DIGESTION AND FOOD. 



17 



FIG. III. 



Section of the 



(Esophagus. 



a, Muscular 

 fibres, running 

 lengthwise. 

 Circular fibres. 



23. The pharynx connects the mouth with 

 the oesophagus. It spreads out like a tun- 

 nel behind the palate, and is open to receive 

 the food. The oesophagus extends from the 

 pharynx to the stomach. It is a soft tube, 

 about nine inches long, and rather less than 

 an inch in diameter. It is covered with two 

 layers of muscular fibres, one of which runs 

 lengthwise, (Fig. III. a, ,) the other winds 

 around it successively from top to bottom. 

 (Fig. III. b.) These muscles have a power 

 of contraction, or of drawing themselves up, 

 like the earth-worm, and again of relaxing 

 themselves, and being stretched out loosely. 

 They draw around the gullet like the string a> 

 of a work-bag, and thus, narrowing the pas- 

 sage, force onward whatever food or other $ f 

 matter there is within it. 



24. When the food is thrust backward by the tongue, it 

 passes into the pharynx, which closes upon it, and propels it 

 downward into the oesophagus. Then the uppermost band of 

 muscular fibres contracts, and closes its upper end, and 

 prevents a return of the food backward. Then the next 

 band contracts, and forces the food onward. Then the third 

 band does the same. Thus, while each one is successively 

 pressing upon the contents of the tube, these are forced on- 

 ward and downward to the stomach. While one band is 

 contracting, that which is next below it relaxes, to admit the 

 entrance of the food. This is the process of swallowing, 

 and is performed by the successive action of these circular 

 muscles. These bands are so well adapted to each other, 

 and work in such harmony, that we are not aware of the 

 steps of this operation. Vomiting is performed upon the same 

 principle, except that the order of contraction is reversed ; the 

 lower fibres first contract, and then those next above, and 

 thus their action forces solid and fluid matters upwards, from 

 the stomach to the mouth. 



