62 PHYSIOLOGY 



the ureter. The amount of water that is removed from 

 the blood in this way depends on the amount that has 

 been drunk. Normally it is about three pints daily. 



Urea. — Urea (N 2 H 4 CO) is formed by the splitting 

 up of proteids. It therefore contains a large amount of 

 nitrogen. As it is an incompletely oxidized substance, 

 the body does not get so much heat in proportion from 

 the splitting up of proteids as it does from the splitting 

 of fats and starches, which give rise to water and carbon 

 dioxide, completely oxidized substances. 



The Sweat Glands. — The sweat glands are very tiny 

 (Fig. 42). They are scattered over the body in the 

 layer of fat immediately under the skin. The " pores " 

 are the openings of minute ducts that lead from them 

 to the surface of the skin. Their secretion is composed 

 normally of water, salt and the products of the sebace- 

 ous glands; but if the kidneys fail to do their work 

 properly the substances ordinarily secreted by them ap- 

 pear in the perspiration. The amount of the secretion 

 may be a quart or more daily, depending on the activity 

 of the kidneys, on the temperature and on the amount 

 of exercise. 



Hygiene of Digestion. — The hygiene of digestion is 

 possibly more important than that of any other function. 

 Many eminent physiologists and physicians attribute 

 the majority of bodily ills to abuse of the digestive sys- 

 tem. Not only must we have palatable food in sufficient 

 quantities, but it must combine the right proportion of 

 proteids, fats and starches. The thought alone of some- 

 thing that we particularly like causes the mouth to 

 water. Exactly the same thing takes place in the 



