THE MECHANICS OF" DIGESTION 1003 



swallowing, because the backward movement of the tongue and upward 

 deviation of the larynx usually suffice to prevent an ingress of food 

 into the respiratory passage. 



At about the level of the closed epiglottidean orifice, the bolus is 

 brought under the influence of the middle and inferior constrictors of 

 the pharynx, the successive contractions of which force it into the 

 upper segment of the esophagus. It has been shown by Kronecker 

 and Falk^ that fluids pass more rapidly and usually do not require a 

 concerted action of the parts just enumerated; in fact, the movements 

 of the back of the tongue generally suffice to direct them through the 

 relaxed upper segment of the esophagus into its lower portion. It is 

 for this reason that some persons, under abolition of the pharyngeal 

 reflexes, are able to pour considerable quantities of water almost 

 directly into the cardia. This also explains the fact that the erosions 

 produced by the hasty intake of corrosive fluids, are usually most 

 severe in the lower esophagus. 



It has been shown by Cannon and Moser^ that the progression of 

 semi-solid food through the esophagus takes place much more leisurely, 

 and is effected by peristaltic waves which proceed from above down- 

 ward. It will be remembered that the smooth musculature of this 

 membranous tube is arranged in two layers, namely, as an inner 

 circular and an outer longitudinal coat. A peristaltic wave, however, 

 does not consist solely of a contraction of the circular fibers, but pre- 

 sents itself in all instances as a progressive wave of constriction which 

 is anteceded by a wave of relaxation, the bolus being driven ahead of 

 the contracting band of muscle tissue in the direction of least resistance. 

 But since the upper and even the middle segments of the esophagus 

 contain a few strands of striated muscle tissue, it cannot surprise us to 

 find that the progress of the bolus is more rapid above than in the vi- 

 cinity of the cardia. According to Schreiber,^ the entire act of peristal- 

 sis for semi-solid food consumes about 6 seconds, about one-half of 

 this period being occupied by the passage of the bolus through the lower 

 segment of the esophagus. 



A very appreciable retardation also results at the cardiac sphincter 

 which guards the gastric orifice of the esophagus. This circular 

 ring of smooth muscle tissue relaxes only under the gradually increas- 

 ing force of the newly arrived bolus. Obviously, this mechanism pre- 

 vents the sudden ingress of the food into the stomach as well as its 

 immediate projection into the fundic portion of this organ. ^ On 

 listening over the region of the cardia when fluid is taken, two sounds 

 are heard, the first of which is produced by its sudden projection 



1 Archiv fiir Anat. und Physiol., 1880, 296. 



2 Am. Jour, of Physiol., i, 1899, 435, and Eykmann, Pfluger's Archiv, xcix, 

 1903, 513. 



3 Archiv fiir exp. Path, und Pharm., xlvi, 1901, 414. 



^ Beaumont's observations upon Alexis St. Martin, also Hertz, Guy's Hosp. 

 Rep., London, 1907. 



