THE DIGESTIVE SYSTEM 



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way because the lungs evolved from the 

 floor of the mouth cavity. This meant that 

 both food and air must pass through the 

 pharynx and could only be accomplished 

 by providing some way of closing one when 

 the other was functioning. The apparatus 

 is very simple among amphibians but be- 

 comes more complex in the higher forms, 

 particularly in mammals. Here an elaborate 

 system of nerves and muscles have evolved 

 to provide a smooth crossing of food and 

 air. The inadequacies of this system are 

 familiar to anyone who has experienced the 

 difficulties resulting when particles of food 

 get into the air passages. 



Let us see how the process of swallowing 

 takes place in man. When the food reaches 

 a pasty consistency in the mouth, it is 

 forced into the pharynx (back of oral cav- 

 ity) by the tongue where a series of events 

 follow in rapid succession which propel it 

 to the stomach through the small esopha- 

 gus. When food or fluids reach the pharynx 

 a chain of impulses is initiated which brings 

 about the pulling forward of the larynx and 

 a tipping of the epiglottis to prevent food 

 from passing into the trachea, together with 

 a simultaneous opening of the upper end of 

 the esophagus (Fig. 17-5). Once all this is 

 started, the food is beyond recall. When it 

 enters the esophagus a peristaltic wave car- 

 ries it quickly to the stomach. Peristalsis is 

 accomplished by the thick muscles which 

 make up the wall of the esophagus. The 

 presence of food in this collapsed tube 

 causes the longitudinal muscles to relax just 

 in front of the bolus ( ball ) and at the same 

 time causes the circular muscles to constrict 

 just behind it with the result that the food 

 is pushed along the tube (Fig. 17-9). This 

 is a rapid action, requiring only 5 to 6 sec- 

 onds for food and less for fluids. Further- 

 more, it has nothing to do with gravity be- 

 cause a horse can swallow uphill, as indeed 

 it must almost always do, and even a human 

 can drink in an inverted position if he de- 

 sires. Swallowing is initiated only by the 



presence of food or fluids in the pharynx; 

 thus it is impossible to swallow twice in 

 rapid succession when no foods or fluids are 

 taken into the mouth. Yet when one drinks 

 fluids one can swallow continuously. This 

 means that swallowing can only occur 

 when food or fluids are in the pharynx. 



THE STOMACH 



After passing through the esophagus, the 

 food drops into the stomach, which is a sac- 

 like expansion of the digestive tube de- 

 signed not only to store considerable food 

 (about two and one-half quarts) but to 

 start the digestion of proteins (Fig. 17-2). 

 It is a thick-walled muscular sac lined with 

 gastric glands. These are minute, slender 

 pockets in the soft mucosa with tiny open- 

 ings through which the gastric juice flows 

 into the cavity of the stomach. In addition 

 to the circular and longitudinal muscle lay- 

 ers of the esophagus, the stomach possesses 

 an oblique layer. With this elaborate sys- 

 tem the stomach becomes an efficient mix- 

 ing or churning organ. 



The stomach is usually divided into two 

 general regions: the cardiac region which 

 immediately follows the esophagus, and the 

 pyloric region which is followed by the 

 small intestine. The anatomy and activity 

 of the two portions vary somewhat. The 

 portals of entry and exit to the stomach are 

 guarded by valves consisting of thickened 

 circular muscles which, when strongly con- 

 tracted, completely close both of these 

 openings. Such valves in a tube are called 

 sphincters. Besides these two in the stom- 

 ach, the cardiac at the entrance and the 

 pyloric at the exit, there are two others 

 along the digestive tube: one where the 

 small intestine joins the colon (ileocaecal) 

 and the other at the end, the anal sphincter. 

 These valves are important in retaining the 

 food in its proper place until digestion is 

 complete. 



As food is swallowed the stomach gradu- 



