202 DIGESTION MASTICATION, INSALIVATION, DEGLUTITION. 



is very large. It seems impossible that the fluid thus incorporated with the 

 food should not have an important influence on the changes which take 

 place in the stomach, although it must be confessed that information on this 

 point is very meagre, except as regards the digestion of starch. 



It is undoubtedly the abundant secretion of the parotid glands which 

 becomes most completely incorporated with the food during mastication and 

 which serves to unite the dry particles into a coherent mass. The secre- 

 tions from the submaxillary and sublingual glands and from the small 

 glands and follicles of the mouth, being more viscid and less in quantity than 

 the parotid secretion, penetrate the alimentary bolus less easily and form a 

 glairy coating on its exterior, agglutinating the particles near the surface 

 with peculiar tenacity. 



When the processes of mastication and insalivation have been completed, 

 and the food has passed into the pharynx, it meets with the secretion of the 

 pharyngeal glands, which still farther coats the surface with the viscid fluid 

 which covers the mucous membrane in this situation, thus facilitating the 

 first processes of deglutition. 



It has been observed that the saliva engages bubbles of air 'in the ali- 

 mentary mass. In mastication, a considerable quantity of air is mixed with 

 the food, and this facilitates the penetration of the gastric juice. It is well 

 known that moist, heavy bread, and articles that can not become impregnated 

 in this way with air, are not easily acted upon in the stomach. 



DEGLUTITION. 



Deglutition is the act by which solid and liquid articles are passed from 

 the mouth into the stomach. The process involves first, the passage, by an 

 automatic movement, of the alimentary mass through the isthmus of the 

 fauces into the pharynx ; then a rapid contraction of the constrictors of the 

 pharynx, by which it is forced into the oesophagus ; and finally, a peristaltic 

 action of the muscular walls of the oesophagus, extending from its opening at 

 the pharynx to the stomach. 



Physiological Anatomy of the Parts concerned in Deglutition. The parts 

 concerned in this process are the tongue, the muscular walls of the pharynx 

 and the oesophagus. In the passage of food and drink through the pharynx, 

 it is necessary to completely protect from the entrance of foreign matters a 

 number of openings which are exclusively for the passage of air. These are 

 the posterior nares and the Eustachian tubes above, and the opening of the 

 larynx below. 



The tongue a muscular organ capable of a great variety of movements 

 is the chief agent in the first processes of deglutition. A study of the 

 muscles which are brought into action in deghitition^ would involve an ana- 

 tomical description so elaborate as to be out of place in this work. The 

 movements of the tongue, however, will be described in connection with the 

 mechanism of deglutition. 



The pharynx, in which the most complex of the movements of deglutition 

 take place, is an irregular, funnel-shaped cavity, its longest diameter being 



