THE METABOLIC MACHINERY OF ANIMALS 275 



in order to transport or to utilize this stored material must first 

 break it down into simpler soluble compounds so that it may pass to 

 the cells of the organism where it is utilized. A somewhat similar 

 situation occurs in animals since complex protoplasmic material of 

 animal or vegetable nature is taken in by the organism, broken down 

 into simpler units, and then utilized or stored in the cells of the 

 body during the normal processes of metabolism. This breakdown 

 of foods is known as digestion, the intricacies of which make a fas- 

 cinating study. 



There are a number of important and interesting problems which 

 present themselves at this point. The most important problems 

 involved are : What is food and how is it digested ? How is it 

 disposed of after absorption? How is energy released? How are 

 waste products removed? Briefly, they center around questions 

 which we should answer, for it is both interesting and profitable 

 to understand something of the human machine. Consequently, 

 although other animals are mentioned, the fact should not be lost 

 sight of that we have a selfish interest and are anxious to know about 

 ourselves. The answers to these stimulating questions will be found 

 in the discussions that follow. 



SECTION A. INTAKE DEVICES AND HOW THEY 



FUNCTION 



Foods and Their Uses 



Any substance taken into the body that can be utilized for the 

 release of energy, for the regulation of body processes, or for the 

 building and repair of tissues falls into the category of food. If this 

 broad definition of food is adopted, then water, inorganic salts, vita- 

 mins, carbohydrates, proteins, and fats should be included. Food 

 substances may be further subdivided into those capable of releasing 

 the latent or potential energy bound up within the molecule and those 

 which, though non-energy producers, are still essential to life. Energy 

 which is so essential to the metabolism of an organism is largely 

 secured through the breakdown of a complex series of molecules into 

 simpler ones. Non-energy producers are equally as essential to the 

 well-being of the organism since water and inorganic salts, for ex- 

 ample, are necessary for the maintenance of the normal composition 

 of tissue. 



