1002 DIGESTION 



the entrance of the food into the upper extremity of the esophagus. 

 The third is restricted to the esophagus and terminates with the 

 arrival of the food in the cardiac end of the stomach. It is also to be 

 noted that the first is effected by striated muscle, and constitutes, there- 

 fore, a voluntary act, whereas the last two are due almost wholly to 

 the contraction of smooth muscle tissue and are, therefore, involuntary 

 or reflex in their nature. In spite of these functional differences, how- 

 ever, deglutition is a continuous act and no pauses occur between its 

 successive phases. Further, the initiation of the first invariably means 

 the completion of the third, although some persons may acquire a 

 limited volitional control over the second. 



Immediately before the beginning of the first stage, the process of 

 mastication is suspended. Respiration is arrested after a slight con- 

 traction of the diaphragm, constituting the so-called "respiration of 

 swallowing." The lips are closed and the maxillae closely approxi- 

 mated. The tip of the tongue is then elevated and pressed against the 

 the inner aspect of the upper gum. The muscles effecting this move- 

 ment are the inner longitudinal strands of the tongue which are con- 

 trolled by the hypoglossal nerve. This elevation then progressively 

 involves the entire tongue from before backward, forcing the bolus in the 

 same direction through the fauces. This movement brings into play the 

 muse, mylohyoideus (nerv. trigeminus) which raises the hyoid bone, 

 as well as the muse, styloglossus, muse, palatoglossus and, in an in- 

 direct manner, also the muse, stylohyoid (nerv. facialis). The latter 

 elevate the back of the tongue, so that its inherent muscle strands may 

 progressively obstruct the posterior extent of the oral cavity. 



As soon as the bolus has been forced through the fauces, it is 

 brought under the control of the three sphincters of the pharynx which 

 direct it into the upper extremity of the esophagus. This process 

 necessitates a temporary obstruction of the nasal and laryngeal 

 cavities. The closure of the first is brought about by the simultaneous 

 contraction of the levator palati and palato-pharyngeus muscles, the 

 uvula being at this time forced in contact with the posterior pillars, and 

 the latter in turn with the upper posterior wall of the pharynx. The 

 closure of the epiglottidean orifice necessitates the elevation of the 

 hyoid bone and an upward and forward movement of the larynx. The 

 former is brought about by the contraction of the geniohyoid, anterior 

 belly of the digastric and mylohyoid, and the latter, by the contraction 

 of the thyrohyoid. At this time, the back of the tongue is pulled 

 backward by the contraction of the styloglossus, thereby forcing the 

 epiglottis downward across the laryngeal orifice. A still firmer 

 closure of this passage is effected by the contraction of the reflector 

 epiglottis and aryepiglotticus, as well as by the constriction of the 

 glottis itself. Stuart and McCormick, 1 however, have show r n that the 

 removal of the epiglottis does not seriously interfere with the act of 



1 Jour, of Anat. and Physiol., 1892; also: Kanthak and Anderson, Jour, of 

 PhysioL, xiv, 1893, 154. 



