160 TEXT-BOOK OF PHYSIOLOGY 



of the tongue. It is triangular in shape, wide in front, narrow behind, and 

 directed downward and backward. It is bounded above by a thin plate of 

 cartilage, the epiglottis, placed just behind the tongue and so arranged that 

 it can easily be depressed and elevated. 



The Esophagus. The esophagus, the continuation of the deglutitory 

 canal, extends downward from the lower border of the cricoid cartilage for a 

 distance of from 22 to 25 centimeters, to a point opposite the ninth thoracic 

 vertebra, where it expands into the stomach. Its walls are composed of an 

 internal or mucous and an external or muscle coat, united by areolar tissue. 

 The muscle coat consists of an external layer of longitudinal fibers arranged 

 in three bands and of an internal layer composed of fibers arranged circularly 

 in the upper part and obliquely in the lower part of the esophagus. In the 

 upper third the fibers are striated; in the middle third they are a mixture of 

 both striated and non-striated; in the lower third they are entirely non- 

 striated. 



The muscle-fibers surrounding the esophago-gastric orifice are arranged 

 in the form of and play the part of a sphincter muscle, and for this reason 

 may be termed the sphincter cardia muscle. By its action it prevents 

 under normal conditions a return of food into the esophagus. 



The deglutitory act may be for convenience divided into three stages, viz. : 



1. The passage of the food from the mouth into the pharynx. 



2. The passage of the food through the pharynx into the esophagus. 



3. The passage of the food through the esophagus into the stomach. 



In the first stage the bolus of food is placed on the superior surface of the 

 tongue. The mouth is then closed and respiration is momentarily sus- 

 pended. The tip of the tongue is placed against the posterior surfaces of the 

 teeth. The tongue, by reason of its intrinsic musculature, then arches from 

 before backward against the roof of the mouth and by the contraction of the 

 hyoglossus muscle pushes the bolus of food through the isthmus of the fauces 

 into the pharynx. This completes the first stage. It is a voluntary effort 

 and accomplished partly by the tongue, though, as shown by Meltzer, mainly 

 by the mylohyoid muscles. 



The second and third stages, or the passage of the food through the 

 pharynx and esophagus into the stomach, have been attributed until quite 

 recently entirely to peristaltic movements of their musculature. 1 It has been 

 stated that with the passage of the food through the isthmus of the fauces the 

 posterior wall of the pharynx advances and seizes the food, which, in 

 consequence of a rapid peristaltic movement running through the constrictor 

 muscles from above downward is transferred to the esophagus; that with the 

 entrance of the food into the esophagus a similar peristalsis, varying in rapidity 

 in different sections in consequence of a change in the character of its 

 musculature, gradually transfers the food into the stomach. There can be 

 but slight doubt that by this method the bolus of food, especially if it is of firm 

 consistence and of a size sufficient to distend the esophagus, is transferred 

 into the stomach, but that it is not the case with liquids has been demon- 

 strated by Kronecker, Falk, and Meltzer. 



In 1880 the first of these experimenters made the observation that the 



1 Peristalsis may be defined as a progressive wave-like movement which passes over different 

 portions of the walls of the alimentary canal. Its effect physiologically is the propulsion of its 

 solid and semisolid contents. It is characterized by a contraction of the muscle-fibers behind 

 the object and an inhibition or relaxation of the muscle-fibers in front of it. (Bayliss and Starling.) 



