2 lines behind the cornea, where rupture has occasionally occurred. The openings in this coat are, that for the

The

The following parts would be pierced by a needle from before backwards :conjunctiva, anterior elastic cornea, laminated cornea, posterior elastic cornea, the aqueous humour and its sac, capsule of lens, lens, capsule. needle, if in the axis of the eye, would traverse the hyaloid canal, and emerge through the punctum aureum. If laterally, the following coats would be pierced, vascular, nervous, and Jacob's layers of the retina, pigmentary, arterial, and venous layers of choroid, and the sclerotic.

insertion of the cornea, for the entrance of the optic nerve, and small appertures to transmit the ciliary nerves and vessels which supply the organ. The sclerotic is cartilaginous in many animals, and bony in the seabream, as discovered by Dr. Jacob.

The Cornea is exquisitely transparent, hence the term "pellucida," applied to it in contra-distinction to the "cornea opaca," or sclerotic. It is circular posteriorly, but in front, being overlapped above and below by the sclerotic, is about 7 of an inch transversely, according to frequent measurements, and a little less from above downwards. The cornea and sclerotic thus form respectively and 4 of the circumference of the globe. Bowman states that its anterior and posterior surfaces form similar and equi-distant curves; whereas Jacob regards it as a meniscus lens, thicker in the centre, and, as noticed by Haller, it certainly magnifies letters.

The cornea can be divided into 5 layers: 1. The conjunctiva, a mucous epithelium which is prolonged from over the sclerotic; it is most readily demonstrated by plunging the eye in boiling water, when it can be peeled off as a coagulated film. The eye of the cuttle-fish has no other cornea than that which the common integument forms; the serpent's slough includes a skin from the cornea, and in the mus typhlus or mole-rat, according to Jacob, hairs grow from it. Some cutaneous eruptions and phlyctenular ophthalmia raise up the conjunctiva; and particles of iron and other foreign bodies are often impacted in it: this is the only layer of the cornea which is sensitive. 2. The anterior elastic lamina was first clearly described by Bowman. It is hyaline, exquisitely transparent, and of high refractive power, so that it sparkles in water. It is scarcely acted on by any reagent, but yields, by prolonged boiling, a matter more like chondrin than gelatin, and peculiar in being soluble in excess of its precipitants. Although thicker (2000) than the posterior elastic lamina, it is much less easily

separated, as it is tied to the next layer by many curved fibres; but by prolonged maceration I have often succeeded in separating it in the horse's eye. 3. The proper or laminated cornea consists of many fibrous layers, the spaces between which are tubular, as demonstrated by Bowman, who with mercury has injected these corneal tubes." They are probably nutritive canals for the cornea, which so far from being non-vascular heals after operations with surprising rapidity. Neither is it without nerves, for microscopists have lately found branching nerve-tubes in this layer. Owing to the laminated arrangement, the cataract needle is more apt to slip between the layers than to pierce them if the operator be not careful. This layer becomes opaque by stretching it, or by pressure on the globe of the dead eye. 4. The posterior elastic lamina or membrane of Demours and Descemet is quite similar to that already described, and with it admirably preserves the due curvature of the cornea. 5. The epithelium of the aqueous humour constitutes the last plane of the cornea, and it can scarcely be traced farther in the anterior chamber. The anterior and posterior elastic laminæ seem to me to be but the basement layer of the conjunctiva and membrane of the aqueous humour respectively, and thus the cornea consists of 3 essential structures the conjunctiva prolonged from the sclerotic, the proper cornea continuous with this coat, and the membrane of the aqueous humour, all somewhat modified to suit special purposes. The five layers can be distinguished in the annexed section by Mr. Richardson, who in an able paper (Dublin Journal, February, 1859) has discussed the changes which staphyloma produces. The second figure illustrates the confused condition of the cornea and anterior chamber in that disease.

The Choroid and its appendage, the iris, form the next plane, a very fine areolar web-sometimes named the "tunica arachnoidea oculi," or "membrana fusca"-in

some places intervening. To convey clear ideas of this coat and the method of demonstrating it, I cannot do better than quote Dr. Jacob's accurate description:

"This membrane is of a deep brown or black colour, being stained with the colouring matter called the black

Conjunctiva.

Anterior elastic lamina.

Laminated

cornea.

Posterior elastic lamina.
Membrane of aqueous

humour.

Healthy and Staphylomatous Cornea.

pigment; but when this is removed, it exhibits a high degree of arterial and venous vascularity. Its external surface is comparatively rough, coarse, and flocculent, and obscured by the cellular membrane which connects it to the sclerotic. The inner surface, which is in contact with the retina, presents a very different appearance. It is soft and smooth, and when minutely injected resembles the more delicate mucous membranes, and exhibits a remarkable degree of minute villous vascularity. The external surface being composed of the larger branches of arteries, veins, and nerves, may be torn away from the soft, smooth, and more closely interwoven inner layer; or the inner layer may be partially dissected up from it, with some care, especially in the eyes of the larger quadrupeds. This manœuvre having been executed by Ruysch, and preparations so formed displayed by him, the inner layer has been denominated the tunica Ruyschiana. But this is a mere anatomical artifice. There is no natural division into two layers, the soft, smooth, and highly vascular inner surface being formed by the ultimate subdivision and distribution of the larger branches of vessels, which exhibit themselves separately on the outside. It is a condition somewhat analogous to that of the skin, where the soft, smooth, villous external surface presents so remarkable a contrast to the rough internal surface, with its layer of cellular membrane uniting it to the subjacent parts."

b

The arteries are the short ciliary branches of the ophthalmic, and the veins, which are branched like the boughs of the weeping willow (vena vorticosa), end in the ophthalmic vein. The inner layer is a sheet of beautifully regular hexagonal cells, discovered by Wharton

Pigment Cells of Choroid. a. Horizontal view. b. From side.