MUCOUS MEMBRANES. 



1529 



FIG. 1283. 



Epithelium 



nerves and the blood-vessels and the beginnings of the lymph-radicles. Thus it will 

 be seen that the general structure of a mucous membrane corresponds closely with 

 that of the integument, the protecting epidermis of the latter being represented by 

 the epithelium of the former, while both the corium and the tunica propria include 

 the connective-tissue basis over which the epithelial layer stretches. A stratum of 

 submucous tissue, corresponding with the subcutaneous layer in the skin, connects 

 the mucous membrane with the surrounding structures. 



The epithelium may be squamous or columnar, simple or stratified. Its char- 

 acter is usually determined by the conditions to which it is subjected ; thus, where 

 covering surfaces exposed to mechanical influences of foreign bodies, it is commonly 

 stratified squamous, as in the upper part of the digestive tract. Where, on the other 

 hand, the mucous membrane is concerned in facilitating absorption, as in the intestinal 

 tube, the epithelium is simple columnar in type. In localities in which the existence 

 of a current favors the function of an organ, either as a means of freeing the surface 

 from secretion or particles of foreign matter, as in the respiratory tract, or of propul- 

 sion through a tube, as in the epididymis or the oviduct, the epithelium is of the 

 ciliated columnar variety. Modifications of the epithelial cells, due to the presence 

 of pigment or of secretion, distinguish certain mucous membranes, as those clothing 

 the olfactory region and the large intestine respectively. 



The tunica propria or stroma consists of interlacing bundles of fibro-elastic tissue 

 which support spindle or stellate connective-tissue cells. The latter usually lie within 

 the uncertain clefts between the 

 stroma bundles, which may be re- 

 garded as lymph-spaces. In many 

 localities the surface of the tunica 

 propria is beset with numerous ele- 

 vations or papilla, over which the 

 epithelium extends. Such irregu- 

 larities, when slight, may not modify 

 the free surface of the mucous mem- 

 brane, since the epithelial layer com- 

 pletely fills the depressions between 

 the elevations ; when more pro- 

 nounced, the papillae or folds of the 

 connective tissue produce the con- 

 spicuous modelling of the surface 

 seen in the papillae of the tongue 

 or the rugae of the vagina. The 

 papillae contain the terminal loops 

 of the blood-vessels and the nerves 

 supplying the mucous membrane. 

 Where especially concerned in ab- 

 sorption, the mucous membranes often gain increase of surface by cylindrical eleva- 

 tions, or villi, as conspicuously seen in the small intestine. These projections, 

 consisting of the stroma covered by epithelium, contain the absorbent vessels, or 

 lacteals, in addition to the blood-capillaries. 



A more or less well-defined line separates the epithelium from the subjacent 

 tunica propria. This demarcation is the basement membrane, or membrana propria, 

 a detail which has been variously interpreted. Usually the basement membrane 

 appears as a mere line beneath the epithelium, and is then, probably, formed by the 

 apposition of the basal processes of the epithelial cells. When surrounding glandular 

 tissue it is better developed, presenting a distinct and much more robust structure. In 

 these positions the basement membrane is probably a product of the tunica propria and 

 occurs in two types, sometimes being homogeneous, at other times reticular (Flint 1 ). 



In many localities the deepest part of the mucous membrane, next the submu- 

 cous tissue, is occupied by a narrow layer of involuntary muscle, the muscidaris 

 mucosee. While not everywhere present, it is especially well developed in the intes- 

 tinal tract from the gullet to the anus, and in places consists of two distinct layers, 

 1 American Journal of Anatomy, vol. ii., No. i, 1902. 



Section of mucous membrane of oesophagus. X 55. 



