MUCOUS MEMBRANE. 
Where mucous membranes are not destined 
to move on the parts they cover, the areolar 
tissue beneath them is very scanty. This is 
the case in the nasal cavities, even in the por- 
tions furnished with a great substratum of 
bloodvessels. But where much motion is re- 
quired, as where a muscular lamina underlies 
the mucous, and the enclosed cavity is liable to 
vary in its dimensions, the areolar tissue is co- 
pious, and very similar in its elements and in 
the size of its interstices to the ordinary forms. 
Examples of this are seen in the whole alimen- 
tary tract. 
But it is under the cutaneous part of the mu- 
cous system that this tissue assumes its highest 
developement. Elsewhere its object is to pro- 
mote freedom of movement, or to confer elasti- 
city. Here it answers both these purposes, and 
in addition gives a great capacity of resistance 
against external pressure and violence. The 
former end is attained by the structure called 
Subcutaneous fascia, which is a large quantity 
of this tissue in its ordinary form. The two 
latter are effected by that more condensed part 
to which the term of cutis has been given. This 
last is the structure to which the submucous 
areolar tissue of the intestinal canal mainly cor- 
responds, as may be shown by an examinatioh 
of the submucous tissue of the mouth, pharynx, 
and esophagus, which holds an intermediate 
place. To describe its modifications in different 
Situations would be to encroach too much on 
the province of another article (see Skin), and 
a few general remarks must here suffice. 
The framework of the cutis may be said to 
consist entirely of a modified form of the areolar 
tissue. Both elements are enormously deve- 
loped, but especially the yellow fibrous one. 
The fibrille of this are thicker than elsewhere, 
and branch and inosculate with great freedom, 
enclosing interstices open on all sides, and 
giving passage to the wavy bands of the white 
fibrous element as well as to vessels, nerves, 
the ducts of the sweat-glands, the sebaceous 
glands, and the roots of the hairs. These in- 
terstices are in general very close, but they vary 
with the size of the parts which occupy them. 
On the deep surface of the cutis the yellow 
fibrous element changes gradually into that of 
the subcutaneous fascia, or that of ordinary 
areolar tissue. It cannot be doubted that the 
skin chiefly owes its elasticity and toughness to 
this remarkable developement of the yellow 
fibrous element. 
Lopographical view of the mucous system in 
man.— Referring the reader to the article Skin 
for a detailed description of that part of the 
mucous system, and its immediate dependen- 
cies, I shall now proceed to point out some of 
the more remarkable varieties of the internal 
tracts. These tracts have been usually com- 
prehended under two general divisions, the 
_ gastro-pulmonary, and the genito-urinary. The 
_ former is continuous with the skin at six points, 
the two eyelids, the two nostrils, the mouth, 
and the anus; the latter at a single one, the 
orifice of the urethra in the male, and the labia 
_ pudendi in the female. Besides these, there 
are two smaller tracts, the mammary, each of 
495 
which is subdivided into several, which open 
separately on the skin. 
The description of the gastro-pulmonary 
tract may be commenced at the lips. It covers 
their inner surface, the cheeks, gums, tongue, 
and palate, and extends into the labial, buccal, 
and larger salivary glands, of which it consti- 
tutes the chief mass. It passes over the arches 
of the palate, (where its involutions form the 
tonsils,)and lines the pharynx, Eustachian tubes, 
and the cavities of the tympana. Penetrating 
into the nose by the posterior nares, it lines 
all the passages and chambers of that organ, 
and advances along the nasal duct to the lachry- 
mal sac. Thence it may be traced along the 
canaliculi to the front of the eye, where it takes 
the name of tunica conjunctiva; covers the 
posterior surface of the eyelids, a certain por- 
tion of the sclerotic, and the cornea, and forms 
the caruncula, the Meibomian and lachrymal 
giands. In these complicated portions of its 
course, the membrane shares more or less in the 
construction of the five organs of special sense, 
and is the essential seat of two of them, taste 
and smell. From the pharynx it spreads in 
two directions; first, into the larynx, trachea, 
tracheal glands, and bronchial ramifications, 
until it terminates by forming the air-cells of 
the lungs; secondly, into the alimentary 
canal. Here it lines the esophagus, stomach, 
and intestinal tube, as far as the anus, and it 
penetrates along the excreting ducts of the liver 
and pancreas, into the inmost recesses of those 
glands, to form their secreting surface. 
The genito-urinary tract may be traced along 
the urethra into the bladder, ureters, and pelvis 
of the kidneys; and thence into the substance 
of those organs as far as the Malpighian bodies, 
the extremities of the uriniferous tubules.. In 
connection with the urethra, processes pass to 
the glands of Cowper; and, in the male, 
into the interior of the prostate, the vesicule 
seminales, vasa deferentia, and tubules of the 
testes. In the female, the vagina, uterus, and 
Fallopian tubes receive a lining from it, which, 
at the fimbriated extremity of those canals, be- 
comes continuous with the serous membrane of 
the abdomen.” 
The very remarkable differences presented to 
the eye by different parts of this system have 
been a source of great difficulty to anatomists, 
who, on other grounds, believed them to be 
nearly allied ; and it would appear that hitherto 
no satisfactory explanation has been given of 
the anatomical conditions on which this variety 
depends. This deficiency I shall now endea- 
vour in some degree to supply. From the ex- 
aminations I have made, I have been led to 
consider in a distinct and separate manner the 
several elementary tissues already mentioned, 
composing the simple mucous membrane, and 
* This remarkable exception to a general fact 
has long attracted attention. As a mere anatomi- 
cal difficulty, it has lately received curious illustra- 
tion from Henle’s discovery of the existence of an 
epithelium on serous and other allied surfaces. But 
its true explanation can_ probably only be attained 
by a study of its morphology, joined with that of its 
final cause. 
