82 



ELEMENTARY PHYSIOLOGY 



By means of the skin we part with water, a small quantity ol 

 mineral salts, and a little carbonic acid gas. 



The kidneys are the source of a considerable loss of water, 

 and a very large quantity of urea. 



The loss by the skin varies very considerably. The exu- 

 dation through its pores is commonly known as perspiration or 

 sweat. In hot weather, or during violent exercise, we perspire 

 very freely ; so much so that the skin becomes covered with 

 globules of water, containing carbonic acid gas and urea in 

 solution. But in cool weather, and during rest or the taking of 

 gentle exercise, the liquid exuded evaporates into the air as fast 

 as it reaches the surface, and consequently the perspiration is 

 insensible. 



From the above we learn how the four chief elements of the 

 body oxygen, carbon, hydrogen, and nitrogen, are lost. The 

 loss of hydrogen and oxygen in the form of water is very variable, 

 and is replaced largely by the water contained in our foods and 

 drinks. The loss of carbon and nitrogen is repaired by the use oi 

 organic food-substances, and amounts to an average of 4,500 grains 

 of carbon and 300 grains of nitrogen daily in an adult person, 

 taking moderate exercise. 



The high temperature of the tody (98-4 F.) is produced 

 and maintained by chemical action. This action is chiefly 

 oxidation, very closely resembling that which takes place when a 

 candle burns (see page 78). The materials to be oxidised are 

 supplied through our foods. A constant supply of oxygen gas is 

 also obtained by the blood as it circulates through the lungs ; and 

 we may regard the carbonic acid gas given off as the product of 

 the continuous oxidation of carbon in the body. 



If we cease to take food for any considerable time the body 

 gradually wastes away, and in a few days a marked change is pro- 

 duced in the general appearance. The discharge of waste matter 

 by means of the lungs, skin, and kidneys still continues, and every 

 muscular exercise adds to it. As no food is being taken, the 

 temperature of the body is maintained by the oxidation and 

 consequent wasting away of the tissues. After a few days the 

 temperature decreases, till, at about the ninth or tenth day, the 

 body has lost about forty per cent, of its weight, and is reduced 

 to a temperature of about 70 F, when death from starvation 

 puts an end to the painful existence. 



Before considering the manner in which the waste of the body 

 may be most advantageously repaired, it will be necessary to learn 

 the nature of the various food-substances of which we make 

 use. 



Food-substances may be divided into organic and inorganic. 



