196 HUMAN PHYSIOLOGY 



Vomiting. When the stomach is to be emptied 

 through the cardia instead of in the normal way the 

 antrum is said to contract while the opening to the 

 esophagus yawns. The diaphragm bears strongly down 

 and the abdominal muscles spasmodically grip the 

 viscera. The sudden pressure projects more or less of 

 the gastric contents up the esophagus. The respiratory 

 passages are protected as in swallowing by the tucking 

 of the larynx under the tongue and the swinging of the 

 soft palate to the back of the pharynx. The chest is 

 alternately expanded and compressed, the expansions 

 enlarging the esophagus for the stomach to fill and the 

 compressions hurrying the accumulation to the exterior. 

 This reflex is one in which the breathing muscles are far 

 more actively employed than those of the stomach. 

 Its value in removing substances that might prove 

 poisonous is perfectly clear but it occurs under many 

 circumstances when it seems to serve no useful purpose. 



Salivary Digestion in the Stomach. The digestion 

 of starch by the saliva is limited by the development of 

 acidity in the stomach. The enzyme of the ^saliva 

 is destroyed by acid in a minute percentage. The es- 

 sential question is how long a time may pass before the 

 acidity is developed throughout the mass of the food. 

 This mass, we have seen, lies nearly motionless in the 

 fundus. The incoming gastric juice works its way 

 from the walls of the stomach gradually toward the 

 center. Minute by minute the sphere dominated by 

 the acid will be larger and the core in which salivary di- 

 gestion can go on will be more restricted. Yet it is likely 

 that for half a hour or more after an average meal some 

 space remains in which starch digestion is in progress. 



The transformation of starch by the saliva is now 

 believed to constitute an important part of the digestive 

 process, something which was not held to be true a few 

 years ago. Tests which have been made upon pre- 

 scribed portions of food swallowed by individuals and 

 recovered after various intervals have shown that a 



