304 - Multicellular Animals, Especially Man 



WAVE OF 

 CONSTRICTION 



BOLUS OF FOOD 



WAVE OF 

 RELAXATION 



t?S& 





changes. The emptying of the stomach is 

 relatively rapid (about 10 minutes) in the 

 case of a drink of plain water; but some 

 meals may take 3 to 4 hours in passing 

 through the stomach, depending on the qual- 

 ity and quantity of the food. 



The Size and Shape of the Stomach. The 

 capacity of the stomach varies according to 

 its contents, and during the ingestion of a 

 meal the muscular wall is capable of expand- 

 ing to a maximum capacity of about 3 quarts, 

 in an average individual. Then during the 

 digestion of the meal, the stomach gradually 

 shrinks, as the semidigested food is passed, 

 little by little, into the small intestine. Fin- 

 ally the gastric cavity is practically obliter- 

 ated, whereupon further peristaltic waves, in 

 which the stomach merely squeezes down 

 upon itself, give rise to "hunger pangs," and 

 it is time to eat again. 



The anatomy of the stomach is best de- 

 scribed in terms of the half-filled organ such 

 as is outlined in Figure 16-19. Such a stom- 

 ach is distinctly J-shaped, and permits one to 

 identify the three main parts of the stomach. 



(1) The fundus is a small, bulbous, gas-filled 

 chamber, which occupies the vertical limb of 

 the J, above the opening from the esophagus; 



(2) the cardiac region consists of the re- 

 mainder of the vertical limb; (3) the pyloric 



Fig. 16-18. Peristalsis in the esopha- 

 gus. Three stages in the downward 

 movement of a semisolid bolus of food 

 are shown. 



region extends from the "bend of the )" to 

 the point where the stomach joins the duo- 

 denum, or first part of the small intestine. 

 Guarding the entrance and exit channels of 

 the stomach are ringlike bands of muscle: 

 (1) the cardiac valve, which encircles the 

 esophageal opening and prevents a regurgita- 

 tion of food into the esophagus; and (2) the 

 pyloric valve, which prevents food from pass- 

 ing out of the stomach into the duodenum, 

 until the proper time. Both of these valves 

 are typical sphincter valves, such as are found 



FUNDIC PART OF STOMACH 

 DIAPHRAGM 



CARDIAC PART 

 OF STOMACH 



PYLORIC PART 

 OF STOMACH 



Fig. 16-19. The stomach and duodenum in relation 

 to the liver and pancreas in man. 



