Chap. XIV] THE DIGESTIVE SYSTEM 261 



be performed, the muscles draw the pharyngeal bag upward and 

 dilate it to receive the food ; they then relax, the bag sinks, and 

 the other muscles contracting upon the food, it is pressed down- 

 ward and onward into the oesophagus. 



THE (ESOPHAGUS, OR GULLET 



The oesophagus is a comparatively straight tube, about nine 

 inches (225 mm.) long, which commences at the lower end of the 

 pharynx, behind the trachea. It descends in front of the spine, 

 passes through the diaphragm, and terminates in the upper or 

 cardiac end of the stomach. 



Structure. — The walls of the oesophagus are composed of three 

 coats : (1) an external or muscular, (2) a middle or areolar, and 

 (3) an internal or mucous, coat. The fibres of the muscular coat 

 are arranged in an external longitudinal and in an internal circular 

 layer. Contraction of the outer layer produces dilatation of the 

 tube; contraction of the inner, constriction. Consequently this 

 arrangement is of importance in the movements which carry the 

 food from the pharynx to the stomach. These movements are 

 called peristaltic and consist of contraction of the longitudinal 

 fibres, followed by contraction of the circular fibres. The areolar 

 coat serves to connect the muscular and mucous coats. The 

 mucous membrane is disposed in longitudinal folds which disap- 

 pear upon distention of the tube. 



Function. — The oesophagus serves (1) to connect the pharynx 

 with the stomach, and (2) to receive the food from the pharynx 

 and by a series of peristaltic contractions pass it on to the 

 stomach. 



Regions of the abdomen. — That portion of the alimentary 

 canal which is below the thorax is contained in the abdomen. 

 For convenience of description, the abdomen may be artificially 

 divided into nine regions by drawing the following arbitrary 

 lines : — 



1. Draw a circular line around the body at the level of the 

 tenth costal cartilages. 



2. Draw another circular line at the level of the anterior superior 

 spines of the ilia. 



3. Draw a vertical line on each side from the centre of Poupart's 

 ligament upward. 



