492 
difficult to reject the belief that it is intimately 
concerned jn the glandular function, and varies 
in correspondence with it. 
To the preceding summary account of these 
three principal kinds of epithelium much might 
be added respecting the intermediate forms. 
This, however, does not appear to be required 
in so general a description. The spheroidal 
and the prismatic are seen blended in the speci- 
men I have figured from the human membrana 
bay fp (fig. 281). 
Mf the non-ciliated and ciliated epithelium.— 
The true scaly variety appears never to be 
clothed with cilia. The prismatic epithelium 
is that which most commonly bears these vibra- 
tile organs. They are placed on the free extre- 
mities of the prisms in the respiratory tract and 
in the uterus and Fallopian tubes. The true 
piped epithelium is always without cilia. 
is is a general fact, and one of great import- 
ance. But those varieties which seem iuterme- 
diate between the spheroidal and the other two 
forms are often furnished with cilia ; of which 
examples may be seen in the Malpighian bodies 
of the kidney, in the mucous membrane of the 
frog’s mouth, and in that of the human tympa- 
num (fig. 281). In all cases the cilia, when 
Fig. 284. 
Various particles of epithelium from the frog’s mouth. 
a, b,c, small particles that have not reached the 
surface. They appear to present three stages or 
periods, showing a subdivision of the nucleus and 
a formation of two cells out of one ; d, three fully 
developed particles, with cilia on their free sur- 
face; e, f,g, other complete particles, showing 
cilia on that part only which has formed a portion 
of the general surface of the membrane. 
Magni : 
‘agnified 400 diameters, 
they exist, are developed only on that aspect of 
the particles which forms a portion of the gene- 
ral surface of the membrane. 
It is as yet entirely unknown by what pro- 
cess the cilia are produced and nourished ; 
whether the particles, with their cilia, are shed 
from time to time, and are succeeded by others, 
(as is most probable,) or whether the same 
organs remain, and merely change their com- 
ponent elements. (On the subject of Crit in 
general the reader is referred to Dr. Sharpey’s 
excellent article.) 
Of the elementary tissues appended to the 
mucous system.—The two elementary tissues 
now described may be considered as the more 
MUCOUS MEMBRANE. 
essential constituents of the mucous system, or 
as forming the simple mucous membrane. 
simple mucous membrane envelopes the rest of 
the body. It contains within its own substance © 
neither vessels nor nerves, but is, strictly spea 
ing, extra-vascular. By modifications, chiefl 
of the epithelial element, it is in itself ca 8 
of presenting variety of appearance and 
properties in different situations, But in im-— 
mediate connection with its deep surface, tha 
is, with the basement membrane, there are cer- 
tain tissues common to almost ony eee of the 
frame, but here assuming a peculiar arrange. 
ment and office, and by their diversities ii 
various localities, occasioning the most compli- 
cated varieties of outward form, of structure, 
and of function. ” 
These appended tissues are minute blood- 
vessels, a lymphatic network, nerves, and - 
lar tissue. 
It has been already stated that in many parts 
the simple mucous membrane, by its innume- 
rable minute involutions over an extensive sur- 
face, is formed into a com brane. 
Into the composition of this (of which a good 
example is afforded by that of the stomach) the 
appended tissues enter more or less largely, but 
they are likewise, in addition, generally spread 
out in great abundance as a layer underneath 
the compound membrane. This layer has been 
commonly termed submucous cellular vane 
(sometimes tunica nervea,) in the case of in 
ternal surfaces, and cutis vera or dermis in the 
case of the skin. : J 
Bloodvessels. — These may be said to be 
universally present under the sim, mucous 
membrane, with the exception ps of th 
cornea, where vessels, in the normal state, have 
not yet been demonstrated. The capillaries, 
their simplest form, appear to be arranged as 
plane network, such as that of the rectum 0 
the frog (fig. 285). The interstices of th 
network vary much in size and shape in dii 
rent localities. The most copious supply ¢ 
blood distributed to any such membrane is t 
afforded to the air-cells of the lungs in all a 
mals. Here this plane capillary plexus hi 
areole scarcely exceeding the diameter of 
vessels themselves. Where the membrane th 
supply is folded, however irregularly, th 
follow its surface, and bence result many vai 
ties in their arrangement and inosculations. 
even seems to be for the purpose of gainin 
great freedom of inosculation between the 
illaries that the extraordinary complexity 
en given to many of the simple m 
brane, especially in the secreting glands. | 
many foldings from somewhat distant parts 
the membrane are there brought into imm 
diate proximity to one another, and are 
lied by the same or closely connected ves 
This is remarkably exemplified in the tes 
kidney, and liver. The capillary system of 
these, as well as of other solid glands, may 
styled a solid plerus, being extended in ey 
direction, and presenting areole of nearly eq 
size in whatever plane a section of it be 
The liver presents the most perfect instance: 
such a solid plexus, and in it the vessels ar 
aiilt 
