MUCOUS MEMBRANES : EPITHELIUM. 61 



of the ordinary areolar tissue on which the membrane rests ; 

 this layer is copiously furnished with blood-vessels, but it is 

 seldom supplied with many nerves. Thus the mucous mem- 

 brane lining the stomach possesses in health so little sensi- 

 bility, that we are not aware of the contact of the substances 

 taken in as food, unless they are of an acrid character, or of 

 a temperature very diiferent from that of the body; and 

 though the mucous membrane lining the air-passages is very 

 susceptible of certain kinds of irritation, yet it has but little 

 ordinary sensibility in the state of health, except near the 

 entrance to the windpipe. The large supply of blood which 

 these membranes receive, has reference to their active partici- 

 pation in the functions of secretion and absorption. One 

 secretion is common to all, that of the mucus by which they 

 are covered ; this serves to protect them from the irritation 

 that would otherwise be produced by the contact of solid or 

 liquid substances, or even of air, with their free surfaces; 

 and we see the results of its deficiency, in the inflammation 

 which attacks the membrane, sometimes proceeding to its 

 entire destruction, when from any cause the secretion is 

 checked, as it sometimes is by injuries of the nerves sup- 

 plying the part. 



40. In every mucous membrane, as in the skin, the fibrous 

 texture is bounded on the free surface by basement-mem- 

 brane, beyond which no blood-vessels pass. And the surface 

 of the basement-membrane is covered by cells, arranged either 

 in a single layer or in multiple layers, constituting the 

 Epithelium. This, although answering to the Epidermis in 

 structure and position, has a very different character ; for its 

 cells neither dry up nor become horny ; nor do they adhere 

 in such a manner as to form a continuous membrane, except 

 in the interior of the mouth and oesophagus (gullet), where 

 the epithelium is endowed with somewhat of the firmness 

 of cuticle, in order to resist the abrading contact of hard 

 substances. The epithelium cells of mucous membranes are 

 commonly somewhat flattened ; but in some situations, as on 

 the villi of the intestinal canal (fig. 9, d), they have more of 

 a cylindrical, or rather conical shape, their smaller extremities 

 being in contact with the basement-membrane. The epi- 

 thelial cells are frequently cast off, like the epidermic, espe- 

 cially from the parts that are most concerned in secretion ; 



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