Biology of Man 473 



remembered that the mere addition of water to a food will not result in 

 digestion, but the specific action of digestive enzymes is necessary (con- 

 sult table on Summary of Digestion, showing the enzymes and their 

 roles). 



After foods are digested and absorbed they can be made into a part 

 of the living protoplasm. This phenomenon is called assimilation. In 

 all probability assimilation, as well as other phenomena, is affected by 

 certain specific vitamins, some of the better known being listed in a 

 summary to expedite their mastery (Figs. 369 to 372). Vitamins may 

 be considered as essential accessory substances present in variable 

 amounts in different foods. Each vitamin has its unique function in the 

 maintenance of normal body processes (consult summary of vitamins). 

 Unlike other food components, such as carbohydrates, fats, proteins, 

 and minerals, they do not provide energy or build tissues directly. Their 

 function is to enable the body to use other foods properly in addition to 

 performing other very essential functions. Vitamins vary in their chemi- 

 cal composition, their solubility in fat or water, their resistance to heat, 

 and their inactivation by oxygen. The term vitamin, which was coined 

 when vitamins were erroneously thought to be amines (containing an 

 amine, NH2) essential to life (vita), is still used today but may even- 

 tually be supplanted when more is learned about them and their chemi- 

 cal compositions are completely ascertained. Minerals are nutritional 

 substances (inorganic) which, in combination with other food constitu- 

 ents, promote the formation and maintenance of various parts of the 

 body structure. Each mineral not only aids in metabolic processes but 

 actually forms a part of certain body fluids and tissues. Calcium aids 

 in blood clotting and bone formation, while iron enters into the con- 

 struction of the hemoglobin of the red blood corpuscle. 



The human digestive system (Figs. 234 and 236) consists of: (1) 

 The mouth with its numerous taste organs, a muscular tongue, thirty- 

 two teeth (two incisors, one canine, two bicuspids, and three molars in 

 each half of each jaw), and three pairs of salivary glands (the parotid, 

 submaxillary, and sublingual glands) for the secretion of saliva (Fig. 

 236). (2) A tubelike esophagus with two layers of circular and longi- 

 tudinal muscles for the peristaltic movement of foods. (3) The stomach 

 with its three layers of muscles (the circular, oblique, and longitudinal), 

 the anterior cardiac part (nearest the heart) in which foods are stored, 

 and the posterior pyloric part principally for digestion. The stomach 

 secretes gastric juice and mucin. (4) The small intestine for digestion 

 and absorption which is composed of the duodenum (one foot long), the 



