DIGESTION IN THE MOUTH. 287 



the sixteen hours of abstinence sixteen thousand grammes are secreted ; 

 in all, fifty-six thousand grammes, or one hundred and twelve pounds. 

 This is certainly an inside estimate. In these animals, also, a less amount 

 is secreted with wet and green food. This immense amount of fluid is 

 again absorbed, and is, therefore, not lost to the economy. The part 

 which each gland plays in the secretion of this volume of fluid is also 

 determinable, and is a point of interest, since we already know that the 

 chemical composition and the function of the different secretions are not 

 uniform. The volume of saliva poured out depends on the dryness of 

 the food, and not, as has been claimed, upon the amount of starch which 

 it contains, indicating that the mechanical uses of the saliva are of 

 greater importance than its chemical functions. 



The volume of the special salivary secretions cannot be computed 

 from the volume of the glands. Thus, the parotids of the horse are 

 four times as large as the submaxillary glands, and yet they secrete 

 twenty-four times as much saliva. The parotid of the ox is scarcely as 

 large as the submaxillary, and yet it secretes four or five times as much 

 saliva as the latter. In the horse the parotid furnished seven-tenths of 

 the total amount of fluids poured into the mouth, a fact which may be 

 readily determined by means of oesophageal fistulse, conjoined with 

 closure of the parotid duct. The submaxillary has been determined by 

 the above method to furnish about one-twentieth of the total salivary 

 secretion. These figures cannot, of course, be taken as being rigorously 

 correct, since the necessary operative procedures must more or less 

 modify the activity of the glands. In the non-herbivora the quantity 

 of saliva is much less. It has been estimated at fifteen hundred 

 grammes in twenty-four hours for a man, while in the dog the parotid 

 has been calculated to contribute twenty-four grammes, the submaxil- 

 lary thirty-eight, and the other glands twenty-four grammes in 

 twenty-four hours. 



6. The Physiological Role of the Saliva. — The uses of saliva are 

 both mechanical and chemical. Mechanically, it assists in the formation 

 of the bolus of food, after having previously aided its mastication, and 

 acts as a lubricant in its passage to the stomach. It aids the apprecia- 

 tion of taste, and by lubricating the surfaces of the mouth and teeth 

 prevents the adhesion of viscid substances, and in man permits the 

 movements of rapid articulation. In the ruminant animals the en- 

 trance of saliva into the paunch is essential for the proper maceration 

 of food, so as to enable its regurgitation to the mouth in rumination. 



The chemical action of the saliva on the food was discovered by 

 Leuchs in 1831, who found that the saliva was capable of converting 

 soluble carbohydrates into dextrin and sugar. The cause of this prop- 

 erty of saliva lies in the presence of ptyalin, the diastatic ferment of 



