Immediately behind the transparent cornea is the aqueous humor which fills up the anterior and posterior chambers which lie between the cornea and the lens. As its name implies, it is very nearly pure water, with a mere trace of albumen and chloride of sodium. As no epithelium exists in front of the iris, or on the anterior surface of the lens, it is most probably secreted by the cells on the posterior surface of the cornea.
The crystalline lens lies opposite to and behind the pupil, almost close to the iris, and its posterior surface is received into a corresponding depression on the forepart of the vitreous humor. In form, it is a double-convex lens, with surfaces of unequal curva ture, the posterior being the most convex. It is inclosed in a transparent capsule, of which the part covering the anterior surface is nearly four times thicker than that at the posterior aspect, in consequence, doubtless, of greater strength being required in front, where there is no support, than behind, where the lens is adherent to the vitreous membrane. The microscopic examination of the substance or body of the lens reveals a structure of wonderful beauty. Its whole mass is composed of extremely minute elongated ribbon-like structures, commonly called the fibers of the lens, which are regarded by Kolliker as thin-walled tubes, with clear, albuminous contents. These fibers are arranged side by side in lamella, of which many hundred exist in every lens, and which are so placed as to give to the anterior and posterior surfaces the appearance of a central star, with meridian lines.
The lens gradually increases in density, and, at the same time, in refracting power, towards the center; by this means the convergence of the central rays is increased, and they are brought to the same focus as the rays passing through the more circumferen tial portions of the lens. (According to Brewster, the refracting power at the surface is 1.3767, and at the center 1.3990.) According to Berzelius, the lens contains 58 per cent of water, 36 of albumen, with min ute quantities of salts, membrane, etc. In consequence of the albumen, it becomes hard and opaque on boiling, as we familiarly see in the case of the eyes of boiled fish. In the adult, its long diameter ranges from Id to iths, and its antero-posterior diameter from ith to ith of an inch; and it weighs 3 or 4 grains.
The vitreous humor lies in the concavity of the retina, and occupies about four fifths of the eye posteriorly. It is inclosed in the hyaloid membrane, which sends numerous processes inwards, so as to divide the cavity into a series of compartments, and thus to equalize the pressure exerted by the inclosed soft gelatinous mass. Between the ante rior border of the retina and the border of the lens, we have a series of radiating folds or plaitings termed the ciliary processes of the vitreous body, into which the ciliary pro cesses of the choroid dovetail, The vitreous humor contains, according to Berzelius, 98.4 per cent of water, with a trace of albumen and salts, and hence, as might be expected, its refractive index is almost identical with that of water.
The appendages of the eye now claim our notice. The most important of these appendages are the muscles within the orbit, the eyelids, the lachrymal apparatus, and the conjunctiva, to which (although less important) we may add the eyebrows.
The muscles by which the eye is moved are four straight (or real') muscles, and two oblique (the superior and inferior). The former arise from the margin of the optic fora men at the apex of the orbit, and are inserted into the sclerotic near the cornea, above, below, and on either side. The superior oblique arises with the straight muscles; but, after running to the upper edge of the orbit, has its direction changed by a pulley, and proceeds backwards, outwards, and downwards. The inferior oblique arises from the lower part of the orbit, and passes backwards, outwards, and upwards. The action of the straight muscles is sufficiently obvious from their direction : when acting collectively, they fix and retract the eye; and when acting singly, they turn it towards their respec tive sides. The oblique muscles•antagonize the recti, and draw the eye forwards; the superior, acting above, directs the front of the eye downwards and outwards, and the inferior upwards and inwards. By the duly associated action of these muscles, the eye is enabled to move (within definite limits) in every direction.