FOOD 199 



The proteids give rise to urea. This is excreted by the kid- 

 neys, and when it is separated from the kidney secretion by 

 chemical means, it appears as a white, crystalline powder. 

 There are also found in the secretion of the kidneys small 

 quantities of phosphate and sulphate of lime, sodium, and 

 potassium. Urea is a substance of great importance, as it 

 is the chief waste substance produced by the process of 

 decay and growth in the cells of the body. Carbon dioxid 

 leaves the body through the lungs, and water leaves it 

 through the lungs, kidneys, and skin. 



351. How the Waste is measured. If a man were to 

 live all day in a small chamber placed upon very deli- 

 cate platform scales, he would find that he lost weight 

 every second of his existence except when taking food, 

 and more rapidly at some times than at others, the amount 

 of loss depending upon the activity of his body. The 

 loss of weight occurs through the excretions which the 

 man gives off. At average temperature of the air and 

 average activity of his body, the day's loss to be replaced 

 by food eaten, water drunk, and air breathed, would be 

 about as follows : 



From the large intestine . 5 oz. excrement. 



From the skin 25 oz. perspiration. 



From the kidneys ... 50 oz. excretion. 



From the lungs . . . . 35 oz. carbon dioxid and water. 



Total . . . . 115 oz. 



The total loss is, then, nearly 8 pounds, three fourths of 

 which is water (nearly 6 pounds). The remainder, except 

 the 5 ounces of excrement, consists of those waste materials 

 (solid or gaseous) which result from the breaking down of 

 the active living protoplasm into simple chemical sub- 

 stances, through the process of oxidation. This amounts to 

 2J\ ounces and is apportioned as follows : perspiration, \ 

 ounce of salts, and a trace of urea ; the kidney secretion, 



