MUCOUS MEMBRANE. 
but which is quite distinct from the cuticle, 
and the great mass of that complicated struc- 
ture to which the terms ‘cutis’ and ‘ dermis’ 
are applied. 
A very strong reason for believing this mem- 
brane to be present in the skin, is the fact of 
its existence in those minute organs, so profusely 
scattered under the cutaneous surface, the se- 
baceous and sudoriferous glands. In fig. 277 
I have represented it in a portion of one of the 
latter, taken from the axilla, where they are 
very large. These glands are nothing more 
than involutions of the external tegument, and 
correspond closely with the labial and allied 
glands connected with the ordinary mucous 
membranes. It is impossible to suppose that 
a structure attaining so marked a developement 
in those parts, should be wanting in the general 
superficies, with which they are, at numberless 
points, directly continuous. ; 
In other situations, where a simple expanse 
of mucous membrane is spread out upon a 
surface of the body, as in the csophagus, 
ei mouth, nose and its sinuses, vagina, 
ladder, &c. (from all of which, however, 
_ there are numerous prolongations called follicles 
and glands, which shew this structure well,) 
a basement tissue such as that described has 
not been shown to exist. Its existence rests 
at present principally on analogy, and it is 
difficult to say whether it be not more or 
less modified. Certain of the peculiarities 
presented by these several parts depend on 
a modified form and greatly augmented mass 
of the epithelial element, but many also on 
varieties in the areolar and vascular tissues 
underlying the mucous tissue, and, properly 
speaking, forming no part of it. These will 
be treated of under the topographical descrip- 
tion of the membrane. 
Of the epithelium.—A very brief period has 
elapsed since it was universally held that most 
mucous membranes wanted epithelium, and 
their analogy with the skin was only maintained 
in this particular by a fancied resemblance 
drawn between epidermis and mucus. One of 
the principal results of microscopic observation, 
conducted with the improved modern instru- 
ments, is that of Henle, proving not only that 
this structure is present throughout the mucous 
System, but that in most situations it is so abun- 
dant as to constitute nearly the whole material of 
the tissue. This fact, as yet so novel, coupled 
with the discovery announced at the same time 
of the occurrence of a lining of analogous cha- 
racter on all internal cavities, makes the study 
of this structure under its varied forms pecu- 
liarly interesting and important. It will readily 
be conceived how wide a field is here opened 
to view, and how premature it would yet be to 
_ attempt to offer a general history of such a 
4 
4 
] 
| 
structure. The numerous questions presenting 
themselves on every side render this impossible; 
and if it were not so, the scope of the present 
_ article would oblige me to confine the descrip- 
_ tion to those forms of epithelium met with 
_ in the mucous system. In acknowledging the 
great obligations I am under to Henle’s admi- 
rable paper on this subject, I may state that 
489 
the following account has been written as much 
as possible from my own observations. 
By the term epithelium is now meant a layer 
of particles or modified cells, furnished with 
nuclei and nucleoli, lining an internal surface 
of an organized body, and by their apposition 
and union constituting a kind of pavement. 
A similar investment to an external surface is 
styled epidermis. Both these, m their ordinary 
forms, will be embraced by the following de- 
scription. 
Epithelium is an organized structure endowed 
with vitality. This is shewn by its form, the 
process of its growth, and the living properties 
it displays. Of these the most eminent is that 
of ciliary motion, which in all the higher 
animals is performed by cilia clothing the free 
surface of epithelial particles. But in very 
many situations, if not in all, the processes 
of nutrition carried on in the epithelial layer 
of the mucous system differ materially from 
those of other organic tissues ; the old elements, 
which in other cases are reconveyed into the 
blood, being here shed on the free surface of 
the membrane, and thus becoming at once 
eliminated from the system. 
The epithelial particles preserve a greater 
resemblance to the form of the development 
cell than most other tissues. In many parts 
they continue to be truly cells throughout their 
existence, and in no instance is the nucleus, 
from which they have proceeded, absorbed. 
In connection with a wide and varied range 
of function, these particles present numerous 
modifications of form, bulk, and texture, the 
leading features of which have been pourtrayed 
by Henle. The following arrangement, how- 
ever, differs in several respects from that pro- 
posed by him,* and is more in accordance with 
what I have myself observed. Founding it on 
the anatomical condition of the particles and 
on their office, I distinguish three varieties, — 
the /amelliform or scaly, the prismatic, and the 
spheroidal. These all run together by imper- 
ceptible gradations. The particles may be also 
divided into non-ciliated and ciliated, the scaly 
being always bald, the prismatic and sphe- 
roidal in some situations furnished with cilia. 
Of the lamelliform or scaly variety—This 
consists of broad flattened particles (or scales, 
properly so called), having an angular outline 
(caused by their lateral apposition) and a 
nucleus, which is generally eccentric. These 
scales form layers of extremely«variable thick- 
ness. They are generally, however, super- 
imposed in great numbers over one another, 
as in the mouth, fauces, and cesophagus of the 
human subject, where they constitute the 
Opaque defensive investment so visible to the 
eye in those parts. 
But the best-known example of this form is 
presented by the cuticle, which from its ex- 
posed position is thicker and denser than any 
internal epithelium. This variety, then, is the 
one which offers the most convincing proof of 
* He divides it into pavement epithelium (or 
the scaly), cylinder epithelium (or the prismatic), 
and ciliated epithelium. See Miiler’s Archiv. 
