THE ALIMENTARY CANAL 347 



submucous (Meissner's) plexus. Its nerve trunks and ganglia are some- 

 what smaller than those of the myenteric plexus. The submucous 

 plexus supplies the muscular and glandular tissues of the mucous mem- 

 brane. 



The Mucous Membrane. The mucous coat or mucosa of the 

 gastro-intestinal canal contains four typical structures, (1) an internal 

 lining epithelium; (2) the muscularis mucosa? which forms the outer- 

 most layer; between these is (3) a tunica propria or corium of diffuse 

 lymphoid or areolar tissue, which serves chiefly for the support of (4) 

 the secreting glands. 



The muscular!* ntucosce usually consists of a double layer of smooth 

 muscle, the outer being longitudinally, the inner circularly disposed. 

 This layer is most highly developed in the esophagus. 



The tunica propria consists of delicate fibro-elastic and reticular 

 tissue whose volume is in inverse proportion to that of the secreting 

 glands. It is most abundant in the esophagus. In the stomach and 

 intestines it is considerably infiltrated by lymphocytes and often con- 

 tains diffuse lymphoid tissue. Small lymph nodules are also found 

 in the deeper part of this membrane; they progressively increase in 

 size toward the lower portion of the tract, where they form the solitary 

 nodules of the intestine. 



The nature of the lining epithelium and the type of glands differs 

 in each succeeding portion of the canal, and must, therefore, together 

 with the other peculiarities of the several subdivisions of the tract, 

 be separately considered. 



THE PHARYNX 



The pharynx may be subdivided, upon histological as well as physio- 

 logical grounds, into (1) an upper respiratory portion, or nasopharynx, 

 and (2) a lower portion, oropharynx and laryngopharynx ; only the 

 latter of these properly belongs to the alimentary tract. Tin- soft palate 

 and uvula form a thin partition between the naso- and on. pharynx. The 

 nasopharynx has already been described as a part of the respiratory 

 system (see Chapter XII). 



The mucous membrane of the lower portion of the pharynx is lined 

 by stratified squamous epithelium which rests upon a thick curium of 

 areolar tissue. The tunica propria is well supplied with thin-walled 

 blood-vessels and lymphatics, and contains many miirus-sivivting glands 

 of the tubulo-acinar type whose secreting portions lie deeply embedded 

 in the connective tissue of the muscular coat. 



