186 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



show how the surface of an organ can be enormously increased without 

 enlarging the organ. 



Of the various components of the gastric juice, mucin secreted by mucous 

 cells makes a slippery protective layer over the lining of the stomach, and the 

 hydrochloric acid kills bacteria and provides an acid medium without which 

 the gastric enzymes will not work. Enzymes work best in a particular acidity 

 or alkalinity, and if this changes, their action is hampered or stopped. The 



Fig. 11.16. Arrangement for recording stomach contractions on a kymograph, 

 the revolving drum that carries a strip of smoked paper. The writing finger, at the 

 top, makes the sharp lifts that record contraction. The other finger records the 

 repeated pressures caused by breathing. (Courtesy, Gerard: The Body Functions. 

 New York, J. Wiley and Sons, 1941.) 



enzyme, pepsin, reduces protein only to the stage of proteoses and peptones; 

 probably no amino acids are formed, and there is relatively little absorption 

 from the stomach (Table 11.3). The action on proteins is a preliminary hy- 

 drolysis, a splitting and absorption of water that expose the products to the 

 enzyme action that occurs after they enter the intestine. The enzyme, rennin, 

 is an aid to pepsin in that it coagulates milk and produces the soluble milk 

 protein, casinogen, upon which pepsin can act. Lipase is an enzyme that acts 

 on the finely emulsified fats of cream and egg yolk. The production of gastric 

 juice is first stimulated by food in the mouth, but when it reaches the stomach, 

 the flow is greatly increased. Experiments on animals have shown that partly 

 digested food stimulates the production of gastrin, a hormone said to be pro- 

 duced by cells of the lining of the pyloric end of the stomach and discharged 

 into the blood stream whenever such food comes in contact with these cells. 

 If an extract of pyloric lining is injected into an animal, its gastric glands 

 begin to secrete within a short time. In cross-circulation experiments a blood 



