472 Animal Biology 



Muscles play an Important role in the movements of many organs as 

 will be observed by a study of such systems as the digestive (Fig. 236), 

 respiratory (Figs. 234 and 244), circulatory (Figs. 237 to 241), excre- 

 tory (Fig. 245), reproductive (Figs. 254 and 255), and special sense 

 organs (Fig. 249). Certain movements are caused by cilia which line 

 various areas, such as those of the nose and reproductive tubes. 



IV. FOODS AND NUTRITION 



A food may be defined as any substance which when ingested in the 

 proper amount is absorbed from the digestive tract and contributes to 

 the normal maintenance of the body. Foods are composed of such or- 

 ganic compounds as carbohydrates, fats, proteins, and vitamins and 

 such inorganic substances as water and various inorganic salts. These 

 food constituents are considered in detail elsewhere and the reader is 

 referred to them. Of all the thousands of organic and inorganic sub- 

 stances, only a few serve as satisfactory human foods, probably because 

 of man's limited number of enzymes by means of which he can digest 

 them. Any animal in utilizing a substance for food purposes must get 

 that substance into a condition which can be absorbed through the small 

 openings in the semipermeable membranes which enclose the absorptive 

 cells of the digestive tract. These openings may be of the size to permit 

 the absorption of water molecules (two atoms of hydrogen and one 

 atom of oxygen) but too small to permit the absorption of other foods 

 the molecules of which are larger. Consequently, certain foods whose 

 molecules are large (because of a great number of atoms which form 

 these molecules) must be changed (digested) before they can be ab- 

 sorbed. For example, the molecule of sucrose (cane sugar) (C10H22O11) 

 must be digested to form two simpler monosaccharide molecules before 

 it can be absorbed, while the simple molecule of glucose (CeHijOe) is 

 absorbed unchanged. Digestion is primarily a chemical process whereby 

 the molecules of foods which are too large to be absorbed are changed so 

 that they may be. This phenomenon is accomplished by hydrolysis 

 which means "a change by the action of water." Hydrolysis is based on 

 enzyme action in which water is added to the complex molecular ar- 

 rangements of foods, thus disassociating the complex molecules into 

 simpler, absorbable ones. The summary of the digestion of foods in 

 man is given so that the stages can be easily memorized. For example, 

 when one molecule of water (H2O) is added to a molecule of sucrose 

 (cane sugar) (G12H22O11), the hydrolytic action results in two separate, 

 absorbable molecules, each with the formula of (C6H12O6). It must be 



