i 7 6 TEXT-BOOK OF PHYSIOLOGY 



different chemic characteristics, as shown by their reaction to various chemic 

 reagents. Though having some resemblance to the proteins 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 proteins from which they are derived. 



From the foregoing facts it may be inferred that in the digestion of pro- 

 teins there is a progressive diminution in the size of the molecules through a 

 series of hydrolytic changes. The molecules of the proteins, which from 

 various causes are coagulated, are transformed into smaller molecules which 

 are non-coagulable, soluble, and diffusible. 



On liquid fat and hydrated starch gastric juice has no appreciable action. 

 It has apparently been demonstrated, however, that when fat in the emulsi- 

 fied state, the state in which it exists in milk, is introduced into the stomach 

 it undergoes a cleavage into fat acids and glycerin, in a manner similar to 

 that which fat undergoes in the intestine under the action of pancreatic 

 juice, as will be stated in a future paragraph. This presupposes the 

 existence of a ferment to which the name lipase has been given. Though 

 the action of saliva on starch is interfered with and even checked by a small 

 percentage of hydrochloric acid it is certain from the results of recent experi- 

 ments, that starch digestion continues for from twenty minutes to a half 

 hour or longer, for the reason that the acid as fast as it is secreted combines 

 with the proteins and is thus rendered inoperative and for the reason also 

 that the food is largely retained in the extreme fundic end of the stomach 

 where the gastric juice is not abundant. After the above-mentioned period, 

 free acid makes its appearance when salivary digestion ceases. 



Notwithstanding the fact that dilute solutions of hydrochloric acid (0.3 

 per cent.) will promptly invert cane-sugar to 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. 



Action of Gastric Juice on Foods. The action of gastric juice on 

 proteins affords a key to its action in the reduction of foods to the liquid or 

 semiliquid condition. It is evident that it will be most active in the digestion 

 of food consisting largely of protein materials, such as meat, eggs, milk, etc. 

 Meat is disintegrated first by the conversion of the proteins 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, undergoes 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 semidigested meat passes into the intestine, where its final 

 solution is effected. 



The white of egg, especially when slightly boiled, is much more readily 



