THE MUCOUS MEMBRANE. 135 



The mucous membrane presents different appearances in 

 the different organs it traverses. It is disposed in longitu- 

 dinal folds, is thick and loosely attached to the muscular 

 coat in the oesophagus. It presents the form of plaits 

 or rugae in the stomach, and of valvulaa conniventes in the 

 upper intestines. 



The structure of the mucous membrane is very analogous 

 to that of the skin, and like it, consists of three membranes, 

 an epithelium, a proper mucous and a fibrous coat. The 

 epithelium corresponds with the cuticle, and consists of 

 nuclei, vesicles, and scales. It has, until very recently, been 

 considered as extending inwardly only to the cardiac ori- 

 fice of the stomach, but by the microscope it now seems to 

 be satisfactorily established as covering the whole extent 

 of the mucous surface wherever found. 



The epithelium presents a variety of 

 forms in different situations. In the 

 mouth (Fig. 20) it assumes the shape of 

 laminae, the nuclei or cytoblasts forming 

 the deepest layer, then upon these are the 

 cells, and upon these again the topmost 

 layer of polygonal scales, which become thin and flattened, 

 and constitute the highest stage of development from the cell 

 germ or nucleus. The nuclei, cells and scales are connected 

 by a glutinous substance, in which are found opaque gran- 

 ules. The scales are constantly exfoliating, and give place 

 to the deeper layer, which in their turn give way to others, 

 and so in perpetual succession, there is a perpetual waste 

 and supply. 



In the stomach and intestines, the epithelium (Fig. 21) 

 has the columnar or cylindrical shape, the apices of the 

 columns resting on the papillary coat, while the bases, by 

 their approximation, form the free surface. Each column 

 has its nuclei, cells and scales, and is produced in the same 

 way as the laminated epithelium, and also undergoing the 

 constant waste and supply. 



FIG. 20. Epithelium scales from inside of the mouth. 



