212 THE STOMACH. 



observed that the food tube and the air tube cross, and that 

 the pharynx is their crossing. There is a spring switch (to 

 borrow a term from the railway) which keeps the track open 

 for the air, which is all the time passing ; but when the food 

 comes along, the switch must be held open until it has passed. 



As we saw when we examined the gullet, it has an outer 

 muscular coat, and an inner mucous coat. The muscular coat 

 has two layers, an inner with circularly arranged fibers, and 

 an outer layer with longitudinally arranged fibers. When the 

 food enters the gullet the muscle fibers, especially the circular 

 fibers, shorten, and by a wave-like action push the mass rap- 

 idly along into the stomach. 



The passage of the food through the gullet may be illus- 

 trated as follows : Let several persons hold a large rubber 

 tube with their hands in contact. Put an egg-shaped piece of 

 wet soap in the tube. The first hand is shut, and pushes the 

 soap along into the part of the tube held by the next hand ; 

 this hand now compresses the tube, while the first hand 

 remains clinched; and so, in turn, the object is pushed the 

 whole length of the tube. 



The first part of swallowing is voluntary, but after the 

 bolus has entered the gullet the action is involuntary. 



The mucous lining of the gullet has many mucous glands, 

 which lubricate the passageway by the mucus which they 

 secrete. 



The Stomach. Just beyond the diaphragm the diges- 

 tive tube widens suddenly, forming the stomach ; the stomach 

 is an oval sac lying just beneath the diaphragm, with the 

 large end to the left and the small end to the right. The 

 smaller end, by narrowing, becomes the small intestine. 

 When the stomach is empty it collapses, as its walls are soft 

 and flexible. When distended it may hold three pints, or 

 even more when abnormally distended. 



