MUCOUS MEMBRANES :-EPITHELIUM. 51 
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 different 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; 
e 2 
