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.

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    muscles will produce a pressure on the eyeball where the sclerotic is thinnest. These anatomical facts have suggested the hypothesis, in which the adaptation of the eye to vision at different distances, is explained by changes in the length of its axes. The thinner portion of the sclerotica, however, is sufficiently firm to retain its form when removed from the eye; and we cannot tear it with the fingers or forceps. The external surface of the sclerotica is rough and cellular. It is covered in front by the conjunctiva. Under this membrane it affords attachment to the tendons of the muscles moving the eye, which are inserted into it in the same way as tendons are attached to bones in other situations. The four recti lie upon the membrane to a considerable extent from before backwards; while, in the space circumscribed by them, it is covered by fat. The internal surface of the membrane is smooth, connected to the choroid by short, soft, and easily lacerated cellular threads, partially tinged by the dark pigment, and hence sometimes called lamina scleroticas fusca. The opposed surfaces of the sclerotica and choroid have a moist appearance, though we do not observe any distinct fluid. The external surface and the substance of the membrane are of a pure white color, without any appearance of red vessels. In the healthy state it contains few vessels circulating red blood. It reddens, however, under the excitement of disease, when the vessels are so numerous as to give the membrane a general red tinge. The white of the eye depends on the sclerotica, which is seen through the semitransparent conjunctiva; in the same way the reddened state of the membrane, when inflamed, or any other morbid changes in its anterior portion are plainly visible externally. When we trace the optic nerve to the eyeball, it seems to be implanted in the back of the sclerotica. The fibrous sheath of the nerve and the tunic are continuous at this point; their substance is intimately blended, although the sclerotic is here much thicker than the nervous sheath. The dispute whether the sclerotica is a production or continuation of the dura mater, through the nervous sheath, is merely verbal. The parts in question are closely analogous in structure, and inseparably united. In consequence of this union, the cornea, the sclerotica, the sheath of the optic nerve, the dura mater, and the periosteum of the orbit, constitute a series of continuous fibrous structures, an arrange- ment which affords explanation of various pathological phenomena. The optic nerve enters the globe by a round aperture in the sclerotica. This has been described as being occupied by a circular portion of membrane perforated by many small openings, and hence called lamina cribrosa. This notion has arisen from observing the effect of squeezing the end of the nerve, when the choroid and retina have been removed from the cup of the sclerotic; the medullary matter is then expressed in numerous minute portions. It is forced out from the terminations of the neurilematous tubes; and a careful dissection will show clearly that the opening in the sclerotica is a simple round foramen. A circular groove \_k, Fig. 2] is seen in the sclerotica, on its inner surface, at its anterior termination, where it joins the cornea and is connected to the ciliary ligament. It has been called the canal of Fontana, having been pointed out by him in the eye of an ox. It is a venous tube, receiving veins of the iris, called circulus venosus iridis by Arnold,1 who regards it as a venous sinus, bearing the same relation to the sclerotica that the sinuses of the head do to the dura mater. He says, farther, that this vessel and its nature were well known to Hovius; that it often receives injection through the arteries, but cannot be filled from the veins, probably on account of valves. 1 Anatomische und physiologische Unlersuchungen, p. 10-14.