92 DIGESTION. 



25. The gastric juice is the proper solvent of certain 

 articles of food, especially those belonging to the albu- 

 minoid class. This solvent power is due to its peculiar in- 

 gredient, pepsin ; in digestion, this substance acts like a 

 ferment ; that is, it induces changes in the food simply by 

 its presence, but does not itself undergo change. The acid- 

 ity of the gastric juice, which is due to lactic acid, is not 

 accidental; for we find that the |flpi cannot act in an 

 alkaline solution that is, one whicn is not acid or neutral. 



fThe quantity of gastric juice secreted daily is very large, 

 probably not less than three or four pints at each meal. 

 Though this fluid is at once used in the reduction 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. \ 



26. Movements of the Stomach. The inner coat- 

 ing of the stomach is the mucous membrane, which, as we 

 have seen, furnishes the gastric juice. Next to this coating 

 lies another, called the muscular coat, composed of invol-* 

 untary muscular fibres, some of which run circularly, and 

 others in a longitudinal direction. These expand to ac- 

 commodate the food as it is introduced, and contract as it 

 passes out, In addition, these fibres are in continual mo- 

 tion while food remains in the stomach, and they act in 

 such manner that the contents are gently turned round 

 from side to side, or from one end of it to the other. 



27. By these incessant movements of the stomach, 

 called the peristaltic movements, the gastric juice comes 

 in contact with all parts of the food. We are, however, 

 not conscious that these movements take place, nor have 

 we the power to control them. When such portions of the 

 food as are sufficiently digested approach the pylorus, it 



25. .What is the office of the gastric juice? Acidity of the gastric juice* 

 Quantity of gastric juice used ? What becomes of it ? 



26. Muscular coat of the stomach? Expansion and contraction of its fibres? 

 Action of the fibres? 



27. Peristaltic movements? What is said of our consciousness of and power 

 over these movements ? Describe the movements of the pylorus. 



