60 PHYSIOLOGY. 



This statement is substantiated very strikingly in cases of poison- 

 ing by carbolic acid and other corrosive substances. The mouth and 

 tongue, from longer contact, are always burned, while the pharynx 

 and oesophagus may escape altogether, or, at most, are but slightly 

 burned. The escape of the latter is due to the rapid transit of the 

 corrosive substance through them. However, the cardiac entrance 

 of the stomach is always very much corroded before the sphincter 

 relaxes for admission into the stomach. 



When the ingested food has been thoroughly insalivated or is 

 semisolid, there begins to be a departure from the three-stage act 

 toward the force-pump action of liquids. When the food is very 

 much liquefied the latter action is very prominent; so that any fixed 

 line for the swallowing of food or liquids does not exist. 



Nervous Control of Deglutition. 



Deglutition is a reflex act. Every reflex act requires an afferent 

 set of sensory nerves, a reflex center, and an efferent set of motor 

 nerves, that of swallowing no less so than any other. The sensory 

 nerves have their terminations in the mucous membrane of the 

 pharynx and oesophagus, including branches of the glosso-pliaryng eal 

 to the tongue and pharynx, branches of the fifth to the soft palate, 

 and the superior laryngeal branch of the vagus innervating the glottis 

 and epiglottis. The reflex center lies somewhere forward in the 

 medulla. The efferent nerves are : branches of the fifth, which sup- 

 ply the digastric, mylohyoid, and muscles of mastication; the facial, 

 which supplies the levator palati ; the glosso-pharyngeal supplies the 

 muscles of the pharynx. Stimulation of central end of the superior 

 laryngeal calls out an act of deglutition. Stimulation of central end 

 of the glosso-pharyngeal arrests it. The inferior laryngeal branch 

 of the vagus innervates the muscles of the larynx, while the hypo- 

 glossal is distributed to the intrinsic muscles of the tongue. Division 

 of the two vagi is followed by paralysis of both oesophagus and 

 stomach, with a very firm contraction of the circular band of fibers 

 guarding the cardiac orifice. Therefore these nerves send motor fibers 

 to the oesophagus and stomach, but inhibitory ones to the cardiac 

 sphincter. So firm is the tetanic contraction of the sphincter that if 

 food is swallowed after division of the vagi it accumulates within the 

 oesophagus, no part of it passing into the stomach. 



The act of swallowing inhibits the vagus center, for a single act 

 of deglutition increases the pulse-rate. This influence upon the heart- 

 beat is dependent upon neither the amount, character, nor temperature 



