416 



ALIMENTARY CANAL 



teeth. A rapid contraction of the mylo-hyoid muscles, which form 

 a floor for the front portion of the mouth {diaphr'agma oris), pushes 

 the tongue up against the hard palate. At the same time the 

 hyoglossas pulls the tongue backwards and the bolus is shot 

 towards the gullet. This closes the voluntary stage of deglutition. 



Several other muscles come into play at this point. As will 

 be seen from Fig. 62 (lower portion), just above the larynx is a 

 busy crossing common to two routes. Gaseous food and gaseous 

 excreta pass to and fro right across the track of the descending 

 bolus. At the moment of swallowing, the nose-to-lung and lung- 

 to-nose traffic is reflexly held up. Further, the escape of the 

 food mass by either of these incorrect routes is prevented as 

 follows : 



(«) The oral pharynx is closed by the action of the pharyngo- 

 palatini muscles, which form the posterior pillars of the fauces. 

 The pharynx is thus drawn to a narrow cleft. Against this narrow 

 opening the soft palate is pressed by the action of the levator and 

 tensor veil palatini nuiscles. (6) The laryngeal aperture is kept 

 closed by the action of the crico-arytcenoideus lateralis, arytcenoidei, 

 and the thyreoarytcenoidei muscles, which pull the aryta^noid 

 cartilages forwards against the back of the epiglottis. Accom- 

 panied by a quick downward motion of the tip of the epiglottis, the 

 bolus is pushed over the back of this structure and is impelled into- 

 the gullet. The following table (LXV.) shows the time relations of 

 the chief muscles engaged in deglutition (from Kronecker and 

 Meltzer and others) : 



TABLE LXV 



III. The stomach. By the impulse imparted to it at its entry 

 into the gullet, aided generally by gravity and to a questionable 

 extent by peristalsis, the bolus is forced down to the gateway of 

 the stomach. This aperture, in common with the exit from the 

 stomach, is guarded by a thick ring of \isceral nmscje. When 



