CHAPTER X. 
ELEMENTARY PHYSIOLOGY — continued. 
Food and digestion. — The principal use of food is to supply 
energy to the body, the muscles of the body being constantly active 
or continually liberating energy by breaking down the complex 
proteins, carbohydrates and fats, and therefore fresh supplies of 
these are necessary to prevent the body living on its own material 
and wasting away. 
Again during growth, the material from which the body is built 
up must be supplied by the food, so that a suitable food must be 
able to yield the necessary amount of energy and also supply the 
materials necessary for growth and repair. 
Before food can be utilised by the body it must be broken down 
to its simplest form and rendered soluble, and the process of 
preparing the food for absorption is called digestion. 
Before considering the process of digestion some idea of the 
composition of food must be given. Foodstuffs are classified into 
those not yielding energy and those that do yield energy — the former 
acting chiefly as solvents, e.g., water and the inorganic salts. The 
foodstuffs yielding energy are complex combinations of the chemical 
elements — carbon, hydrogen and oxygen, with or without nitrogen, 
sulphur, phosphorus and iron — that is to say, they are of the same 
nature as the materials found on the analysis of dead protoplasm. 
The energy-yielding foodstuffs are further subdivided into nitrogen- 
containing compounds and non-nitrogen-containing compounds. 
The proteins belong to the former, and the carbohydrates and fats 
to the latter. The proteins are important in that they are the only 
source of the nitrogen and sulphur necessary for the construction 
and repair of living tissues ; they are therefore the essential organic 
constituents of the food. Proteins leave the body as carbonic acid, 
water and urea. 
The difference between the carbohydrates and fats is that in the 
former the oxygen and hydrogen exist in the same proportion as in 
water ; carbohydrates and fats leave the body as carbonic acid and 
water. 
Digestion, — In the elephant the food is grasped by the trunk 
and placed in the mouth where, by the act of chewing, it is 
thoroughly broken up and mixed with the saliva. 
