THE MOUTH SWALLOWING ; SALIVARY DIGESTION 65 



nated with food particles and bacteria. The deposit 

 should be removed by the dentist at regular intervals. 



The three glands furnish slightly different varieties of 

 saliva. Mucin, the essential compound in mucus, is pres- 

 ent in thfTsecretion of the two lower glands and not in 

 that of the parotid. It gives to the saliva from the sub- 

 maxillary and sublingual glands a ropy, mucilaginous 

 character, which, of course, becomes more apparent when 

 evaporation has concentrated the solution. This is illus- 

 trated when the mouth is dried by rapid breathing during 

 exercise and becomes furred with the residue of salivary 

 mucin. This constituent of the saliva probably makes it 

 superior to water as an agent for molding the food into 

 pellets. The most interesting property of the secretion, 

 its power to hydrolyze^tarch^may be discussed to more 

 advantage aTEerwtf have followed the food to the stomach. 

 Clearly, there is not time enough for much digestive change 

 in the mouth of a person of average habits. 



Swallowing. The transfer to the stomach is a more 

 complex matter than is likely to be realized. It involves 

 an interruption of breathing and the protection of the nasal 

 passages and the larynx against the intrusion of food. The 

 first purpose is effected by the swinging back of the soft 

 palate against the back of the pharynx. The second is 

 accomplished by the drawing forward of the larynx toward 

 the chin, a movement which can be plainly felt. By it 

 the larynx is tucked under the root of the tongue and over- 

 laid by the epiglottis. The same motion serves to widen 

 the upper part of the esophagus, which is not usually ap- 

 preciably open. With the parts in this position the bolus 

 of food is crowded back from its original seat upon the 

 tongue and urged through the pharynx by the successive 

 contraction of the bands of muscle which surround it. 

 As soon as it is fairly within the esophagus the soft palate 

 is lowered, the larynx is allowed to emerge from its covert, 

 and the breathing can be resumed. Such quick and well- 

 ordered adjustments give evidence of co-ordinated reflex 

 action, the contact of the food morsel with one spot after 



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