THE DIGESTIVE SYSTEM 



307 



Seven cavities open into the pharynx, the mouth, the two nasal 

 passages, the two Eustachian tubes, the larynx, and the esophagus. 

 Three divisions may be distinguished, oral, nasal, and laryngeal. The 

 palatine tonsils lie in the oral portion, the nasal passages and Eustachian 

 tubes open into the nasal portion, while the larynx opens into the laryngeal 

 portion. When food enters the pharynx the entire pharynx is raised by 

 the contraction of the stylo-pharyngeal muscles while the constrictor 

 muscles of the pharynx squeeze the bolus towards the esophagus. The 

 nerve supply of the pharynx comes chiefly from the glossopharyngeal. 

 Like the mouth cavity, most of the pharynx is Hned by stratified squamous 

 epitheHum. The nasal portion, however, is lined with ciliated columnar 

 epithelium. 



Since the pharyngeal region is closely associated with the respiratory 

 organs of vertebrates, the description of the evolution and development 

 of the pharynx is omitted here and will be found in the following chapter. 



The Esophagus 



The esophagus is that portion of the alimentary canal which extends 

 from the pharynx to the stomach. It is nearly ten inches in length and is 

 the narrowest part of the digestive tract. From the pharynx it passes 

 just beneath the back bone through the mediastinum and diaphragm 

 to the cardiac region of the stomach. 



The wall of the esophagus consists of the four layers characteristic 

 of the digestive tract, tunica mucosa, tunica submucosa, tunica muscularis, 

 and tunica adventitia ; but the serous layer which covers the stomach and 

 intestine is wanting in the esophagus, since the body cavity lined by the 

 serosa does not extend into the neck. The tunica mucosa includes not 

 only the stratified squamous epithelium which fines the esophagus, but 

 also a connective tissue tunica propria and a muscularis mucosae, a thin 

 layer of longitudinal muscle fibers. The muscular coat of the esophagus 

 consists of striped fibers in the upper third, while those of the lower two- 

 thirds are smooth. In the contracted state of the esophagus, the mucosa 

 is thrown into longitudinal folds fike those of the stomach. (Fig. 258) 



The submucosa is a layer of loose connective tissue containing glands 

 and many blood and lymph vessels. The tunica muscularis consists of an 

 inner layer of circular muscles and an outer longitudinal layer. The 

 connective tissue between them contains a plexus of sympathetic nerve 

 fibers. By the wave-fike peristalsis of the circular muscles food is con- 

 veyed from the pharynx to the stomach. 



Development of the Esophagus. Beginning with the fourth week, 

 the esophagus develops as an elongation of the fore-gut between pharynx 

 and stomach. Its single-layered columnar epithelium becomes gradually 

 converted into a stratified squamous epitheHum like that which lines 



