DIGESTION. 



91 



-c 



and, under ordinary circumstances, permits only such 



matters to pass it as have first been properly acted upon 



in the stomach. Coins, buttons, and the like are, however. 



readily allowed to pass, because they can be of no use if 



retained. The soft and 



yielding texture of this 



organ the stomach 



indicates that it is r^h 



designed to crush antT 



comminute solid articles 



of food. 



24. The Gastric 

 Juice. We have seen 

 how the presence of food 

 in the mouth excites the 

 salivary glands, causing 

 the saliva quickly to 

 flow. In the same man- 

 ner, when food reaches 

 the stomach, its inner 

 lining, the mucous mem- 

 brane, is at once excited 

 to activity. (At first its 

 surface, which while the 

 stomach is empty pre- 

 sents a pale pink hue, 

 turns to a bright red 

 color, for the minute 

 blood-vessels which 

 course through it, are filled with blood. )( Presently a clear, 

 colorless, and acid fluid exudes, drop by drop, from its nu- 

 merous minute glands or "tubules," until finally the surface 

 is moistened in every part, and the fluid begins to mingle 

 with the food. This fluid is termed the gastric juice. \ 



FIG. 23. THE ORGANS OF DIGESTION. 



O, O3ophagus. I, Small Intestines. 

 S, Stomach. C, Large Intestines. 



L, Liver. P, Pancreas. 



M, Pylorus. N, Spleen. 



G, Gall-bladder. 



24. What is meant by the gastric juice ? 



