THROAT AND ESOPHAGUS 123 



may be resumed. Practically no digestion takes place in the 

 pharynx due to the facts that food is swallowed quickly and no 

 enzymes are secreted here. 



The esophagus is a comparatively straight tube, about nine 

 or ten inches long. It descends in front of the spine, passing 

 through the diaphragm and terminating in the stomach. It has 

 one set of muscles extending around it circularly and another layer 

 of muscles longitudinally arranged. By a series of contractions of 

 the circular and the longitudinal muscles, food is passed by a series 

 of wavelike movements into the stomach. Once the top ring of 

 muscles is stimulated by the swallowing of the food, a wave of 

 contraction goes through the full length of the gullet, causing 

 the food to enter the stomach. This series of rhythmic, wave- 

 like contractions of circular and longitudinal muscle fibers which 

 affect successive portions of the tube downward is called peri- 

 stalsis. The constricted portion is always preceded by an area 

 of relaxation which renders the contraction more effective in 

 forcing the contents onward. The direction is normally the 

 same, and the action is under the control of the nervous system. 

 This movement might be compared with placing a large marble 

 in a narrow rubber tube and forcing it through by successively 

 pinching the tube. 



During the processes of mastication, moistening, and swallow- 

 ing, the food is reduced to a soft, pulpy condition. Any starch 

 it may contain begins to change into sugar in the mouth, but 

 it remains in the mouth, throat, and esophagus so short a time 

 that digestion cannot be completed in them. 



Questions and Suggestions 



1. How does the body cavity of the frog differ from the cavities 

 in man's body? 



2. Name three purposes of the digestive process. 



3. Compare the alimentary canals of the worm, frog, and man, 

 stating all similarities and differences. 



