What is the nature of the superficial layer of the cornea It is composed of two lamella. The more superficial constitutes a very fine but firm epithelium. According to Valentin, after sixteen or twenty-four hours' maceration, the epithelium separates from the cornea. The cells have in this case lost a little in transpa rency and are somewhat distended. The nuclei appear more or less swollen by the action of the water. The other lamella situated under neath the epithejium is more loose in its co hesion, and is '*hat Valentin considers the same structure as the papillary layer described by him in the ocular conjunctiva. Valentin says that a chorion or fibrous layer does not exist in the conjunctival extension over the cornea. The bloodvessels derived from the sclerotic conjunctiva run merely betwixt the papillary body and the surface of the proper substance of the cornea. They are very deli cate and extremely difficult to inject.
Romer* has described the arteries of the con junctiva cornea from injections. The fine twigs of the arteries of the sclerotic conjunctiva unite together around the margin of the cornea into a vascular wreath or circle. From this there arise very numerous branches which run from the circumference towards the centre of the cornea, and in their course make two or three very fine subdivisions. Their ends bend distinctly inwards, and appear to penetrate the proper substance of the cornea.
Having thus shown on the surface of the cornea the existence of an epithelium and a structure, called by Valentin a papillary body, similar to what is found on the surface of the sclerotic conjunctiva, as also a stratum of blood vessels, we must admit a cellular support for those vessels, however delicate. If so, the bloodvessels and cellular tissue would consti tute the essential elements of a chorion.f We can only explain the development of those ex tensions of membrane like sclerotic conjunctiva, over the cornea by supposing an irregular and undue development or hypertrophy of these elements.
The question, " Does the conjunctiva' ex tend over the cornea?" may be considered as answered in the affirmative by the above ana tomical demonstration. Morbid anatomy now comes in advantageously with its corro borative evidence. " Nothing is more in favour," says Eble, " of the existence of a con junctival layer on the cornea than the microsco pical structure of this membrane, for there is the greatest resemblance between the structure of the sclerotic conjunctiva and the investment of the cornea."' Eble thus retracts the opinion he formerly expressed against the existence of a conjunctival layer of the cornea.
Externally the sclerotica overlaps or en croaches on, more or less, the edge of the cornea. In certain constitutions, and especially in old persons,t I have observed that the overlapping part of the sclerotica is thicker and more opaque than usual—perhaps also en croaching more extensively on the cornea. The
conjunctiva covering the overlapping sclerotica, especially when the latter is to any considerable extent, appears in its independent form with its chorion fully developed, and although it ad heres to the subjacent overlapping part of the sclerotica very closely by cellular tissue, it by no means presents the same intimate union with the subjacent structure and the same rudi mentary state which the conjunctival extension over the transparent cornea has. In an eye before me in which the overlapping sclerotica is to some considerable extent at the upper edge of the cornea, I easily raised up in a fold and then separated by dissection the perfectly developed conjunctiva from over the part. The conjunctiva covering the overlapping part of the sclerotica has a vascular connexion with the latter no otherwise than by the anastomoses of the proper vessels of each—a vascular con nexion, which indeed subsists between the scle rotica and conjunctiva elsewhere. The dispo sition just described is connected with a point in the pathology of the eye, viz. the bluish white ring which is observed to encircle the cornea more or less completely in certain inter nal inflammations of the eye, and so frequently in what is called arthritic iritis that it has been considered a diagnostic of it, but certainly with out just grounds.
Before explaining the cause of the appear ance, I would request it to be remembered that the insertion of the ciliary ligament is at some little distance from the apparent margin of the cornea; that the vessels which form the red zone of the sclerotica in the internal inflammations of the eye, and in inflammation of the proper substance of the cornea, are vessels which send branches inwards to the iris, opposite the ciliary ligament, branches outwards to anastomose with those of the conjunctiva, and lastly, branches which, following the original direc tion, go to be distributed to the proper sub stance of the cornea. These vessels are not apparent in the healthy state, and one set of them only may become apparent in inflamma tion. Thus in inflammation of the iris, they will be apparent only as far as opposite the in sertion of the ciliary ligament. Between this and the clear part of the cornea is the opaque overlapping part of the sclerotica, which of course not being in the way of the progress of the vessels towards the inflamed part, remains white as usual, and the cornea not being affected the minute branches to its proper sub stance remain unenlarged and unseen. Hence the overlapping part of the sclerotica is seen in contrast between the abruptly terminating red sclerotic zone on the one hand, and the trans parent cornea (appearing dark on account of the dark structure behind it) on the other, forming the bluish white ring.