I 3 o PHYSIOLOGY FOR BKGINNERS CHAP. 



acid and water ; animals on the other hand cannot do this, 

 hence we see why animals are dependent for their lives on 

 plants. 



Daily Loss. When a man is neither gaining nor losing 

 weight, the amount of material leaving the body must be equal 

 to the amount entering it during the same time. The body, 

 as we shall see, loses material by the lungs, skin, and kidneys, 

 and it gains material by the lungs and from the food taken into 

 the alimentary canal, about one-tenth of which does not enter 

 the substance of the body at all, but merely passes along the 

 alimentary canal and leaves the body again as faeces. The 

 object of the food is to supply the daily loss. The daily loss 

 is the loss of substances containing chiefly the elements carbon, 

 nitrogen, hydrogen, and oxygen. Let us consider the daily loss 

 and supply of the elements carbon and nitrogen. A man 

 taking only a small amount of exercise, loses in a day about 

 4000 grains of carbon, most of which is lost as carbonic acid 

 by the lungs ; for we saw that about eight ounces (3840 grains) 

 might so be accounted for. Of nitrogen he loses in a day 

 only about 300 grains, which leaves the body chiefly in the 

 urea of the urine. 



Daily Supply ^ixed Diet. The man's daily food 

 must supply the 4000 grains of carbon and the 300 grains 

 of nitrogen, and the carbon and nitrogen should be in the 

 right proportion in his diet, so that he need not have to take 

 in more carbon than he wants in order to get the necessary 

 amount of nitrogen, or more nitrogen than he wants in order 

 to get the necessary carbon. If a man were to live on bread 

 alone he would have to eat more than four pounds daily in 

 order to get 300 grains of nitrogen, for bread contains only a 

 small amount of proteid, in which form alone the nitrogen can 

 be taken up. This amount of bread would contain nearly 9000 

 grains of carbon, so that he would get 4000 to 5000 grains of 

 carbon more than necessary. If a man were to live on lean 

 meat only, which is rich in proteids and therefore in nitrogen, he 

 would have to eat six pounds to get the daily amount of 4000 

 grains of carbon. This amount of lean meat contains about 

 1000 grains of nitrogen, so that there would be a waste of 600 

 to 700 grains of nitrogen. If a man were to live on pure pro- 

 teids only, on albumin, for instance, he would have to eat a 



