200 ELEMENTS OF PHYSIOLOGY 



I ounce of salts and \\ ounces of urea; the lungs, 25 

 ounces of carbon dioxid gas. 



352. Variation in the Amount of Food. Suppose the 

 man in the little room on the scales were to work, for in- 

 stance, turn a hand mill or swing dumb-bells, would the 

 amount of waste be increased? Would the increase 'be 

 chiefly urea (from proteid) or carbon dioxid (from carbo- 

 naceous food)? If it were to turn very cold, the loss of 

 heat from his body would be greatly increased, and the 

 waste correspondingly increased. If he were to think 

 very hard, the increase would be marked. If he were to 

 remain perfectly quiet in body and calm in mind, the waste 

 would be decreased ; likewise, if the temperature became 

 mild. The effect upon the waste would not always be 

 perceptible until some hours after its cause, for it takes 

 time for the circulation and organs of excretion to remove 

 the waste after the material of the waste has been formed. 



353. The Most Important Class of foods is the proteids. 

 They are substances consisting of nitrogen, carbon, hy- 

 drogen, oxygen, sulphur, and phosphorus. As sugar and 

 starches, fats and oils, contain no nitrogen, the tissues can- 

 not be built from them alone. Proteids not only build 

 tissue, but are a source of some heat and energy, so life 

 could be sustained on this class of foods alone; but it 

 would not be the best and healthiest way of living, because 

 in order to get enough carbon, there would be taken in an 

 excess of nitrogen, and the digestive organs would be 

 given extra work which would be of no value to the body. 



354. Proportions of Nitrogenous and Carbonaceous Food 

 needed. The carbon dioxid gas which is daily lost by the 

 body contains 8 ounces of carbon, while the urea which is 

 lost contains a little more than \ ounce of nitrogen. The 

 body thus loses carbon and nitrogen in the ratio of 

 about i to 15. The food, of course, must supply these 

 elements in the same proportion. If one foodstuff be 



