DEGLUTITION 279 



By contraction of the external pterygoid muscle of one side the jaw is moved 

 toward the opposite side. In the depression of the lower jaw, the articular 

 tubercle is moved forward in a line which is concave upward, at first slowly, 

 then rapidly and at last slowly again. 



The lips, cheeks, and tongue cooperate to bring the new or incompletely 

 masticated portions of the morsel between the teeth, to facilitate saturation 

 of it with the saliva and finally to form the mass into a bolus to be swallowed. 



2. SUCKING 



The buccal cavity is capable of being closed air-tight. It can also be 

 enlarged without admission of air, and in this way suction can be produced 

 by which fluids may be drawn into the mouth. 



When the buccal cavity is closed so as to be air-tight, the tip of the tongue 

 lies against the teeth and the alveolar process of the upper jaw; the base of the 

 tongue is raised on both sides against the back teeth and the neighboring parts 

 of the upper jaw; the lower surface of the tongue rests on the edge of the lower 

 jaw, and the soft palate hangs down loosely over the root of the tongue. Holding 

 the jaws perfectly still with the parts in this position, a negative pressure of 

 from 2 to 4 mm. of Hg. prevails in the mouth (Mezger, Bonders). One can 

 easily convince himself that the cavity is really air-tight, for if he depress the 

 lower jaw without opening the mouth, the cheeks are drawn between the rows 

 of teeth. 



By drawing the tongue back or by lowering the jaw, the buccal cavity is 

 enlarged and a suction is thus produced, which becomes still stronger if the 

 tongue be drawn downward. 



The space inclosed by the mouth parts in the act of sucking is about 77 c.c., 

 three-eighths of which is due to the depression of the jaw, and five-eighths 

 to the lowering and flattening of the tongue. 



The power of suction in the human mouth is very great. It is possible 

 by repeated efforts to develop a negative pressure of 700 mm. of Hg. 



A child in nursing grasps the nipple of the breast with the lips in such 

 a way that the mouth is closed air-tight. In the absence of teeth this is 

 facilitated by special, membranous, and very vascular prominences on the 

 edge of the gums of both jaws. These structures are found in the position 

 of the four canines and, especially in the lower jaw, are united by a mem- 

 branous seam projecting 1-3 mm. high. 



3. DEGLUTITION 



We include under deglutition all those processes by which the bolus of 

 food is propelled from the mouth into the stomach. It is a very complicated 

 reflex process in which many muscles cooperate. 



After the bolus is formed and is placed on the back of the tongue, the 

 swallowing reflex is elicited by stimulation of the sensory nerves in the back 

 of the mouth. In the ape the bolus, thrown into the pharynx, excites the 

 reflex act in gliding over the tonsils (Kahn). In man no definite part of 



