130 



DIGESTION 



once excited to activity. At first, 'its surface, which while 

 the stomach is empty presents 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, color- 

 less, and acid fluid exudes, drop by 

 drop, from millions of little tubes in 

 the inner surface of the stomach, 

 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. 



28. The gastric juice dissolves cer- 

 tain articles of food, especially those 

 belonging to the albuminoid class. 

 This solvent power is due to its 

 peculiar ingredient, pepsin; in diges- 

 tion, this substance acts like a fer- 

 ment that is, it induces changes in 

 the food simply by its presence, but 

 does not itself undergo change. The 

 acidity of the gastric juice, which is 

 due to hydrochloric acid, is not acci- 

 dental; for we find that the pepsin 

 cannot act in an alkaline solution 

 that is, one which is not acid or neu- 

 tral. The quantity of gastric juice 

 secreted daily is very large, probably 



not less than three Or four pints at FIG. 29. ALIMENTARY CANAL in- 

 , , r _, i ,1 n -i eluding Gullet, Stomach, Small 



each meal. Though thlS fluid IS at and Large Intestines 



once used in the digestion of the 



food, it is not lost ; since it is soon re-absorbed by the stomach, 

 together with those parts of the food which it has digested and 

 holds in solution. 



28. What is the office of the gastric juice ? Acidity of the gastric juice 2 Quantity of 

 gastric juice used ? What becomes of it ? 



