses the muscles, 

 and therefore more oxygen is brought to them, the glycogen they contain 

 is oxidized, carbon dioxide is formed, and water set free. The carbon 

 dioxide is carried away by the blood, and is discharged partly by the 

 lungs and partly by the skin; the water passes off by the kidneys. But 

 other circumstances besides exercise affect the production of carbon dioxide. 

 The quantity thrown off by the body is always increased after food. It is 

 increased when the temperature of the surrounding medium is lowered, for 

 since the temperature of the body is nearly 100 F., the oxidation of oil, of 

 glycogen, and of proteids is necessary in order that it should be maintained. 

 When, therefore, the external temperature falls, more of these substances 

 must be ingested and oxidized to develop the required heat. The culmi- 

 nation of this is seen in the Esquimaux, who keep up their temperature 

 during the severity of an Arctic winter by consuming large quantities of 

 oily substances, the oxidation of which produces more heat in proportion to 

 Ibs. ;n weight than any other kinds of food. 



In warm climates the amount of heat required to maintain that of the 

 body is comparatively small; hence the propriety of reducing the quantity 



