326 VETEKINAEY PHYSIOLOGY 



The quality of the sound depends upon the overtones which 

 are made prominent by resonance in the pharynx, nose, and 

 mouth. By varying the shape and size of these cavities, and 

 more especially of the mouth, the quality of sound may be 

 considerably altered. 



In animals the voice varies with the age and sex, and it may 

 be modified by castration. 



II. FOOD AND DIGESTION 

 I. FOOD 



The great use of food is to supply energy to the body. The. 

 muscles of the body are constantly active ; they are constantly 

 liberating energy by breaking down the complex molecules 

 of proteins, fats, and carbohydrates, and hence a constant 

 fresh supply of such material is necessary to prevent the body 

 living on its own material and wasting away (see p. 69). 

 During the growth of the body, too, the material from which 

 it is to be built up, and the energy used in its construction, 

 must be supplied by the food. 



Hence a suitable food is one which will yield the necessary 

 amount of energy, and will supply the materials necessary 

 for repair and for growth. 



But food must also supply the water and salts required to 

 keep the various constituents of the body in solution, so that 

 the essential chemical changes may go on. 



Nature of Food 



Food may be divided into foods not yielding energy and 

 foods yielding energy. 



A. Food-stuffs not yielding Energy. 1. Water is the chief 

 constituent. Since it is daily given off in large quantities by 

 the kidneys, lungs, skin, and bowels, it must daily be supplied 

 in sufficient amounts or the chemical changes cannot go on 

 and death supervenes. 



