GENERAL CONSIDERATIONS ON THE DIGESTIVE APPARATUS. 393 



a. The innermost is called the mucous membrane, because of the mucus 

 with which its free surface is always covered. It is continuous with the skin at 

 the natural openings ; and from its similarity of organization, it has been named 

 the internal or re-entering skin, or internal tegumentanj membrane. 



It should be remarked, however, that the comparison of the mucous membrane 

 with the skin only holds good at the two ends of the apparatus — anteriorly, from 

 the mouth to the stomach ; posteriorly, at the margin of the anus. 



A mucous membrane comprises a superficial or epithelial layer and a deep 

 portion which constitutes the dermis, or chorion {corium). 



The epithelium^ is a very thin, inert pellicle, entirely composed of epithelial 

 cells united by an almost insignificant quantity of amorphous matter {Mastema). 

 The cells are flat or polygonal, round or cylindrical, polyhedral, or very irregular 

 in shape. In consequence of these diverse forms, there is pavement (or flat, 

 simple, tesselated, or squamous), spherical (or spheroidal), and cylindrical or conical 

 (or columnar) epithelium. If the cells are furnished with small filiform appendages, 

 named vihratile cilia, the epithelium is then designated ciliated. When the cells 



Fig. 210. 



Fig. 211. 



Fig. 212. 



COLUMNAR EPITHELIUM. 



1, Nucleus of the cell ; 

 2, membrane of the 

 cell raised from its 

 contents by the ab- 

 sorption of water. 



COLUMNAR CILIATED EPITHELIUM 

 (magnified 310 DIAMETERS). 



a, Nucleated cells resting on their 

 smaller extremities ; 6, cilia. 



SQUAMOUS EPITHELIUM FROM 

 THE MOUTH. 



The large scale is magnified 

 310 diameters, and exhibits 

 a nucleus with nucleolus in 

 the centre, and secondary 

 nucleated cells forming the 

 body of the scale. 



are arranged in a single layer on the surface of the corium, the epithelium is said 

 to be simple ; it is stratified (or laminar) when the cells are arranged in strata 

 upon each other. In stratified epithelium, the shape of the cells is not the same 

 on the surface and beneath it, and it is named after the form of the superficial 

 layer : examples — stratified tesselated epithelium., stratified cylindrical epithelium. 



The mucous dermis, or corium, is composed of connective tissue, the thickness, 

 elasticity, vascularity, and sensibility of which varies with the situation and the 

 function of organs. The corium is thin and almost destitute of elastic fibres 

 when applied to the bony walls of a cavity ; on the contrary, it is thick, elastic, 

 and slightly adherent when it lines organs which — like the stomach, oesophagus, 

 and intestines — are capable of increasing or diminishing in capacity. The fasciculi 

 of the connective tissue in the deeper layers of the corium are loosely united, but 

 nearer the surface they lie closer ; sometimes they form, under the epithelium, 

 an amorphous surface-layer, the basement (or limitary) membrane. In certain 

 places (intestine), the dermis has a layer of smooth muscular fibres which can be 

 resolved into two planes (the muscularis mucosce). 



The sub-epithelial face of the corium is scarcely ever smooth, but offers 

 minute prolongations named villosities, or papillm, which are very varied in their 

 form and size ; it is more or less marked by depressions, named follicles. The 

 villi are observed on the deep-seated mucous membranes (intestine) ; they are 



