THE DIGESTIVE SYSTEM 93 



Ptyalin acts on starch, dextrin, and other carbohydrates, 

 converting them into simpler bodies. The first change is a 

 conversion of the huge starch molecule into dextrin and then into 

 maltose. Like other enzymes, pytaline 1 is not destroyed or used 

 up in producing its reaction, but if heated to 60°C. it loses its power. 

 It is most active in a neutral medium at the body temperature. 

 In the presence of the free hydrochloric acid of the stomach 

 ptyalin is rapidly rendered inert. The chief part of salivary 

 digestion actually occurs in the stomach despite the hydrochloric 

 acid of the gastric juice, which requires some time to penetrate 

 the food. This is due to the low motility of the cardiac portion 

 of the stomach, which prevents the food from mixing with the 

 hydrochloric acid very promptly, and to the fact that this secre- 

 tion is confined to the fundus and pyloric regions. It would 

 seem fitting, on account of the large amount of carbohydrates in 

 his food, that the horse's saliva contain a large amount of ptyalin, 

 but examination shows that saliva taken from the parotid duct 

 is unable to convert a starch mixture into a reducing sugar except 

 in a very limited extent and after a considerable time. 



(B) Digestion in the Stomach. — The stomach is popularly 

 supposed to be the chief organ of digestion. While this may 

 be true in man, in the horse and in the ox the stomach acts 

 principally as a temporary storehouse for food, which is under- 

 going preparation for the more complex digestive changes 

 designed to take place in the intestines. 



Gastric juice is secreted by the glands of the stomach. The 

 difficulty of obtaining a pure sample from the horse and ox has 

 prevented detailed study of the secretion from these animals. 

 The physiologically important constituents are lactic and hydro- 

 chloric acids, water, and the enzymes, pepsin and rennin. Fresh 

 gastric juice also contains much mucin, but this has only a passive 

 part in digestion. It forms a thick, viscid covering for the food 

 shortly after it is introduced into the stomach. 



Lactic and hydrochloric acids are found free in the gastric 

 juice and give the secretion an acid reaction. By inhibiting 



1 To show the presence and action of ptyalin, some starch paste should be 

 prepared. Five c.c. of the paste may be placed in each of as many test-tubes 

 and filtered saliva added in different quantities. By keeping them at various 

 temperatures and testing from time to time with iodin and Fehling's solu- 

 tions the progress of digestion in the mouth may be well demonstrated. 



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