52 PHYSIOLOGY 



cell-walls. As water is a liquid it passes through easily 

 and carries with it everything it holds in solution. In 

 its natural state water always contains substances in 

 solution which are good for the body. We should there- 

 fore drink large quantities of it, but we should be careful 

 that it is pure and uncontaminated by disease germs. 



Salts. — Salts are contained in the water that we drii 

 and in almost everything that we eat. Table salt, 

 sodium chloride, is the most important of these, for it 

 constantly lost from the body through perspiration an< 

 other excretions, and it is necessary to the compositioi 

 of the blood and the maintenance of the right propor- 

 tion of water in the tissues. Animals that live on grain* 

 poor in salt often travel many miles in order to reach 

 salt licks. As salts are so readily dissolved in water, 

 they undergo no change before entering the cells of the 

 body. 



Carbohydrates. — Carbohydrates include cellulose, 

 starch and sugar. They are manufactured by plants 

 from water and carbon dioxide. They are therefore 

 found in varying amounts in most plant tissues. As 

 animals eat the plant tissues they are also found in the 

 bodies of animals. They consist of carbon, hydrogen 

 and oxygen in such proportions that there is always 

 twice as much hydrogen as oxygen. They are called 

 carbohydrates, a name that means carbon watered, 

 because two parts of hydrogen and one of oxygen form 

 water. 



Hydrocarbons. — Hydrocarbons include all fats 

 and oils. They are found in the tissues of both plants 

 and animals. They contain carbon, hydrogen and oxy- 



