MUCOUS MEMBRANE. 
sult from the predominance of their epithelial 
element over the others. The ducts terminate in 
vesicles, very similar in the figure they assume 
to those of the lungs, but nearly filled up 
with epithelial particles. The basement mem- 
brane is very delicate. The capillary vessels 
encircle the vesicles, and are comparatively few 
in number, whence the pale colour of these 
glands. The areolar tissue forms capsules for 
those aggregations of vesicles, termed lobules, 
but does not penetrate between the individual 
vesicles. 
_ The mammary glands derive their extreme 
denseness and toughness, as well as their white 
colour, principally from the areolar tissue, in 
which the proper glandular membrane is en- 
closed. This tissue penetrates more abundantly 
between the minuter subdivisions of the gland 
than is observed in any other instance. It 
thus affords support, at the same time that it 
permits and facilitates movement of one part 
of the organ on another. It is also of such a 
hature as to readily allow of distension during 
_ lactation. 
General outline of the functions of the mu- 
_ cous system.—By its external anatomical posi- 
tion, this system is subservient to four great 
_ functions: the reception of impressions from 
without, the defence of the body from external 
injurious influences, the absorption of foreign 
particles, and the separation of such as are for 
any reason to be eliminated. It may almost 
be said to be the peculiar seat of these func- 
tions, which, however, are distributed in a very 
* unequal manner over its different regions. 
Reception of external impressions—The skin 
and mucous membranes appear everywhere 
fitted by their nervous supply to receive im- 
pressions, which, being conveyed to the ner- 
yous centre, may there excite a reflexion of 
Stimulus along motor nerves, without the in- 
tervention of consciousness. Common sensa- 
tion, or that which in its most exalted form 
becomes touch, exists in all parts of the cuta- 
neous surface, within the mouth, for some dis- 
tance within the nostrils, and (with the excep- 
tion of the pharynx and esophagus) in general, 
wherever the epithelium is of the true scaly 
variety. Where the sense of touch is most 
ect, the simple membrane is observed to be 
involuted into the form of papillz for the pur. 
hae of crowding a larger number of nervous 
ps into a given space. Taste and smell, 
which are nearly allied to touch, are the other 
ea senses of which the mucous system is 
seat. The sensations of hunger and thirst 
seem also referrible to this tissue. 
Defence, from external influences—One chief 
division of the mucous system, viz. the skin, 
derives its main characteristics from its adap- 
tation to this function, and those parts of the 
Mucous membranes which are most exposed to 
_ the contact of irritating substances approach 
_ the most nearly to the skin in their structure. 
Their Seg is scaly and in thick lamine, 
their submucous areolar tissue abundant, dense, 
d resisting. The nervous endowments of 
_ Such surfaces, whether excito-motory or sen- 
sorial, mainly contribute to the protection of 
499 
the animal. And, on the external tegument, 
the developement of hairs, nails, &c. in their 
endless modifications of form, position, and 
structure, serve, with few exceptions, the same 
important purpose. In some parts of the mu- 
cous membrane peculiarly obnoxious to pres- 
sure, there are special glands for the lubrication 
of their free surface. 
Absorption of external material.— Every 
particle, entering the body from without, is ab- 
sorbed, in the first instance, through some por- 
tion or other of the mucous system. What is 
now known of the nature of this function in 
general, renders it certain that every part of the 
mucous system would form an absorbing sur- 
face, if favourably circumstanced for doing so. 
But as the extraneous material, to be absorbed, 
must be brought into contact with the absorb- 
ing surface, often by some special and com- 
plicated means, this function is chiefly limited 
to certain distinct districts of the system. With 
few exceptions the glands are not suited 
either by their position or structure to receive 
the contact of extraneous substances, and even 
many portions of the mucous membranes are 
incapacitated in the same manner, as, for ex- 
ample, most of those lining the excretory pas- 
sages of the glands. The secretions which, in 
a healthy state, are the only substances brought 
into contact with these surfaces, are, it is true, 
occasionally modified by a partial absorption 
of their constituents; but, generally speaking, 
this occurs to a very slight extent. Once 
formed, they usually traverse the channels, 
leading to the outlets of the body, unchanged. 
The simplest condition under which this 
function presents itself appears to be that ex- 
hibited by the respiratory surface, which, whe- 
ther it be arranged as lungs or gills, is con- 
cerned with aeriform particles, and absorbs and 
secretes through the self-same structure. The 
skin also is a very active absorbing surface, 
and appears, by the best observations, to be 
provided with a close net-work of lymphatics, 
which I have already stated to be most probably 
situated immediately under the basement mem- 
brane. It does not appear that the existence 
of the lymphatic pores, described by MM. 
Breschet and Roussel de Vauzéme as opening on 
the free surface of the cuticle, has been confirmed 
by any subsequent anatomist. I have sought 
in vain for any such system of vessels in the 
cuticle, and I believe those distinguished ob- 
servers must have been deceived by the irre- 
gular lines of union between the epidermic 
particles. It is true that the thickness and tex- 
ture of the layers of epidermic seales are little 
calculated to allow of their being permeated 
by foreign material, whether fluid or gaseous ; 
and, therefore, it is not likely that absorption is 
effected to any great extent either through their 
substance or interstices. It seems more con- 
sonant with facts to suppose, that this process, 
especially in respect to solid matters, is carried 
on by the simple membrane of the sudoriferous 
ducts, with which external particles would 
easily be brought into contact through their 
open extremities. But as these ducts traverse 
the thickness of the cuticle, and in that part of 
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