MUCOUS MEMBRANE. 
ally gaining ground, that all the glands fur- 
nished with excretory ducts have a very close 
relation to the former, in which their ducts for 
the most part open. Still, it does not appear 
that the proofs of this alliance have been hi- 
therto, by any author, deemed sufficient to in- 
duce him to blend these several parts under a 
description common to themall. Even Miiller, 
in whose philosophical work on the glands is 
contained so much new and important evidence 
of this relation, continues thus to sever them 
in the late edition of his Physiology. But, in- 
deed, although much weight is to be granted 
to the arguments drawn from continuity and 
occasional convertibility of structure, course of 
developement, rough analogies of composition 
or of function, and sympathies under disease, it 
must be allowed that Pitherto that most im- 
portant of all proofs has been all but wanting, 
which, as I shall endeavour to show, is capable 
a being derived from minute anatomical ana- 
lysis. 
The researches which I have hitherto been 
able to make on this subject are still so incom- 
plete, that I should have gladly delayed their 
publication for some time longer, had the pro- 
gress of this work admitted of it. As itis, I 
shall state the conclusions to which I have 
been led, and the grounds they rest upon, 
{pointing out, as far as possible, where farther 
examination is demanded,) with the hope of 
thereby giving a clearer and more satisfactory 
view of the structure and relations of this im- 
portant class of tissues than could be otherwise 
accomplished. 
I shall point out that the skin, mucous mem- 
branes, and secreting glands, consist of certain 
elements, which the anatomist may detect and 
discriminate, some of which are essential to 
their tissue, others appended or superadded,— 
and that the broad characteristic distinctions 
between these structures, appreciable to ordi- 
nary sense, as well as the innumerable grada- 
tions by which they every where blend insensi- 
bly with one another, are solely due to various 
degrees and kinds of modifications wrought in 
the form, quantity, and properties of these re- 
spective elementary parts. 
The skin is the outer tegument of the body ; 
the mucous membranes form its internal invest- 
ment, and are continuous with the skin. The 
duets of all glands are continuous either with 
skin or mucous membrane, and their true 
secreting portion, as already described, (see 
Guanp,) is merely a further prolongation of 
the same tissue. These offsets, like the great 
mucous tracts, are in the direction of the inte- 
nor of the body ; they form follicles and tubes 
of infinite variety, and, however complicated, 
May still he regarded, in a certain sense, as ex- 
_ ternal to all other textures. Thus the mucous 
_ System may be described as a great and un- 
interrupted membrane, every where perfectly 
_ closed, in which the rest of the animal, or the 
_ parenchyma, is enclosed. This membrane has 
_ two surfaces, the one free, superficial or exter- 
hal, the other attached, deep, or parenchymal. 
It is on the parenchymal surface that the ap- 
pended structures (viz. blood- and lymphatic 
485 
vessels, nerves, and areolar tissue) are found 
in more or less profusion. 
The functions of the mucous system, nume- 
rous and diversified as they are, all bear a dis- 
tinct reference to its really external anatomical 
position, and by this circumstance they are 
associated together: the principal are sensation, 
absorption, secretion, excretion, and defence of 
the parts lined by it against the contact of 
foreign bodies. 
A glance at the opinions that have prevailed 
concerning the structure and relations of the 
mucous membranes, will exemplify, more clearly 
perenne than any other course, how imperfect 
ave been the means employed, until a very 
recent period, in researches into minute or 
structural anatomy. The distribution of their 
bloodvessels had indéed been studied with 
brilliant success by Ruysch, Lieberkiihn, and 
others, by the aid of injections, the admirable 
delicacy of which no modern art has surpassed ; 
and somewhat of their extensive connexions, 
general properties, and even of their texture, 
had been divined from rough dissection, mace- 
ration, and observations on the mode of their 
developement and on their morbid states. But 
the ignorance that really prevailed, as to their 
intimate structure, is abundantly evinced by the 
number of disputed questions, the absence of 
precision of detail, and the substitution of loose 
and unwarranted analogies in its stead. Within 
the last five years discoveries have been made 
which throw a new and most important light 
on the whole subject, and when viewed in con- 
nection with one another, must be considered 
to have greatly simplified our knowledge re- 
specting it. These discoveries, due chiefly to 
Boehm, Boyd, and Henle, result from exami- 
nations of recent specimens with the micro- 
scope, and those of the last observer, which are 
especially valuable, were made with high pow- 
ers employed upon a single tissue (the epithe- 
lium) in different forms and situations. It is 
this kind of research that promises the most 
enlarged and trustworthy results to any one 
who will follow it in a spirit of due caution 
against hasty generalization, and which has 
already done so much in the present day to- 
wards a complete remodelling of our ideas, 
both concerning the elements of organization 
and their union to form compound tissues. 
Before proceeding to a description of the 
anatomical elements of the mucous system, it 
is necessary to premise that a great portion of 
the membranes, usually termed mucous, are 
glands of a complicated structure, arranged in 
a membranous form, consisting of a closely 
packed mass of secreting tubules, which open 
on the general surface, and are essentially invo- 
lutions of it. The bloodvessels and other ap- 
pended tissues occupy the intervals of these 
tubules, and so approach the surface; but, ne- 
vertheless, they always remain on the deep or 
parenchymal aspect of the mucous tissue. So, 
the same membranes present projections, which 
are nothing more than hollow evolutions of the 
same mucous tissue, into which the appended 
tissues are extended. The same remarks apply 
strictly to many regions of the skin. Hence it 
