DEGLUTITION 165 



glossal nerves. The tongue and cheeks serve to bring and keep 

 the food between the teeth. 



Deglutition. Swallowing begins as a voluntary act and insen- 

 sibly becomes an involuntary reflex. The voluntary part 

 of the swallowing consists in the manipulation of the bolus 

 of food so that it lies on the upper surface of the tongue. The 

 anterior part of the tongue is then elevated against the hard 

 palate. The elevation travels backward with considerable 

 pressure from tip of tongue to the base, when a sudden sharp 

 contraction of the mylohyoids places the bolus under such 

 pressure that it is shot backward through the isthmus of the 

 fauces and into or through the pharynx. Simultaneously 

 with the passage of the food through the pharynx a large 

 number of muscles by their coordinated contractions have 

 closed off the openings from mouth to posterior nares and to 

 the larynx. The levator palati and tensor palati muscles 

 have raised the soft palate; at the same time the contraction 

 of the superior constrictor has brought the posterior wall of 

 the pharynx forward to meet the soft palate. The soft palate, 

 the uvula and the posterior wall of the pharynx form, together, 

 a sloping surface which completely closes the pharynx from 

 the posterior nares. 



Immediately after the contraction of the mylohyoids the 

 larynx is pulled upward and forward by the contraction of 

 the thyrohyoid muscle and by the elevation of the hyoid bone. 

 The base of the tongue is at the same time drawn backward 

 by the stylo- and palatoglossus. The epiglottis is thus caused 

 to come into contact with the orifice of the larynx. The glottis 

 is also closed by the approximation of the vocal cords, brought 

 about by the contraction of the lateral cricoarytenoid muscles. 

 That the epiglottis is not absolutely necessary to swallowing 

 has been determined by surgical procedures. 



A bolus of food, if of same size and consistency, is now 

 grasped by the constrictors of the pharynx and moved into 

 the esophagus, along which it is carried by peristalsis. Peri- 

 stalsis consists of a constricting wave preceded by a wave 

 of inhibition. In conformity with the character of the muscle 

 tissue found in the wall of the esophagus, peristalsis moves 

 more rapidly in the upper than in the lower portion. In man 

 the peristaltic wave reaches the cardiac sphincter guarding 



