l8 8 TEXT-BOOK OF PHYSIOLOGY. 



Characters of Peptones. The peptones resulting from the diges- 

 tion of different proteids, though resembling each other in many re- 

 spects, yet possess different chemic characteristics, as shown by their 

 reaction to various chemic reagents. Though having some resem- 

 blance to the proteids from which they are derived, they are to be 

 distinguished from them by the following general characteristics : 



1. They are not coagulable either by heat or by nitric acid. 



2. They are soluble in water, either hot or cold, and in acid and 



alkaline solutions. 



3. They are diffusible, passing through animal membranes with 



great rapidity .^It has been demonstrated that peptones diffuse 

 about twelve times as rapidly as the proteids from which they 

 are derived.^ 



Neither on fat nor starch has gastric juice any appreciable effect, 

 and when these substances are introduced into the stomach they 

 pass into the intestine unchanged. Notwithstanding the fact that 

 dilute solutions of hydrochloric acid (0.3 per cent.) will promptly 

 invert cane-sugar into dextrose and levulose, and that gastric juice 

 will accomplish the same result in test-tubes, there is no strong 

 evidence for the belief that the inversion of cane-sugar takes place to 

 any marked extent in the stomach under normal conditions. The 

 starch, however, which has been subjected'to the action of the saliva 

 still continues to be converted into maltose during the first fifteen to 

 thirty minutes or possibly longer. Eyenjhough gastric juice is j>eing 

 secreted and though hydrochloric acid solutions with a strength of 

 0.3 per cent, will arrest the action of ptyalin, starch digestion con- 

 tinues for the reason that the acid combines with the proteids and 

 is thus rendered inoperative. After the above-mentioned period, 

 free acid makes its appearance when salivary digestion ceases. 



Action of Gastric Juice on Foods. The action of gastric 

 juice on proteids affords a key to its influence in the reduction of 

 foods to the liquid or semi-liquid condition. It is evident that it will 

 be most active in the digestion of food consisting largely of proteid 

 materials, such as meat, eggs, milk, etc. Meat is disintegrated first 

 by the conversion of the proteids of the connective tissue, which have 

 been more or less gelatinized by cooking, into peptones. The 

 sarcolemma of the muscle-fibers which have been thus separated 

 is in a similar manner attacked and converted into peptones. The 

 true muscle or sarcous substance, consisting largely of myosin, un- 

 dergoes a corresponding change. If the quantity of meat be not too 

 large and the gastric juice be secreted in proper amount, it is possible 

 that all the meat will be digested in the stomach. It is quite probable, 

 however, that this is not the case and that a portion of the semi- 

 digested meat passes into the intestine, where its final solution is 

 effected. 



