-§ 
~ 
MUCOUS MEMBRANE. 
apparatus at the extremity of each secreting 
tube, apparently designed to furnish a flow of 
water down the canal.* A large quantity of 
water is evidently required in this secretion as 
a menstruum for the salts and proximate prin- 
ciples it contains; and there is no doubt, from 
the analogy of other glands, that the walls of 
the tubes are the membrane secreting these 
substances. Now the epithelium constitutes at 
least 4%ths of their thickness, and is the only 
of them with which the water can come 
into contact. It therefore seems highly pro- 
bable that this fluid is provided in the manner 
described, in order to dissolve, out of the 
epithelial particles, the peculiar principles which 
they have previously assimilated from the 
blood. 
In support of this general position it may 
be observed, further, 1. That the epithelium, 
which constitutes so large a portion of the true 
glands, is solid and bulky, usually character- 
ized by its finely granular texture, and in this 
respect contrasts strongly with that lining the 
vascular system, which is of extreme delicacy 
and transparence. The exceptions to this re- 
mark contirm its importance. In the air-cells 
of the lungs, the secretions of which are ga- 
seous and not solid, the epithelium is of great 
tenuity, and in the Malpighian capsules of the 
kidney, which appear to serve principally as 
receptacles for the aqueous fluid that escapes 
from the bare capillaries within them, this 
Structure is either wanting or consists of per- 
fectly transparent particles. In many inter- 
mediate varieties, too, there appears traceable a 
correspondence between the bulk of the nu- 
cleated particles and the activity of the secreting 
function ; of which the scaly form in general 
may be mentioned as an instance. 2. That 
many peculiar substances are secreted into the 
interior of nucleated cells, although prevented, 
by the position of those cells, from escaping 
from the body. Such are various fats and fixed 
oils, colouring matters, &c. 3. That this func- 
tion of abstracting somewhat from the blood, 
and elaborating it, seems the most probable 
one that can be assigned to the thymus and 
thyroid bodies, the spleen, and supra-renal 
capsules, and specially to the nucleated par- 
ticles forming so large a portion of these several 
Structures. On the whole there seems much 
weight of evidence in favour of the proposi- 
tion “that secretion is a function very nearly 
allied to ordinary growth and nutrition: that 
whereas these are a combination of two func- 
tions, assimilation of new particles and rejection 
of old, the old being reconveyed into the blood, 
secretion consists in a corresponding assimila- 
_ tion and rejection, but the old particles are at 
Once thrown off from the system without re- 
entering the blood. According to this view, all 
effete material received into the blood, from the 
old substance of the various organs, must be 
_ Teassimilated by an organized tissue, specially 
_ designed for the purpose, viz. the epithelium, 
_ before it can be eliminated : and all substances 
* Phil. Trans. Pt. I. 
1842, p. 73. 
( article REN. as 
See also the 
503 
thrown off from the system, but designed for 
an ulterior purpose, must in like manner be 
assimilated in order to their separation.” It 
places in a strong light a principle of great im- 
portance in physiolugy, the subordination of 
the bloodvessels and their contents to the tis- 
sues among which they are distributed. 
The function of secretion may therefore be 
considered to be universal over the mucous 
system, and its different activity in various si- 
tuations to be dependent on, as it certainly is 
closely associated with, differences in the ar- 
rangement and structure of the epithelial ele- 
ment. The basement membrane, from being 
absent from the lobules of the liver, seems a 
tissue of inferior (perhaps of no) importance 
in respect of this function, and probably is 
chiefly subservient, wherever it exists, to the 
mechanical support of the epithelium. 
There are probably ¢hree ways in which the 
secretions are finally separated from the body : 
and these three ways appear to have a reference 
to the chemical qualities of the product, and to 
their effete or non-effete character. 1. The par- 
ticles assimilated into the nucleated cell may 
be thrown off by virtue of minute chemical 
changes occurring in it, without the cell itself 
being altered in form. In this case the nu- 
cleated cells will be permanent, or only very 
slowly renewed, and the secretion will be 
formed, or at least perfected, by the passage 
of its elements through the cells. 2. The nu- 
cleated cells, as they arrive at their full size, 
may undergo a slow change in the arrangement 
of their elements, and gradually disappear by 
a kind of solution or deliquescence, thus form- 
ing the secretion. 3. The nucleated cells, when 
mature, may be cast off at once, and entire, 
with their contents. The two last modes are 
attended with a continual formation of new 
cells. 
It would appear that, in general, where the 
secretion is formed by the rejected chemical 
elements of the cells (1), or by the destructive 
solution of the cells (2), it is effete; but that, 
when formed chiefly by the separation of cells 
that are mature and contain much organic 
matter (3), it is destined for ulterior purposes 
in the economy. Of the first the kidney seems 
to be an example, of the second the liver, of 
the third the lining membrane of the stomach. 
The varieties in the qualities of the products 
secerned by different portions of the mucous 
system are only referrible to varieties in the 
elective powers of the tracts which respectively 
furnish them, and admit of being most readily 
explained by the view of the nature of secre- 
tion already advanced. It is unnecessary, in 
this place, to enter on a particular description 
of the boundaries of these several tracts, and 
I shall only offer a few observations on the 
nature and extent of that secretion which has 
given its name to the structures here treated of. 
The term mucus, like so many others trans- 
mitted from an early period, was originally 
employed to denote an exaggerated and partial 
condition, was subsequently applied more 
loosely and widely in a generic sense, and 
now requires to be reduced to a more definite 
