252 



THE DIGESTIVE SYSTEM 



Lymphoid tissue occurs in considerable abundance in the oral 

 mucous membrane. Areas of diffuse lymphoid tissue are of fre- 

 quent occurrence and small lymphatic nodules are occasionally 



a c d ~b 



FIG. 212. FROM A SECTION THROUGH THE LIP OF AN INFANT. 



a, cutaneous surface; 6, epithelium of the oral mucosa; c, layer of striated muscle; 

 c?, layer of mucous glands. Hematein and eosin. Photo, x 10. 



found. The lymphatic vessels form a plexus in the tunica propria, 

 which empties into larger vessels in the submucosa. 



Secreting glands occur in considerable abundance in all por- 

 tions of the buccal mucous membrane except that covering the 

 gums. The glands are of the tubulo-acinar type and produce 

 either a pure mucous secretion or, in the case of the larger ones, a 

 mixed mucous and serous secretion. The ducts of the glands are 

 lined by columnar cells which, near the mouth of the duct, offer a 

 gradual transition to the stratified epithelium of the mucosa. The 



