POSTERIOR ELASTIC LAMINA.

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nearly smooth refracting surface to the impinging rays of light; and by the frequent movements of the eyelids, the particles which are decaying and losing their place are brushed away, and escape by the nose. In a learned and interesting paper by Dr. Mackenzie,* you will find described a method of seeing in your own person the nature of the corneal surface. This epithelium is rapidly renewed, if scraped off.+

We may now, gentlemen, turn our attention to the posterior elastic lamina of the cornea, a layer which has been long known as the membrane of Demours or of Decemet, or as the elastic lamina of the cornea, or as the corneal part of the membrane of the aqueous humor.

This layer is very easily detached by scraping from the hinder surface of the lamellated tissue of the cornea, for it adheres but slightly to this tissue, and sends no filaments among the lamellæ, as the anterior elastic lamina does. It is a uniform, transparent, homogeneous layer, considerably thinner than the anterior elastic lamina (being only from 1-2000th to 1-3000th inch thick), but, like it, not affected by maceration, by boiling, or by the action of acids. Though very hard, and capable of resisting much pressure, and giving a crisp sound when divided by the scissors, yet it is very brittle and easily torn, and its fragments then show a remarkable tendency to curl up on all sides into rolls, and always with the anterior or naturally convex and attached surface inwards in the roll, so that it would appear to be formed or laid down in situ in a curve precisely the reverse of that which its elasticity inclines it to assume.

When an ulcer has destroyed the lamellated tissue, it sometimes happens that, for a short time, the posterior elastic lamina is thrown forwards into the breach, by the aqueous humour behind it, and forms there a small pellucid vesicle, which, however, almost always soon gives way by rupture, allowing the humor to escape, and the iris

* On the Vision of Objects on and in the Eye. Edinb. Medical and Surgical Journal, No. 164.

Viz. in about three days. The instantaneousness with which the bloodvessels of the neighbouring conjunctiva, and even of the whole eye, dilate and become turgid with blood, when this epithelium is abraded, has often excited my astonishment. The share of the nervous system in this phenomenon is partly indicated by the severe and apparently inordinate pain which attends so slight an injury.

to fall forwards against the opening. This morbid state illustrates very well the properties of the layer now under consideration.*

With regard to the behaviour of this posterior lamina at the margin of the cornea, much difference of opinion-I might say, much uncertainty-prevails; some holding that it is reflected in a modified form over the whole of the anterior and posterior chambers of the eye, others believing it to terminate with the cornea; but none, as far as I am aware, having given a full and accurate account of its actual condition, which is one of considerable importance to a correct knowledge of the physiology of the organ, and to the understanding of some of its diseases.

Marginal plexiform tissue of this lamina.-This layer, then, will be found to terminate at the border of the cornea in the form of plexiform fibres of the yellow elastic kind, or that variety which is allied to itself in essential characters. In this respect it resembles the anterior elastic lamina. The plexiform fibres spring only from its anterior surface, or that towards the lamellated cornea. They begin to appear a very short distance from its edge, and, as they arise, the lamina itself becomes thinner, and is at last altogether spent. They all pass irregularly outwards, occupying, of course, a position between the posterior elastic lamina and the lamellated cornea; and are finally reinforced by those fibres which come from the thin and extreme edge of the lamina. Immediately beyond this edge, therefore, at the rim of the anterior chamber, there is a layer of open plexiform fibres, passing outward, or from the axis of the eye, and

* In a case recently under my care, the posterior elastic lamina saved the ulcer from perforating. Mr. James M... ..., æt. 52, had at the end of June a central ulcer, which was touched with lunar caustic, and he took quinine. By the middle of July it had gradually penetrated to the deepest layers of the cornea, and the posterior elastic lamina was laid bare, and bulged slightly into it from below, especially when trifling pressure was made upon the globe for the sake of testing the condition. Under a continuance of the tonic treatment this ulcer gradually filled up, the aqueous humor not having escaped; and I saw him in February following with an opacity on a level with the rest of the cornea, but with its central portion (answering to the previously exposed posterior elastic lamina) of a denser white than the border.

When this lamina is thus exposed at the bottom of an opaque milky ulcer, centrally placed, the dark pupil may become visible through it, and look like a particle of dirt, deceiving the surgeon into an attempt to remove it, in making which he inevitably opens the anterior chamber.

EPITHELIUM OF THE AQUEOUS HUMOR.

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being the continuation or representative of the posterior elastic lamina. The posterior of these fibres then curve backwards to the iris, and become inserted into its anterior surface at its greater circumference, in the form of small pillars; and near their insertion they begin to resemble the white fibrous rather than the yellow fibrous tissue in chemical and other qualities.

I have found these pillars of the iris much more evident in some animals than in others; but time will not allow me to enter on comparative details. They exist in all mammalia, and have their analogues in other classes. They are in contact with the aqueous humor, where they form the rim of the anterior chamber. A needle may be passed underneath them from the anterior chamber, so as to suspend by them a considerable fragment of the eyeball.

The great portion, however, of the fibrous continuation of the posterior elastic lamina goes not to the iris, but to the ciliary circle,a name by which anatomists refer to a flattish circle of gray semitransparent tissue, which intervenes between the ciliary processes of the choroid and the sclerotica, immediately behind its junction with the cornea, about which extraordinary differences of opinion prevail, but which I shall hope to show you, in a subsequent lecture, is muscular. For convenience, therefore, I will now assume that it is such, and term it the ciliary muscle. This muscle arises, then, from the fibrous tissue coming from the posterior elastic lamina,-the fibrous tissue passing in a sheet backwards to the anterior region of the ciliary processes, and giving origin on its outer surface, or that turned from the anterior chamber, to the fibres of the ciliary muscle, which then clothe the outer surface of the choroid for about one-eighth or one-tenth of an inch, as far as opposite to the ora serrata.

There are still other fibres derived from the posterior elastic lamina -viz. those placed most anteriorly, and which were the first to take origin from it. These, after a short course outwards, become separated from the sclerotica by a narrow space all round, known as the sinus circularis iridis, and which has been considered as a venous canal; afterwards they pass to be united firmly to the sclerotica beyond this sinus, and in so doing share principally in its formation. But there also exists here a series of circular fibres, those just described being more or less radiating the circular lie outside the others, are opaque, white, and stiff, contributing to the formation of the circular sinus, and to that firm union, the ciliary ligament,

:

which subsists between the ciliary processes and the anterior rim of the sclerotica; and which, as a whole, effectually serves to prevent the aqueous humor from escaping into the space between the sclerotica and choroid.

I am aware of the difficulty I must experience in attempting, to give you a clear description of this structure, before I have passed in review those others with which it is associated. What I have now said, however, must suffice for the present, and I shall return to it in connection with the iris and choroid and lens. A few words remain concerning the posterior epithelium of the cornea, or the epithelium of the aqueous humor.

This is so extremely delicate and so perishable a layer, that it has only of late years been recognised, and yet it is very easily prepared for examination. It is a single series of flat epithelial nucleated particles, placed side by side, and united by their margins. Even in large animals the epithelial cells are not in a double layer. It is coextensive with the posterior elastic lamina, which it separates from the aqueous humor. It would appear, however, from what has just been said concerning the conversion of the posterior elastic lamina at its border into fibrous tissue, which in part passes through the aqueous humor to the iris, that this epithelium must cease with the elastic lamina, since there is no longer any stratum on which it can rest. I have not been able to discover the smallest appearance of it upon the pillars of the iris, and I conceive, therefore, that it is limited to the cornea.

I have called this the epithelium of the aqueous humor, because it is the only true epithelium which can be found in contact with that fluid. I shall have to show in a future lecture that the front of the iris has no true epithelial investment, and that the front of the lens is also destitute of such a covering. It seems, therefore, incorrect to speak of the chambers of the eye as lined by a serous membrane, or of the aqueous humor as contained within a proper capsule; and I suppose that practitioners must abandon the name, at least, of that affection, which is now generally termed aquo-capsulitis, even if they continue to regard it as a distinct disease.

In my next lecture I shall proceed to notice some of the morbid states of the sclerotica and cornea; and shall endeavour to connect my remarks as far as I can with the anatomical, and, I fear, rather dry details, which I have had to dwell upon to-day.

LECTURE II.

Blood-vessels and nerves of the sclerotica and cornea-character of the nutritive process in these structures.-Morbid states of the sclerotica and cornea.Sloughing of both corneæ from defective nutrition.-Reparative process in the cornea-Anatomy of a simple ulcer of the cornea -Formation of vessels in the cornea-Effect of general disease on the cornea.-Lymph or pus in the lamellated tissue.-Pustules.-Opacities of the cornea-Development of papillæ on the cornea-anatomy of Staphyloma corneæ.

GENTLEMEN,-In my last lecture I reviewed the structure of the outer tunic of the eyeball, consisting of the sclerotica and cornea, and described the several layers of which the latter is composed. It remains for me, before proceeding to the more internal parts, to make some observations on the nature of the process of nutrition, as it obtains in these structures, and on the bearing of their anatomical construction on the nature and progress of some of their more important diseases.

Both the sclerotica and the cornea are sparingly supplied with the materials of nutrition, as a glance at the arrangement of the bloodvessels will show. The sclerotica is obliquely pierced behind with numerous arteries derived from the ophthalmic, termed the posterior ciliary; but these go almost exclusively to the choroid, only giving a few minute twigs to the sclerotica as they pass. In front, too, the arteries which have supplied the muscles of the eyeball send forwards beyond the tendons small prolongations, which are visible under the conjunctiva, and lose themselves in the sclerotica, within an eighth of an inch from the margin of the cornea. These, however, traverse rather than supply the sclerotica, and anastomose with vessels of the ciliary muscle and iris. Hence, in the most successful injections, the sclerotica itself is with difficulty tinted by the artificial colour, and the