It now remains for its to describe the trans parent media which occupy the interior of the globe, and through which the rays of light must pass before they can reach the retina and form on it the images of external objects. We shall con sider them in the order in which the rays of light strike them.
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 epithe lium 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 pos terior surface is received into a corresponding depression on the fore part of the vitreous humor. In form, it is a double-convex lens, with surfaces of unequal curvature, 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 Tens 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, com monly called the fibres of the lens, which are developed from cells. These fibres are ar ranged side by side in lamellae, 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, toward the centre; by this means the con vergence of the central rays is increased, and they are brought to the same focus as the rays passing through the more circumferential portions of the lens. (According to Brewster, the refracting power at the surface is 1.3767. and at the centre 1.3990.) The lens contains 58 per cent. of water, 36 of albumen, with minute quantities of salts, membrane, etc. In consequence of its proteid constituent, it becomes hard and opaque on boil ing. as we familiarly see in the case of the eyes
of boiled fish. In the adult its long diameter ranges from one-third to three-eighths, and its antero-posterior diameter from one-eighth to one sixth of an inch, and it weighs three or four 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 mem brane, which sends numerous processek inward, as to divide the cavity into a series of com part monis. and thus to equalize the pressure ex erted by the inclosed soft, gelatinous mass. Be tween the anterior border of the retina and the border of I he lens, we have a series of radiating folds er plaiting: termed the ciliary processes of the vitreous body, into which the ciliary proc esses of the choroid dovetail. The vitreous humor contains 98.4 per cent. of water, with a trace of albumen and salts, and hence, as might be ex pected, its refractive index is almost indentical with that of water.
The appendages of the eye now claim our no tice. The most important of these appendages are the muse/es within the orbit, the eyelids, the lachrymal apparatus, and the conjunctira. to which (although less important) we may add the eyebrows.
The muscles by which the eye is moved arc four straight (or recti) muscles, and two oblique (the superior and inferior). The former rise from the margin of the optic foramen 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 backward, outward, and downward. The inferior oblique arises from the lower part of the orbit, and passes backward, outward, and upward. 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 toward their respective sides. The oblique muscles antagonize the recti, and draw the eye forward; the superior, acting above, directs the front of the eye down ward and outward, and the inferior upward and inward. By the duly associated action of these muscles, the eye is enabled to more (within defi• nite limits) in every direction.