nections which form the side walls of the corneal tubes described.

now

I found that I could inject the tubes with size and vermilion quite as definitely, though not so easily, or so as to form so beautiful an object, as with mercury; but this fact was sufficient to prove that the peculiar varicosities of the tubes were not a false appearance due to the tendency which mercury has to collect itself into the globular form.

What 1 have now mentioned is what occurred with the cornea of a large quadruped. I found it far more difficult to make a similar injection of the human cornea, or of that of the cat or smaller animals, and it is not probable that interstices of equal size-perhaps hardly of the same shape and arrangement-exist in the latter specimens. From the greater thinness of the membrane, and the greater proximity of the entire tissue to the vascular arches from which its nutrient supply is drawn, it may be supposed that such a free and ample system of canals may be dispensed with. In the human cornea, however, (as in fig. 3) I have clearly seen a tubular arrangement of the

interstices under favourable circumstances: although, in general, the tissue too readily gives way under the distending force which it is requisite to employ. The length of the canals between the constrictions does not exceed the 12th of an inch, and is for the most part much less, while their width is from 1-500th to 1-600th of an inch: this is in the human cornea.

It might be conceived that these corneal tubes were a modified form of lymphatic vessels, as it is generally thought that a close lymphatic net-work may be injected in a somewhat similar fashion under the skin and other parts. But I have not found the mercury escaping along the lymphatic trunks when pushed from the cornea towards the sclerotica. On the contrary, it requires hard pressure to make it escape from the cornea at all, and then it enters the anterior chamber of eye, or the between the sclerotica and choroid, or even subconjunctival blood

the

space

JUNCTION WITH SCLEROTICA.

15

vessels. Hence it is probable that the corneal tubes do not communicate directly with any other set of vessels or natural channels.

With regard to the use of these corneal tubes, we shall probably not be far wrong in supposing that they serve to promote and facilitate the permeation of this thick non-vascular structure by those fluid portions of the blood which alone have access to it. Whether the special arrangement of the tubes which I have described is concerned in endowing the cornea with its necessary transparency, it does not seem possible to determine. It might be imagined to contribute to hold all the lamella in place, and to prevent derangement of their relative position. A brief account of these, and other points which I shall notice, is given in the third part of the Physiological Anatomy and Physiology of Man, just published by Dr. Todd and myself.

Junction of the sclerotica and cornea.-I have already stated that the lamellated tissue of the cornea is the only one which, properly speaking, is continuous with the sclerotica. This continuity is so perfect that the two textures cannot be torn asunder, or in any way be shewn capable of detachment along the line of junction. Even maceration is not capable of effecting their separation; and if we consider the close affinities of the two structures, and their mode of union, it will be easy to understand the reason of this. In fact, both belong to the class of fibrous tissues, and have very similar physical and chemical properties. The fibrous bundles of the sclerotica, intricately interlaced and intermixed with threads of yellow elastic tissue, become continuous at the border with the lamina of the cornea. The elementary parts of the one join on to those of the other; the interstices, which are irregular and open on all sides in the sclerotica, assume a regular arrangement, and become tubular in the cornea. On the surface of a very thin vertical section of the two structures, carried through their line of junction, the transition of one into the other can be very satisfactorily traced. By acetic acid the sclerotica swells and becomes transparent, and exhibits the yellow fibrous element of its structure, and also sparing nuclei, like those belonging to tendinous parts. By the same agent, the cornea first grows opaque as the arrangement of its parts is interfered with by the swelling of the tissue during the progress of the acid through it, but subsequently it all very nearly resumes its trans

parency, merely displaying here and there on the surface of its lamellæ the elongated nuclei which were previously indistinctly seen, and which correspond closely with those of the sclerotica and other fibrous tissues.

The description of the lamellated cornea will be most conveniently concluded in connexion with that of the antertor elastic lamina; to which I shall, therefore, now direct your attention.

The anterior elastic lamina has not hitherto, as far as I know, been distinguished by anatomists, and yet it seems a structure of a very interesting kind, an acquaintance with which will perhaps enable us to discriminate some morbid phenomena from others with which they have been classed. It is a continuous sheet of homogeneous membrane, nearly similar in essential characters to the posterior elastic lamina of the cornea and the capsule of the lens, being perfectly transparent and glassy, without appearance of internal structure, and being very slightly or not at all influenced by acids. Its thickness in the human eye is from about 1-1200th to 1-2000th of an inch, and it forms an unbroken covering to the whole laminated cornea, giving it that smooth glistening surface which is exposed by scraping off the conjunctival epithelium. This latter rests upon it as the epithelium does upon the basement membrane in other situations; and I may observe that it appears to me to be strictly a highly-developed form of the basement membrane of the mucous system, remarkably modified in this particular part to answer a special purpose.

The manner in which the anterior elastic lamina is united to the lamella which it serves to cover, is very interesting. It must be borne in mind that the anterior surface of the cornea is convex, and that the maintenance of its exact curvature is of primary importance to vision, as it is there that the first inflexion of the rays of light falling on the eye takes place; and further, that the conjunctival epithelium being a soft and fragile substance, must take the figure of the surface on which its rests: hence, probably, the arrangement I am about to mention. The anterior elastic lamina, a firm, resisting, uniform layer, placed in front of the more soft and porous lamellated tissue, is tied down to the anterior lamella, at innumerable points, by filaments of similar texture to itself, which it sends in among them. These, as they penetrate the lamellæ, divide and expand in such a

ANTERIOR ELASTIC LAMINA.

17

manner as to take firm hold of them, and are thus gradually spent

FIG. 4.

Vertical section of the human cornea near the surface. a, anterior elastic lamina; b, conjunctival epithelium; c, lamellated tissue; d, intervals between the lamellæ, showing the position of the corneal tubes collapsed; e, one of the nuclei of the lamellated tissue; g, fibrous cordage sent down from the anterior elastic lamina. Magnified 300 diameters.

among the four or five lamellæ which lie nearest to the surface. It is singular, too, that these filaments are not set vertically, but everywhere in a slanting direction among the lamellæ, so that in a vertical section they appear to cross one another at right angles. This arrangement might, I imagine, be shown, on mechanical principles, to be the best possible for the maintenance of the convexity of the front of the cornea.

It is obvious, from the elaborate manner in which the anterior elastic lamina is thus tied down to the lamellated

texture, that it can hardly be raised as a separate layer; and hence, probably, the reason of its having been hitherto overlooked. In fact, it scarcely admits of being demonstrated, except on the face of a section of the cornea. The anterior elastic lamina becomes exceedingly thin, and disappears, at the margin of the cornea, its attenuation being accompanied by an increase in the number and size of the filaments which it sends down to the lamellated tissue; so that it seems to expend itself by giving origin to these filaments. And from this extreme border, where it ceases to be distinguishable, a great abundance of them runs into the sclerotica, in that slanting course which the elastic lamina would have itself taken, if it had been prolonged in the direction of its own curvature. These filaments mingle with the elements of the sclerotica, and are gradually lost among its middle fibres. The artificial mode in which the margin of the anterior elastic lamina is thus fixed, may be roughly likened to that of the awning of a tent: it is rendered much more obvious if a thin vertical section of the parts at the junction of the sclerotica and cornea be treated with acetic acid.

C

That this lamina, although apparently homogeneous, like a sheet of glass, is very permeable to fluids, as is the capsule of the lens, may be readily shewn by squeezing an eye after the conjunctival epithelium has been scraped off: the small drops which collect on the surface rest upon this lamina after having transuded through it.

The existence of this lamina will help, I think, to explain, what must have often puzzled surgeons, viz. the tenacity with which small particles of steel, or other sharp angular fragments, stick in the front of the cornea only just within the surface. These will often remain for many days, or even weeks, and prove the cause of much inflammation, and yet still be found difficult of extraction; which could hardly be the case if the lamellated tissue and the conjunctival epithelium were the only textures in which such particles could be imbedded.

The conjunctival epithelium of the cornea may be now conveniently adverted to: it is that delicate, soft, almost pulpy layer, which forms the anterior surface of the cornea, and is so easily raised by the knife or needle. It is a continuation of the epithelium of the conjunctiva covering the front of the sclerotica and lining the lids, and consequently of the cuticular investment of the body.

In those animals which lose and renew the cuticle, by a constant process, unmarked by periods of intermission, the superficial particles are gradually shed after arriving at their mature state, while others are as gradually originated in the deepest region, on the tissue which serves as a basis of support, and near which lies the source of their nutriment. This is precisely what occurs on the outer surface of the cornea in the human eye. The epithelial particles are exceedingly transparent, but in position, form, and mode of growth and decay, they bear a close resemblance to the epidermis. In different animals the number of epithelial layers varies according to the size of the eye: in man, they constitute only a triple or quadruple series, altogether not exceeding the 1-500th of an inch in thickness. The deepest, which rest on the anterior elastic lamina as on a basement membrane, are slightly elongated vertically, and stand endwise; the next are angular or subglobular in shape, and the most superficial are flattened scales, more or less overlapping one another, and of a darker hue than the others when seen by transmitted light. The imbricated scales of the surface have their minute inequalities filled up in the natural state by the watery secretion of the lacrymal gland, so as to present a