338 THE HUMAN BOD Y. 



pharynx, any food which has once entered it must be swal- 

 lowed: 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 in 

 which the food is passing along the gullet, and is compara- 

 tively slow. Even liquid substances do not fall or flow down 

 this tube, but have their passage controlled by its muscular 

 coats, which grip the successive portions swallowed and pass 

 them on. Hence the possibilit}' of performing the apparently 

 wonderful feat of drinking a glass of water while standing 

 upon the head, often exhibited by jugglers; the onlookers 

 forget that the same thing is done every day by horses,, 

 and other animals, which drink with the pharyngeal end of 

 the gullet lower than the stomach. The movements of the 

 oesophagus are of the kind known as vermicular or peri- 

 staltic. Its circular fibres (p. 317) 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 fibres, at the point 

 where the food-mass is at any moment and immediately in 

 front of that, contracting, shorten and widen the passage. 

 The Gastric Juice. The food having entered the sto- 

 mach is exposed to the action of the gastric juice, which is a 

 thin, colorless, or pale yellow liquid, of a strongly acid re- 

 action. It contains as specific elements free hydrochloric 

 acid (about .2 per cent), and an enzyme called pepsin 

 which, in acid liquids, has the power of converting the or- 

 dinary non-dialyzable proteids which we eat, into the closely 

 allied but dialyzable bodies called peptones. It also dis- 

 solves solid proteids, changing them too into peptones. 

 Dilute acids will by themselves produce the same changes 

 in the course of several days, but in the presence of pepsin 

 and at the temperature of the Body the conversion is far 

 more rapid. In neutral or alkaline media the pepsin is 

 inactive; and cold checks its activity. Boiling destroys it. 

 In addition to pepsin, gastric juice contains another enzyme 

 which has the power of coagulating the casein of milk, as 

 illustrated by the use of "rennet," prepared from the mu- 



