66 THE INDIVIDUAL ORGANISM 



epithelium, known as the peritoneal layer. 1 Like the stomach and the 

 small and large intestines, the esophagus is a tube ; but unlike the stomach 

 and intestines it is a relatively flabby, thin-walled tube, producing no 

 enzymes and serving merely to connect the mouth with the stomach. 



Digestion in the stomach. The swallowed food is not retained in the 

 esophagus but passes immediately into the stomach, which is closed at 

 its lower end by the pyloric valve, a sphincter muscle at the juncture of 

 the stomach and small intestine. The food is retained in the stomach 

 for some time — usually from 3 to 4^ hours in the case of an ordinary 

 mixed meal. Fluids and semifluids commence to leave the stomach 

 almost immediately after being swallowed. On account of the rates at 

 which they are liquefied, carbohydrate leaves more rapidly than protein 

 and protein more rapidly than fat. While the food is in the stomach, a 

 number of important digestive processes take place. 



The mucosa of the stomach contains a multitude of glands that, when 

 stimulated, pour their secretion, the gastric juice, into the cavity of the 

 stomach. This gastric juice contains hydrochloric acid and three enzymes. 

 The most important enzyme is pepsin, which aids in the digestion of pro- 

 teins. The others are rennin, which coagulates milk, and small amounts 

 of lipase, which aids in the digestion of fats. The walls of the stomach 

 also secrete mucin, a slimy and viscous substance that coats the wall of the 

 stomach and helps to protect it against the action of the gastric juice. 



The muscle layer of the stomach is thick and powerful and forms the 

 greater part of the thickness of the stomach wall. During digestion, 

 waves of muscular contraction begin in the region of the broad upper or 

 cardiac end of the stomach and sweep toward the narrow lower or pyloric 

 end, becoming increasingly powerful as they proceed. There may be 

 several of these waves in progress at one time. Thus the contents of the 



muscle. For this reason, visceral muscle is often called smooth muscle, as opposed to 

 striated. 



Most of the internal organs of the body are essentially tubular in construction, and 

 the walls of these tubes usually possess an outer layer of longitudinal smooth muscle, 

 surrounding an inner and heavier circular layer. Contraction of the longitudinal 

 layer shortens and thickens or expands the tube; contraction of the circular layer thins 

 and elongates it. These muscles, therefore, are arranged in opposing sets like the 

 skeletal muscles, though they are not attached to bony levers. 



Compared to the action of skeletal muscles, the contraction of smooth muscles is 

 slow and prolonged, that caused by a single stimulus lasting some 20 seconds. The 

 action of the visceral muscles is controlled by the autonomic division of the nervous 

 system and is not conscious; these muscles are therefore often called involuntary 

 muscles. 



1 In the neck region, before the esophagus enters the body cavity, the outer layer 

 is formed of connective tissue rather than of peritoneum. In general, the peritoneum 

 not only forms the outer covering of the portions of the alimentary canal lying within 

 the body cavity but lines the entire wall of the body cavity as well. 



