DIGESTION: 365 



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 possibility 

 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 ver- 

 micular or peristaltic. Its circular muscular fibres contract 

 behind the morsel and narrow the passage there; and the con- 

 striction then travels along to the stpmacjh, pushing the food 

 in front of it. Simultaneously tire 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 stom- 

 ach is subjected to the auction of the gastric juice, which is a 

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

 tion. 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 ordinary non-' 

 dialyzable proteids which we eat, into closely allied bodies, 

 some of which are dialyzable and named peptones. It also 

 dissolves solid proteids, changing them similarly. 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 addi- 

 tion to pepsin, gastric juice contains another enzyme (rennin) 

 which coagulates the casein ogen of milk, as illustrated by 

 the use of "rennet," prepared from the mucous membrane 

 of the calf s digestive stomach, in cheese-making. The acid 

 of the natural gastric juice would, it is true, precipitate the 

 casein, but such precipitate is quite different from the true 

 tyre'm, and neutralized gastric juice still possesses this power; 

 moreover, boiled gastric juice loses the milk-clotting property, 

 and a very little normal juice can coagulate a great quantity 

 of milk. The curdled condition of the milk regurgitated by 

 infants is, therefore, not any sign of a disordered state of the 

 stomach, as nurses commonly suppose. It is proper for milk 



