DIGESTION IN THE MOUTH. 277 



These results must not be regarded as absolutely correct, since, even 

 in the horse, the operation of making a fistula interferes with the normal 

 sequence of mastication, as it is always longest on the side which is 

 opposite to the fistula. 



Ellenberger and Hofmeister found that the parotid of one side in one 

 horse secreted 1000 grammes in half an hour, the same amount in another 

 horse in a quarter of an hour, and in a third horse 4000 grammes in two 

 hours — oats, hay, and chopped straw being given as food. During the 

 pauses between the acts of mastication, the parotids of the horse, con- 

 trary to what is the case in the ruminant, are quiescent. 



In ruminant animals this alteration of activity of the parotid glands, 

 although depending upon the side of mastication, is less readily deter- 

 mined than in the horse ; for when a fistula is made the ruminant animal 

 will continuously masticate on the. opposite side to that in which the 

 fistula is present, and the maximum activity of secretion is therefore 

 taking place on the side opposite to the fistula. On the other hand, if 

 two fistulae are made the animal will change the direction of mastication 

 two or three times a minute, and the process of mastication is much in- 

 terfered with and the character of secretion altered. The inequality of 

 the secretion according to the side on which mastication is taking place 

 is also seen in the ruminant animal during the second period of mastica- 

 tion in rumination. 



These experiments seem to show that mastication is the normal 

 stimulant of the parotid glands, though they also secrete during the 

 pauses of rumination. Further, these glands are insensible to other 

 stimulants, such as salt, acids, etc., brought into contact with the mucous 

 membrane of the mouth. Such stimuli produce no sensible secretion in 

 solipedes and no increase in the constant secretion of ruminants. So, 

 also, sight and odor of food have no effect on the secretion of the parotid, 

 even if the animals are in a state of great hunger. 



The character of the parotid saliva also differs from that of mixed 

 saliva and that of the other salivary glands. It is thin, limpid, contains, 

 with the exception of a few epithelial cells, scarcely aiv^ formed elements, 

 and is invariably alkaline, except after prolonged fasting, when the first 

 few drops may have a slightly acid reaction from the contained carbon 

 dioxide. Great variation exists in the estimates of its specific gravity, 

 it having been said to vary from 1003 to 1012. It contains scarcely any 

 mucin ; when heated to boiling it becomes turbid, as also occurs after the 

 addition of alcohol or mineral acids, showing the presence of an albumen- 

 like body. It becomes clearer when C0 2 is passed through it. It con- 

 tains ptyalin. Sulphocyanide of potassium has been said to be absent 

 from the parotid saliva of the horse. The parotid saliva of the dog has 

 a specific gravity of 1004 to 1007, and when heated deposits a slight 



