DIGESTION AND ABSORPTION 357 



by an enzyme called diastase. Here the same action is caused 

 by an enzyme called ptyalin. This ferment, as we can prove, acts 

 only in an alkaline medium at about the temperature of the body. 



How Food is Swallowed. After food has been chewed and 

 mixed with saliva, it is rolled into little balls and pushed by the 

 tongue into such position that the muscles of the throat cavity 

 may seize it and force it downward. Food, in order to reach the 

 gullet from the mouth cavity, must pass over the glottis, the open- 

 ing into the windpipe, or trachea. When food is in the course of 

 being swallowed, the upper part of this tube forms a trapdoor over 

 the opening. When this trapdoor is not closed, and food " goes 

 down the wrong way/' we choke, and the food is expelled by 

 coughing. 



The Gullet, or Esophagus. In man this part of the food tube 

 is much longer proportionately than in the frog. Like the rest of 

 the food tube it is lined by soft and moist mucous membrane. The 

 wall is made up of two sets of muscles, the inside ones running 

 around the tube ; the outer band of muscle taking a longitudinal 

 course. After food leaves the mouth cavity, it gets beyond our 

 direct control, and the muscles of the gullet, stimulated to activity 

 by the presence of food in the tube, push the food down to the 

 stomach by a series of contractions until it reaches the stomach. 

 The gullet passes directly through a muscular partition, the dia- 

 phragm, which is lacking in the frog. The diaphragm separates 

 the heart and lungs from the 

 other organs of the body cavity. 



Stomach of Man. The stomach 

 is a pear-shaped organ capable of 

 holding about three pints. The 

 end opposite to the gullet, which 

 empties into the small intestine, is 

 provided with a ring of muscle 

 forming a valve called the pylorus. 



Gastric Glands. If we open the 

 stomach of the frog, and remove 

 its contents by carefully washing, ^ de rf the ^ ach and 



its wall is Seen to be thrown into showing the folds of the mucous 



folds internally. Between the folds membrane. 



