GENERAL ZOOLOGY 



consists of simple squamous epithelium and functions as a covering mem- 

 brane. Both longitudinal and circular muscle layers are of non-striated con- 

 tractile tissue bound together by fibrous connective tissue, and their contractions 

 produce the muscular movements that mix the food contents of the small 

 intestine and push them along toward the large intestine. Fibrous connective 

 tissue, containing both collagenous and elastic fibers, is the distinguishing 

 tissue of the submucosa and serves to support the numerous vessels carrying 

 blood and Ivmph. This layer also provides the elasticity essential for the 

 expansion of the intestinal lumen, in addition to carrying the circulatory 

 fluids necessary for absorption. The mucous membrane, functioning in secre- 

 tion and absorption, is composed of simple columnar epithelium, which forms 

 the lining of the tract; a layer of reticular connective tissue (lamina propria), 

 which forms the cores of the villi; and a thin layer of non-striated muscle cells 

 (muscularis mucosae), which is the outermost layer lying adjacent to the sub- 

 mucosa. These several tissues are associated to form the small intestine, in 

 which digestion and absorption occur, and each tissue contributes to the func- 

 tion of the whole organ. In addition to the grouping of cells to form tissues 

 and of tissues to form organs, organs are associated to form the systems 

 described in discussions of metabolism, responsiveness, and reproduction. 



Digestion 



Most of the food that a vertebrate eats, or ingests, is not in a form im- 

 mediately utilizable by its cells. Foods, with the exception of water, inorganic 

 salts, vitamins, and a very few lipids and monosaccharides, must be broken 

 down into smaller units before they can be absorbed and utilized by the 

 animal. The processes of chemical breakdown of foods during their passage 

 through the digestive tract are called digestion. The chemical changes in 

 the food are brought about by specific enzymes secreted by cells of the diges- 

 tive organs. As a result of enzyme action, carbohydrates are broken down 

 into hexose sugars, lipids are separated into fatty acids and glycerol, and 

 proteins are reduced to amino acids. 



The purely mechanical activities of the digestive tract are aids to digestion. 

 Chewing separates food into smaller masses which afford more surface area for 

 the action of digestive enzymes. Movements of the digestive tract mix the 

 food particles with digestive juices and propel the food mass from one region 

 of the tract to the next. Digestion will be discussed as it occurs in the 

 successive regions of the digestive tract. You should keep in mind the 

 structure of the digestive system (p. 47). 



In the mouths of mammals the food may be torn apart or ground into fine 

 particles by the teeth, but in many other vertebrates the food is merely held 

 by the teeth and no mechanical disintegration occurs. The frog, for instance, 

 retains food with its teeth until it can be swallowed, and no digestive changes 

 take place in the mouth. In man, however, the sight, the odor, or even the 



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