EYE, ANATOMY AND PHYSIOLOGY OF THE. In this article we shall consider: 1. The structure of the human eyeball, and of certain accessory parts or appendages which serve to protect that organ, and are essential to the due performance of its functions. 2. The most striking modifications which this organ presents in some of the lower animals. 3. The special uses of the various parts of the eye considered as an optical instrument; and 4. The action of the retina.
1. The globe of the eye is placed in the anterior part of the cavity of the orbit (q.v.), in which it is held in position by its connection with the optic nerve posteriorly, and with the muscles 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 composed of segments of two spheres of different diameters. Of these, the anterior, formed by the transparent cornea, has the smaller diameter, and is there fore the most prominent; and hence the anter6-posterior slightly exceeds (by about a line) the transverse diameter. The radius of the posterior or sclerotic segment is about "ths and that of the anterior segment about of an inch.
DI • When the eyes are in a state of repose, their antero-posterior axes are parallel; the optic nerves, on the other hand. diverge considerzthly from their Commissure within the cavity of the skull to the point where they enter the globe; consequently, their direction does not coincide with that of the eye. Each nerve enters the back of the globe at a distance of about -Ph of an in. on the inner side of the antero-posterior axis of the eye.
The eyeball is composed of several investing membranes, and of certain transparent structures, which are inclosed within them, and which, together with the cornea (one of the membranes), 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 skleros, 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 dura mater, the fibrous investment of the brain and spinal cord.
Near the entrance of the nerve, its thickness is about of an in.; from this it dimin ishes to about 4th; but in front it again becomes thicker, from the tendinous insertions of the straight muscles which blend with it. This coat, by its great strength and com paratively unyielding structure, maintains the inclosed parts in their proper form, and serves to protect them from external injuries.
The cornea (so called from its horny appearance) is a transparent structure, filling up the aperture left in the anterior part of the sclerotic. Its circumference is overlaid by the free edge of the sclerotic, which in some parts presents a groove, so as to retain it more firmly; and the connection by continuity of texture between the two structures is so close, that they cannot be separated in the dead body without considerable macer ation.
The cornea, in consequence of its greater convexity, projects beyond the line of the sclerotic; the degree of convexity, however, varies in different persons, and at different periods of life. It is thicker than any part of the sclerotic, and so strong as to be able to resist a force capable of rupturing that tunic.
Although beautifully transparent, and appearing to be homogeneous, it is in reality composed of five layers, clearly distinguishable from one another—viz. (proceeding from the front backwards) 1. The conjunctival layer of epithelium. It is in this epi thelium that particles of iron, stone, etc., forcibly driven against the eye, usually lodge, and it is a highly sensitive membrane. 2. The anterior elastic lamina forming the anterior boundary of the cornea proper; it is not more than nth of an inch in thick ness; 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 anterior lamina, to the exact maintenance of the curvature of the cornea, so necessary for correct vision. S. The posterior epithelium of the aqueous humor, which is probably concerned in the secretion of that fluid.