426 THE HUMAN BODY 



munication with its upper, or respiratory portion, leading to the 

 nostrils and Eustachian tubes. Finally, the isthmus of the fauces 

 is closed as soon as the food has passed through, by the contrac- 

 tion of the muscles on its sides and the elevation of the root of the 

 tongue. All passages out of the pharynx except the gullet are 

 thus blocked, and by a sharp contraction of the mylohyoid 

 muscles, in the floor of the mouth, such great pressure is put upon 

 the food-mass as to shoot it clear through the pharynx into the 

 opening of the esophagus. Liquids or very soft foods, under the 

 impetus given by the contraction of these muscles, are propelled 

 the whole length of the gullet to the sphincter which guards the 

 entrance to the stomach; more solid masses are thrown only into 

 the entrance of the gullet whence the third stage of swallowing 

 conveys them to the stomach. The muscular movements con- 

 cerned in this part of deglutition are all reflexly excited; food 

 coming in contact with the mucous membrane of the pharynx 

 stimulates afferent nerve-fibers in it; these excite efferent nerve- 

 fibers proceeding to the muscles concerned and cause them to 

 contract in proper sequence. The pharyngeal muscles, although 

 of the striped variety, are but little under the control of the will; 

 it is extremely difficult to go through the movements of swallow- 

 ing without something (if only a little saliva) to swallow and thus 

 excite the movements reflexly. Many persons, after having got 

 the mouth completely empty cannot perform the movements of 

 the second stage of deglutition at all. On account of the reflex 

 nature of the contractions of the pharynx, any food which has 

 once entered it must be swallowed: the isthmus of the fauces is a 

 sort of Rubicon; food that has passed it must continue its course 

 to the stomach, although the swallower learnt immediately that 

 he was taking poison. The third stage of deglutition is that by 

 which solid food is passed along the gullet, and is comparatively 

 slow. The movements of the esophagus are of the kind known 

 as peristaltic. Its circular muscular fibers contract behind the 

 morsel and narrow the passage there; and the constriction then 

 travels along to the stomach, pushing the food in front of it. 

 Simultaneously the longitudinal fibers, at the point where the 

 food-mass is at any moment and immediately in front of that, 

 contracting, shorten and widen the passage. This peristaltic 

 wave requires about six seconds in man for its passage along the 



