ALIMENTATION AND DIGESTION 



109 



and comparatively thick in the pyloric, the thickening in the 

 latter region being caused chiefly by muscle fibers circularly 

 arranged. 



21. The gastric juice. The gastric juice is a clear, thin, 

 colorless liquid which contains, among other things, about 

 0.2-0.3 per cent of hydrochloric 

 acid and certain enzymes. Upon 

 starch it has no action whatever, 

 nor has it any action on fats, unless 

 the fat is in the form of an emulsion 

 (that is, very fine drops of oil sus- 

 pended in water, as in milk or may- 

 onnaise dressing) ; indeed, the very 

 limited power of gastric juice to at- 

 tack fat is a matter of considerable 

 importance in dietetics. Its main 

 chemical action is upon the proteins, 

 which under its influence undergo 

 cleavage into proteases and peptones. 

 The proteoses and peptones, like 



the original protein, are polypeptids (p. 102), but of smaller 

 molecular size. They are not coagulated by heat, and most 

 of them are soluble. 



EXPERIMENTS 



Prepare some artificial gastric juice as follows : To one quart of water 

 add 7 or 8 cc. of concentrated hydrochloric acid and to this a little active 

 pepsin, which may be obtained at any drug store. Pepsin is extracted 

 from the stomach and is the most important of its enzymes. A solution 

 of pepsin in the given strength of hydrochloric acid is virtually gastric 

 juice. Try the effect of this on the following substances by placing each 

 in a half tumblerful of the juice. To get the complete effect the mixture 

 should be set aside for twenty-four hours and tests made the next day. 

 Observations should be made during the first hour or two. If the digest- 

 ing mixture can be kept in a warm place (90-100 F.), the action will 

 be more rapid and the results more satisfactory. The digestions can 

 best be carried out in corked 4-ounce bottles, which should be shaken 



FIG. 55. The inner surface of 



the stomach (magnified about 



20 diameters) 



Showing the openings of the 

 glands. The lining glandular 

 membrane is thrown into folds 



