EYE (AS. rays, Goth. augO, OHG. ouga, Ger. Auge, Icel. gaga, 0Sw. ouga, Sw. iiga, Dan. Nor. ale, Lat. oeutus, eye. Gk. 6acre, ossc, the two eyes, OChurch Slay. oko, Slat. ak.an, eye). The organ of sight. In this article we shall consider the structure of the human eyeb.,I1, and of certain accessory parts or appendages which serve to protect that organ. and are essential to the due performance of its functions.
The globe of the eye is placed in the an terior part of the cavity of the orbit, in which it is held in position by its connection with the optic nerve posteriorly, and with the nmaeles which surround it, and by the eyelids in front. It is further supported behind and on the sides by a quantity of loose fat, which fills up all the interstices of the orbit and facilitates the various movements of which the eye is capable. The form of the eyeball is nearly spherical: but on viewing the organ in profile we see that, it is com posed of segments of two spheres of different dinineirrq. Of thew the anterior, formed by the transparent cornea, has the smaller diameter. and is therefore the most prominent, and hence the antero-nnierior slightly exceeds (by about line) the trousverse diameter. The radius of the posterior or sclerotic segment is about nine teen-fortieths, and that of the anterior seg ment about thirteen-fortieths of an inch. When the eyes are in a state of repose, their antero posterior axes are parallel; the optic nerves, on the other hand, diverge considerably from their conunissure within the cavity of the skull to the point where they enter the globe; consequent ly, their direction does not coincide with that of the eye. Each nerve enters the back of the globe at a distance of about one-eighth of an inch on the inner side of the antero-posterior axis of the eye.
The eyeball is composed of several investing membranes and of certain transparent struc tures which are inclosed within them, and which, together with the cornea, act as refractive media of various densities upon the rays of light which enter the eye.
The outermost coat of the eye is the sclerotic ( from Grk. atAap6s, skiros, hard). It is a strong, dense, white. fibrous structure, covering about four-fifths of the eyeball, and leaving a circular deficiency anteriorly, which is occupied by the cornea. Posteriorly, it is perforated by the optic nerve, and it is there continuous with the sheath which that nerve derives from the Jura miter, the fibrous investment of the brain and spinal cord. Near the entrance of the nerve, its thick
ness is about one-twentieth of an inch: from this it diminishes to about one-fortieth: hut in front it again becomes from the tendinous insertions of the straight muscles which blend with it. This coat. by its great strength and comparatively unyielding structure, maintains the inclosed parts ill their proper form and serves to protect them from external injuries.
The cornea (so called from its horny appear ance) is a transparent structure. filling up the aperture left in the anterior part of the sclerotic. Its eircumference is overlain by the free edge of the sclerotic, which in some parts presents a groove, so as to retain it more firmly; and the (.onnection by continuity of texture between the two structures is so close that they cannot be separated in the dead body without considerable maceration. The cornea, in consequence of its grenter convexity, projects beyond the line of the the degree of convexity, however, varies in different persons aud at different periods of life. It is thicker than any part of the sclerotic, and so strong as to be able to resist a form ca pable of rupturing that tunic. Although beauti fully transparent and appearing to be homogene ous, it is in reality composed of five layers, clear ly distinguishable from one another—viz. (pro ceeding from the front backward) : (1) The conjunctival layer of epithelium. It is in this epithelium that particles of iron, stone, etc., for cibly driven against the eye usually lodge, and it is a highly sensitive membrane. (2) The an terior elastic lamina forming the anterior boun dary of the cornea proper; it is not more than one two-thousandth of an inch in thickness, and its function seems to be that of maintaining the exact curvature of the front of the cornea. (3) The cornea proper, on which the thickness and strength of the cornea mainly depend. (4) The posterior elastic lamina, which is an extremely thin membrane, in which no structure can be detected. It probably contributes, like the an terior lamina, to the exact maintenance of the curvature of the cornea, so necessary for correct vision. (5) The posterior endothelium of the aqueous humor, which is probably concerned in the secretion of that fluid. For further details regarding these different layers, the reader is re ferred to Prudden, A Manual of Practical Normal Histology (New York, 1886).