122 THE DIGESTIVE SYSTEM 



products in the digestion of starch. They are simpler products 

 than starch, but not simple enough to be used by the body. The 

 change, due to ptyalin, takes place when there is a slightly alka- 

 line condition. Saliva that is distinctly acidulous hinders or arrests 

 the digestive process. Boiled starch is changed more rapidly and 

 completely than raw, but food is rarely retained in the mouth 

 long enough for the saliva to more than begin the transformation 

 of starch. 



Throat and esophagus. The mouth cavity narrows to form 

 the pharynx or throat cavity. It is shaped somewhat like a funnel, 

 with its narrow or constricted end turned downward to form the 

 beginnings of the esophagus and the windpipe. There are seven 

 openings in the throat cavity, leading to the nose, ears, mouth, 

 larynx, and gullet. Two, in front above, lead into the back of the 

 nose and are known as the internal nasal openings. Two, one on 

 either side above, lead into the ears and form the openings to the 

 Eustachian tubes. One midway in front connects with the mouth. 

 Two below, one opening into the windpipe through the glottis and 

 the other behind the glottis into the gullet. 



The pharynx is a passageway for air from the nose to the glottis, 

 and for food from the mouth to the gullet. When the food is ready 

 to be swallowed, it is brought together on the upper surface of the 

 tongue and pressed backward into the pharynx. Then the muscles 

 in the pharynx contract, drawing the pharynx upward and caus- 

 ing it to dilate to receive the food. The muscles then relax, caus- 

 ing the pharynx to sink and forcing the food into the esophagus. 

 At this instant, breathing is temporarily suspended and the air 

 passages closed against the possible entrance of food. The soft 

 palate is drawn back, thus closing and protecting the nasal pas- 

 sages. The entrance to the w^indpipe is shielded by being pulled 

 forward under the base of the tongue. There is an additional 

 safeguard through the folding down of the epiglottis, a special- 

 ized cover. When food is once within the esophagus, breathing 



