526 THE MECHANICAL PROCESSES OF DIGESTION [CH. XXXV. 



who have lost their teeth severe dyspepsia is often produced, which 

 can be cured by a new set of teeth. 



DEGLUTITION. 



When properly masticated, the food is transmitted in successive 

 portions to the stomach by the act of deglutition or swallowing. 

 This, for the purpose of description, may be divided into three acts. 

 In the first, particles of food collected as a bolus are made to glide 

 between the surface of the tongue and the palatine arch, till they 

 have passed the anterior arch of the fauces ; in the second, the morsel 

 is carried through the pharynx; and in the third, it reaches the 

 stomach through the oesophagus. These three acts follow each other 

 rapidly. (1.) The first act is voluntary, although it is usually per- 

 formed unconsciously ; the morsel of food when sufficiently masti- 

 cated, is pressed between the tongue and palate, by the agency of the 

 muscles of the former, in such a manner as to force it back to the 

 entrance of the pharynx. (2.) The second act is the most complicated, 

 because the food must go past the posterior orifice of the nose and 

 the upper opening of the larynx without entering them. When it 

 has baen brought, by the first act, between the anterior arches of the 

 palate, it is moved onwards by the movement of the tongue backwards, 

 and by the muscles of the anterior arches contracting on it and then 

 behind it. The root of the tongue being retracted, and the larynx 

 being raised with the pharynx and carried forwards under the base 

 of the tongue, the epiglottis is pressed over the upper opening of the 

 larynx, and the morsel glides past it; the closure of the glottis is 

 additionally secured by the simultaneous contraction of its own 

 muscles : so that, even when the epiglottis is destroyed, there is little 

 danger of food passing into the larynx so long as its muscles can act 

 freely. In man, and some other animals, the epiglottis is not drawn 

 as a lid over the larynx during swallowing. At the same time, the 

 raising of the soft palate, so that its posterior edge touches the back 

 part of the pharynx, and the approximation of the sides of the 

 posterior palatine arch, which move quickly inwards like side curtains, 

 close the passage into the upper part of the pharynx and the posterior 

 nares, and form an inclined plane, along the under surface of which 

 the morsel descends ; then the pharynx, raised up to receive it, in its 

 turn contracts, and forces it onwards into the oesophagus. The passage 

 of the bolus of food through the three constrictors of the pharynx is 

 the last step in this stage. (3.) In the third act, in which the food 

 passes through the oesophagus, every part of that tube, as it receives 

 the morsel and is dilated by it, is stimulated to contract : hence an 

 undulatory or peristaltic contraction of the oesophagus occurs, which 

 is easily observable through the skin in long-necked animals like the 



