THE MECHANISMS OF DIGESTION 479 



starchy food is little chewed, the length of time required for the mastica- 

 tion of other foods depending mainly on their nature, but also to a 

 certain degree on the appetite and on the size of the mouthful. 



It can not be too strongly insisted upon that the act of mastication is 

 of far more importance than merely to break up and prepare the food 

 for swallowing. It causes the food to be moved about in the mouth so as 

 to develop its full effect on the taste buds; the crushing also releases 

 odors which stimulate the olfactory epithelium. On, these stimuli depend 

 the satisfaction and pleasure of eating, which in turn initiate the process 

 of gastric digestion (see page 470). 



The benefit to digestion as a whole of a large secretion of saliva, brought 

 about by persistent chewing, has been assumed by some to be much 

 greater than it really is, and there has existed, and indeed may still 

 exist, a school of faddists who, by deliberately chewing far beyond 

 the necessary time, imagine themselves to thrive better on less food than 

 those who occupy their time with more profitable pursuits. 



Deglutition or Swallowing 



After being masticated the food is rolled up into a bolus by the action 

 of the tongue against the palate, and after being lubricated by saliva is 

 moved, by elevation of the front of the tongue, towards the back of the 

 mouth. This constitutes the first stage of swallowing, and is, so far, a 

 voluntary act. Al^put this time a slight ^ap^t.QTy f* v nt T> a / *1n of the 

 diaphragm occurs the so-called respiration) of swallowing and the 

 mylohyoid quicklv__cpntracts. wit]LJ;hej?pnsequence that the bolus passes 

 between the pillars of the fauces. This marks the beginning of the 

 second stage, the first event of which is that the bolus, by stimulating 

 sensory nerve endings, acts on nerve centers situated in the medulla 

 oblongata so as to cause a coordinated series of movements of the 

 muscles of the pharynx and larynx and an inhibition for a moment of 

 the respiratory center (page 351). 



The movements alter the shape of the pharynx and of the various 

 openings into it in such a manner as to compel the bolus of food to pass 

 into the esophagus (see Fig. 153) : thus, (1) the soft palate becomes 

 elevated and the posterior wall of the pharynx bulges forward so as to 

 shut off the posterior nares, (2) the posterior pillars of the fauces ap- 

 proximate so as to shut off the mouth cavity, and (3) in about a tenth of 

 a second after the mylohyoid has contracted, the larynx is pulled up- 

 wards and forwards under the root of the tongue, which by being 

 drawn backwards becomes banked up over the laryngeal opening. This 

 pulling up of the larynx brings its upper opening near to the lower half 

 of the dorsal side of the epiglottis, but the upper half of this struc- 



