MASTICATION. 81 



The frothy nature of saliva, caused by the presence of air 

 bubbles in it, aids digestion by rendering the food through 

 which the saliva becomes mixed during mastication, more 

 easily penetrable by the digestive juices, than it would be if 

 air bubbles were absent. The presence of saliva in the food 

 materially aids digestion in the stomach, as we learn from the 

 experiments of Spallanzani and Reaumur, who "found that 

 food enclosed in perforated tubes and introduced into the 

 stomach of an animal was more quickly digested when it had 

 been previously impregnated with saliva, than when it was 

 moistened with water. Dr. Wright also found that if the 

 oesophagus (gullet) of a dog is tied, and food mixed with 

 water alone is placed in the stomach, the food will remain 

 undigested, though the stomach may secrete abundant acid 

 fluid, but if the same fluid is mixed with saliva, and the rest 

 of the experiment similarly performed, the food is readily 

 digested" (Kirkes). I think we may conclude from these 

 observations, that the food of horses should be supplied to 

 them in a more or less dry state, and that the practice of 

 giving soaked food or boiled food is, as a rule, objectionable. 

 If, however, such food is given, as in case of illness, or for a 

 change, it is well to mix it with a suitable quantity (say I ^ oz.) 

 of common salt, the presence of which in the mouth excites 

 the flow of saliva. 



Saliva, being of an alkaline nature, aids the digestion of 

 fat by forming it into an emulsion, in which the fat is split 

 up into minute particles. 



Saliva contains the ferment ptyalin, which has the property 

 of converting starch into sugar, in which form it is absorbed 

 into the body. The action of the ptyalin of the saliva 

 on starch is of too brief duration to have much effect ; 

 for it ceases soon after the arrival of the food in the 

 stomach, on account of the presence of acid in the gastric 

 juice. The digestive changes -which the food undergoes 



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