DEGLUTITION. 223 



elements of the food received, and to lubricate their passage 

 along the tongue, and the isthmus of the fauces. 



These distinctions, which appear so ingenious and natural at 

 first sight, Schiff has shown to be, perhaps, a little too sharply 

 defined; thus, mastication alone, that is, not accompanied 

 by any gustatory impression, has little or no influence on 

 the parotid secretion : in the case of all the salivary glands, 

 the impression of taste, joined to the masticatory movements, 

 are the most powerful means of producing secretion. 



The quantity of saliva secreted in a day has been variously 

 estimated on account of the intermittent form of secretion. 

 In dogs, it is as much as 1500 grammes. This secretion 

 though more especially sensible during mastication, however, 

 is continuous; because the saliva is necessary to keep the 

 mouth moist, to assist the movements of the tongue (speech), 

 and, as we have already said, for the purpose of deglutition. 

 We shall find that, by means of the saliva, movements of 

 deglutition are produced from time to time and at very short 

 intervals, the purpose of which is to preserve the function of 

 the organ of hearing. 



C. Deglutition. 



When the food has been so mixed with the saliva as to 

 become capable of movements like a fluid, it is subjected 

 to a pressure, which forces it downwards, from the buccal 

 cavity to the cardiac orifice of the stomach ; in other words, 

 it leaves the mouth, and passes through the pharyngeal and 

 cesophageal tubes. The principle governing the movement 

 of the food is the same as that which governs the movement 

 of fluids, that is, excessive pressure at one point and none at 

 all at others, thus destroying the equilibrium of the fluid, 

 and causing it to flow in the direction in which the pressure 

 is slightest. This principle applies to the deglutition of 

 solids, the state of semi-liquefaction into which they are 

 brought imparting to them mechanical properties similar to 

 those of fluids. 



The organs of deglutition consist (Fig. 62), first of the 

 buccal cavity, bounded, above, by the roof of the mouth; 

 at the back, by the velum of the palate; below, by the tongue ; 

 and, in front, by the teeth. After the buccal cavity, we come 

 to the pharynx, at the level of which the alimentary canal 

 communicates with the windpipe ; or, rather the two passages 

 cross each other (communication from above and behind 

 with the nasal chambers the first part of the air passage; 



