MEMBRANE—MENINGES—MICROSCOPE. 
MEMBRANE, (in Anatomy,) Gr. penyiy€ 5 
Lat. Membrana; Fr. Membrane; Germ. die 
Haut. This term is commonly applied to 
designate those textures of the body which are 
disposed or arranged as lamine, destined to 
cover organs, to line the interior of cavities, or 
either singly or by their application one over the 
other, to constitute the walls of canals or tubes. 
Expansion with very slight thickness is the 
main morphological characteristic of mem- 
branes in their ordinary sense. 
I do not intend here to give any classifica- 
tion of the membranes: the term is extensively 
used in descriptive as well as in general ana- 
tomy ; and anatomists differ materially as to 
the degrees within which they limit its meaning. 
Anatomists hitherto have been content to adopt 
the gross anatomy of the textures as the basis 
of their classification, a circumstance which 
has given rise to much error, as well as to great 
variety of opinion. Now, aided as we are by 
excellent microscopes, and by the light which 
they have thrown upon the minute anatomy of 
the tissues, we should only admit that classifi- 
cation which is based on an ultimate or even 
roximate anatomical analysis. As these points 
will be all fully treated of in the article Tissue, 
' reference is made to it for the details respecting 
the membranes. 
(R. B. Todd.) 
MENINGES.— This word signifies mem- 
branes ; it is specifically applied to those mem- 
branous expansions which cover and more or 
less protect the brain and spinal cord, and in 
this sense is best interpreted by the German 
word Hirnhaut. The term is in common use 
on the continent, but not so frequently em- 
ployed by British anatomists, although always 
understood by them in the sense above given. 
It appears to have been thus applied first by 
Galen, who distinguished pnyuye waxyvrepn, 
or the dura mater, and pxviyé Acwrn, or the 
pia mater. 
The description of the membranes of the 
brain and spinal cord will be found in the 
article Nervous CenTREs. 
(R. B. Todd.) 
MICROSCOPE, (susxgos, small,and cxomsw, 
to look at,) an instrument for aiding the eye in 
the examination of minute objects. Although 
a description of the structure and uses of this 
instrument cannot be considered as strictly 
belonging to a work like the present, yet the 
knowledge of them is so closely connected with 
its general objects that it has been deemed ad- 
visable to make it an object of special attention. 
The applications of this instrument to the pur- 
poses of the anatomist and physiologist are so 
numerous, that a whole treatise might easily be 
written upon them alone. We are not aware, 
until we come to think on the subject, how 
much of our knowledge of what takes place 
within the living body is dependent upon its 
revelations. To take a familiar illustration,— 
the capillary circulation might be, in some 
degree, guessed at by tracing the ramifications 
of the bloodvessels as far as they could be dis- 
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331 
cerned with the naked eye; but we shouldjhave 
known extremely little of it without the micro- 
scope. Our whole knowledge of the early 
processes of development in plants and animals 
is gained by the same assistance. Not only is 
much of that, which ranks as established ana- 
tomical or physiological truth, founded upon 
microscopic researches, but similar researches, 
which are being prosecuted at the present time, 
are yielding a harvest of discovery still richer in 
amount, whilst not less important in its cha- 
racter. 
We propose, in the present article, to take a 
general view of the principles, optical and me- 
chanical, which are concerned in the construc- 
tion of the microscope; and then to give an 
outline of the results of some of the most re- 
cent enquiries in which it has been profitably 
employed,—confining ourselves chiefly, how- 
ever, to those which concern the origin and 
formation of the principal organized structures. 
If it be thought that the former portion is too 
much extended, we have only to say, that we 
know of no single treatise to which we can 
refer our readers for a large part of the informa- 
tion which we desire to convey; and that we 
have therefore judged it desirable to make the 
article complete in itself. 
I. OpricaAL PRINCIPLES GOVERNING THE 
CONSTRUCTION OF MICROSCOPES. 
All microscopes, except those which operate 
by reflection (to be hereafter noticed), depend 
for their operation upon the influence of convex 
and concave lenses on the course of the rays of 
light passing through them. _ This influence is 
the result of the well-known laws of refraction 
—that a ray passing from a rare into a dense 
medium is refracted towards the perpendicular, 
and vice versa. When, therefore, a pencil of 
parallel rays passing through air impinges upon 
a convex surface of glass, the rays will be made 
Fig. 144. 
l 
a 
: 
G 
| 
: 
: 
A B, parallel a7 of light falling upon the convex 
> 
Qa 
Ss 
Q 
Qa 
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surface F, B, D the centre, D B, D B, radii, 
which are the perpendiculars to the curved sur- 
face at the several points ; BC, course of the 
rays if uninterrupted ; B E, their course in con- 
sequence of the refraction they have undergone, 
converging to a focus at E, 
to converge, for they will be bent towards the 
centre of the circle, since the radius is the per- 
pendicular to each point of curvature. The 
central ray, as it coincides with the perpendi- 
cular, will undergo no refraction; the others 
will be bent from their original course in an 
increasing degree in proportion as they fall ata 
distance from the centre of the lens; and the 
effect upon the whole will be such, that they 
