1 66 Physiology. 



Breathing and Swallowing. It is to be observed that 

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

 is their crossing. As we are swallowing only a small part 

 of the time, the passageway naturally stands open to the 

 air; and when we swallow, the parts are, by muscular 

 effort, temporarily arranged for this work. 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 

 close the air passage and hold open the food passage until 

 the food has passed. 



Structure and Action of the Gullet. The gullet has 

 an outer muscular coat and an inner mucous coat (see 

 Fig. 72). The muscular coat has two layers, an inner 

 with circularly arranged fibers, and an outer layer with 

 fibers running lengthwise. When the food enters the gul- 

 let the muscle fibers, especially the circular fibers, shorten, 

 and by a wave-like action push the mass rapidly along into 

 the stomach. The first part of swallowing is voluntary ; 

 but after the mouthful has entered the gullet the action is 

 involuntary. The mucous lining of the gullet has many 

 mucous glands which make the passageway smooth by the 

 mucus which they secrete. 



Illustration of Passage through the Gullet. The passage of the food 

 through the gullet may be illustrated as follows : Let several persons 

 hold a large rubber tube with their hands in contact. Put an egg-shape,^ 

 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 Stomach. Just beyond the diaphragm the digestive 

 tube widens suddenly, forming the stomach ; the stomach 



