A treatise on the diseases of the eye / by W. Lawrence.

  • Sir William Lawrence, 1st Baronet
Date:
1854
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on the diseases of the eye / by W. Lawrence. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

    49/996 (page 39)
    properly so called, being denominated cornea lucida; and the sclerotic, cornea opaca. The sclerotic covers the posterior five-sixths of the globe, leaving in front a circular vacancy filled by the cornea. Behind, and a little on the inner or nasal side of the axis of the eyeball, it is perforated by the optic nerve. The long and short ciliary arteries and veins, the vense vorticosse, and the ciliary nerves also perforate the membrane. The firmness of the sclerotic is such that it retains its figure when divided and emptied of its contents; small portions cut out of the membrane preserve their form, and regain it immediately on the pressure being removed, after they have been squeezed together. When this coat is removed, the other component parts of the globe are so much softer and more delicate in their texture, that the ball no longer retains its regular figure. The sclerotic coat belongs to that class of membranes which anatomists call [white] fibrous [tissue],1 such as the dura mater and periosteum. Like such structures, it is white, or bluish white, somewhat glistening, and possesses some elasticity. It is made up of the same kind of fibres which compose the tendons and ligaments; and these fibres are so closely compacted and interwoven, that, in the ordinary state of the part, they can scarcely be distinguished;2 they are much less fibrous than in certain portions of the dura mater, and we might sup- pose the tunic to be made of condensed cellular texture. If, however, we dis- sect it carefully, we shall observe, in some parts, the glistening, shining aspect, which denotes a fibrous structure; while in many animals, the fibres being more loosely interwoven, are quite obvious; and in certain forms of disease, when the coat is distended, and rendered thinner, the fibrous structure is very clear. This organization gives the sclerotica that firmness which fits it for covering the softer parts of the globe. It has nothing to do with vision, but is merely subservient to the purpose of supporting, connecting, and protecting the more vascular and delicate structures composing the interior of the eye. The density and resistance of the sclerotic are so considerable, that no force which can be applied with the fingers or forceps will lacerate it; when, therefore, the vascular parts within the globe are actively inflamed, this coat becomes so tense as almost to convey the idea of the hardness of stone. The sclerotica is not of uniform thickness throughout. If we divide the globe into an anterior and a posterior half, by a vertical section, the sclerotica will be found much thicker in the latter than in the former division. The thickest part of the membrane is at the entrance of the optic nerve, and on the adjacent posterior aspect of the globe ; it is here about one-twentieth of an inch in thickness. Tracing it for- wards, we find it becoming gradually thinner, to the extent of one-third or one- half of the measure just mentioned. The thinnest portion of the membrane is at the middle, or greatest diameter of the globe: it becomes again a little thicker where it joins the cornea in front. Thus the combined action of the four recti [' There is also a good deal of delicate yellow elastic tissue mixed with the white in the sclerotica.—Bowman.] 2 Arnold states, as the result of careful microscopical examination, that the sclerotica really consists of condensed cellular texture. He found no appearance of primitive fibres or cylinders, but merely a cellular mass permeated by an extremely fine and close net- work of lymphatics, and he has represented these in the 2d figure of the 1st plate of his in- structive and interesting work, Anatomische und 1'Iiysiologische Untersuchungcn iiber das Augc des Menschen. 4to. 1832. Mascagni had already observed and delineated the same facts, Prodromo delta grande Anatomica, tab. 6 and 14. The microscopical researches of Arnold on the cellular texture, had led him to conclude that it consists of lymphatics, with vesicles of fat in greater or smaller number, and bloodvessels of various size. Mas- cagni had previously formed the same conclusion which had been confirmed by Fohmann, from his injections of the lymphatics. The serous and fibrous membranes, being composed of cellular texture, exhibit the same structure, so far as the lymphatics are concerned.— Arnold's Untersuchungcn, Einlcitung.