182 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



MOUTH 



SOFT PALATE 



TRACHEA 



Fig. 11.11. The passageways for air and food, showing the cross ways in the 

 pharynx that must be taken separately by each. When air has the road the trachea 

 (windpipe) is open and the esophagus is closed. When food has the road, the 

 epiglottis covers the trachea and the esophagus is open. (Reprinted from The 

 Machinery of the Human Body by Carlson and Johnson, by permission of The 

 University of Chicago Press. Copyright 1948.) 



into the esophagus, always slippery with mucus. There circular muscles grip 

 it and urge it along the short passage to the stomach (Fig. 11.12). In the 

 upper part of the mammalian esophagus the rapid contractions of striated 

 muscles extend all the way to the stomach, an arrangement well adapted to 

 the amazing rapidity with which animals swallow their food, a long established 



Wave of muscular contraction 

 grasps the bolus of food 



Wave of relaxation opens 

 the tube before it 



Contraction squeezes tube 

 behind it 



Six seconds from mouth to stomach 



• 



1 



# 



Fig. 11.12. It takes about 6 seconds for a bolus of solid food to pass from 

 the mouth to the stomach. A wave of contraction follows a bolus of food; a wave 

 of relaxation opens the way in front of it. 



I 



