THE ACT OF SWALLOWING. 277 



the genioglossus muscle from the middle line (internal mental spine) backward 

 and outward, the direction of whose traction the tongue must naturally follow. 

 The tongue in killed animals sometimes exhibits fibrillary twitchings for an entire 

 day. 



THE ACT OF SWALLOWING (DEGLUTITION). 



The propulsion of the contents of the alimentary canal is effected 

 by a motor process whereby the canal contracts upon the contained 

 mass; and as this contraction progresses throughout the entire length of 

 the tube, the contents are pushed on before it. This movement is 

 called peristalsis. 



The first and most complicated act of this movement is deglutition, 

 in which the following stages can be distinguished : 



1. The mouth is closed by the orbicularis oris muscle (facial nerve). 



2. The jaws are pressed together by the muscles of mastication 

 (trigeminal nerve); in this way the lower jaw becomes a fixed point, 

 permitting the action of the muscles passing from the lower jaw to the 

 hyoid bone. 



3. The tip of the tongue, the back of the tongue, and the root of the 

 tongue are successively pressed against the hard palate, and in this 

 way the contents of the mouth Hjolus or fluid) are forced toward the 

 pharynx. 



4. When the bolus has passe^j^e anterior palatine arches, having 

 been made slippery by the mucus of the tonsillar glands, its return % 

 the mouth is prevented by the contraction of the palatoglossus muscles 

 lying in the anterior palatine arches, which bring these arches firmly 

 in contact with each other, like the scenes in a theater, and by the 

 back of the tongue, which is elevated by the styloglossus muscle. 



5. The bolus now lies behind the anterior palatine arches and the 

 root of the tongue, within the pharynx and exposed to the succes- 

 sive action of the three constrictor muscles of the pharynx, which 

 push it onward. The action of the superior constrictor muscle, which 

 contracts first, is always combined with horizontal elevation, through 

 the elevator of the veil of the palate (facial nerve), and tension of 

 the soft palate, through the tensor of the veil of the palate (trigeminal 

 nerve; otic ganglion). The superior constrictor, through the pterygo- 

 pharyngeal muscle, presses the posterior and lateral pharyngeal wall 

 firmly against the posterior edge of the veil of the palate horizontally 

 elevated and made tense like a cushion (Passavant's cushion), while the 

 edges of the posterior palatine arches are at the same time approximated 

 through the palatopharyngeal muscles. In this way the nasopharyngeal 

 cavity is closed, so that food is prevented from passing readily upward 

 into the nasal cavity. 



In persons with congenital or acquired defects of the soft palate, food can 

 enter the nose during the act of deglutition. 



The elevation of the veil of the palate can be readily demonstrated by intro- 

 ducing a fine probe through one nostril, along the floor of the nasal cavity, until 

 its posterior extremity rests upon the veil of the palate. With every movement of 



~ from the nostril, is 



nected with a gas-pipe, the other with a burner. Every movement of deglutition 

 is attended by movement of the flame. 



