MEMBRANE MEN1NGES MICROSCOPE. 



331 



MEMBRANE, (in Anatomy,) Gr. 

 Lat. Membrana ; Fr. Membrane ; Germ, die 

 Hunt. This term is commonly applied to 

 designate those textures of the body which are 

 disposed or arranged as laminae, 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 

 proximate 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 |U,ijnyi ira.xvrtpi), 

 or the dura mater, and iwv\y% fairm, 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, 0*xgc ? , small, and crxomu, 

 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- 



cerned with the naked eye ; but we should have 

 know?i 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. OPTICAL 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 dt use 

 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. 



A B, parallel rays of light falling upon the convex 

 surface F, B, G ; D the centre, D B, D B, radii, 

 which are the perpendiculars to the curved sur- 

 face at the several points ; B C, 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 at a 

 distance from the centre of the lens; and the 

 effect upon the whole will be such, that they 



