486 
becomes necessary to guard against confound- 
ing such membranes with the simple 
mucous tissue, of which these and all other por- 
tions of the mucous system consist. 
Of the ultimate structure of the mucous 
system.—It has been already stated that the 
mucous tissue is essenti.lly an uninterrupted 
membrane in which the other tissues of the 
animal are contained. A very cursory attempt 
will serve to shew how much more easily it 
admits of being separated and examined ‘in 
certain situations rather than in others. This 
variety depends chiefly on the difference of its 
arrangementand connexions in different regions. 
In the testis and kidney, the capillary vascular 
rete, spread over its parenchymal surface, has 
no intimate attachment to it, and the appended 
areolar tissue is in very small quantity. In 
the testis especially, this latter may be said 
to be almost wanting as an investment to the 
individual tubules, being, as it were, disposed 
as a collective covering to the entire organ, 
sending partial septa itte its interior, and 
bearing the name of Tunica Albuginea. In 
the kidney, a more intricate vascular rete in 
a great measure supplies the place of areolar 
tissue. Hence, in these viscera, the simple 
mucous element allows of being isolated with 
remarkable facility. In the liver, its isolation 
is almost impracticable, owing to its lying in 
the interstices of a capillary plexus that may be 
termed solid, from its being extended uniformly 
in every direction. The intricacy of the inter- 
lacement of the mucous and vascular elements 
in this organ is sufficient to explain the total 
ignorance that prevails concerning the mode 
of termination of the biliary ducts, and con- 
cerning their size, shape, and connexions in the 
lobules of the gland. In many other true 
glands, the mucous tissue may be submitted to 
examination without much difficulty; examples 
of which may be seen in the pancreas and 
salivary glands, with those allied to them, such 
as the duodenal glands of Brunner, the buccal, 
palatal, arytenoid, tracheal, &c.; and in the 
sudorific glands of the skin. In the compound 
membranes also, as that of the alimentary 
canal below the cardia, and the more highly 
developed parts of the skin, the mucous ele- 
ment may be generally distinguished from the 
tissues in connexion with it in a satisfactory 
manner. But in the plane expansions of the 
simple membrane which line the bony cavities 
of the nose and ear, its isolation and the demon- 
stration of its structure are far more difficult, 
for reasons which will be afterwards explained, 
and our knowledge of it here still rests partially 
on the ground of analogy. 
In the mucous tissue there are two structures 
that require to be alee | described, viz. 
the busement membrane and the epithelium. 
The basement membrane is a simple homoge- 
neous expansion, transparent, colourless, and 
of extreme tenuity, situated on its parenchymal 
surface, and giving it shape and strength. This 
serves as a foundation on which the epithelium 
rests. The epithelium is a pavement composed 
of nucleated particles adhering together, and 
of various size, form, and number. The fol- 
MUCOUS MEMBRANE. 
lowing general observations on these element; c y 
rts will receive illustration as we advance. 
either the one nor the other is peculiar to the 
mucons tissue in the sense either of being 
invariably present in it, or of not being found 
elsewhere. There are certain situations of the 
mucous system where no basement membrane 
can be detected, and others from which the 
epithelium is absent. Both, however, are never 
absent together. Avain, a structure apparently 
identical with the basement membrane is met 
with in numerous textures besides the mucous. 
and all internal cavities, whether serous, 8} yno . 
vial, or vascular, or of anomalous kind, (as thost 
of the thymus, and thyroid body) are lined by 
an epithelium. Ss 
In the ensuing description these circum: 
stances will be enlarged upon, and the excep- 
tions and local peculiarities pointed out, as fa 
as I have been able to ascertain them. 
present, I would say that these two elem 
are generally present. The most in n 
questions in animal physiology are involved it 
the determination of the nature and offices 
these two elementary pes of the mucou 
tissue. The discovery of them is, however, t 
recent, and our knowledge of their history ; 
yet too incomplete, to allow of any ce 
conclusions on the subject. Both prese 
various modifications in different situatio 
the study of which is of great im w 
reference to their function. It will now be o 
business to descend to a particular account 
each. va 
Of the basement membrane.—-The basem¢ 
membrane of the mucous tissue, as displ 
in the kidney, is an extremely thin, transpare 
and homogeneous lamina, simple and entin 
without any aperture or appearance of structu 
(fig. 273, c). It forms the parenchymal wall” 
the uriniferous tubules; gives them their siz 
shape, and stability ; is in relation, on the one 
hand, with the vascular system of the org 
and on the other with the epithelial lini 
It is simply in contact with the capi 
plexus, which is fixed chiefly by their t 
tual interlacement; but the epithelium adh 
to it by an organic union. When detac 
from the vascular rete which it traverses, 4 
deprived of its epithelium, it readily wrn 
(fig. 273, c); and such is its tenuity, thi 
is sometimes only by the folds thus oceasi 
that it becomes visible at all. The epithe 
readily separates from it after a slight ma 
tion, and also in many diseased states 
organ, such as inflammation and Bright 
ease. Though this basement tissue is S¢ 
cate, its presence or absence in any fre 
of a separate tubule may always be ascer 
by the aspect of the marginal outline; 
be linear and well defined, the basement 
brane is present, but if irregular and br 
the epithelium only (fig. 273, a, b). Tt: 
times happens that when the epithelium 
seem to be altogether detached, the base 
membrane retains, scattered evenly Ov 
surface and at some distance apart, a nu 
of roundish marks, of the size and aspeé 
the nuclei of epithelium particles. 
