The retina is remarkably complex, consisting of ten distinct layers, from within outward, supported by connective tissue. These are as follows, viz.: 1. Membrana limitans interna. 2. Fibers of optic nerve. 3. Layers of ganglionic corpuscles. 4. Molecular layer. 5. Internal granular layer. 6. Molecular layer. 7. External granular layer. 8. Membrana limitans externa. 9. Jacobson's membrane, or layer of rods and cones. 10. The layer of pigment cells.

The most important of these, however, is the layer of rods and cones in the external portion of the retina. The rods are straight, elongated cylinders extending through the entire thickness of Jacobson's membrane. They consist of an external portion, which is clear, homogeneous, and highly refracting, and of an internal portion which is slightly granular and less refractive; the outer end of each rod is in direct contact with the pigmentary epithelium lining the choroid, while the inner end, tapering to a fine thread, pierces the external limiting membrane and passes into the external granular layer. The cones consist also of two portions, the inner of which is somewhat thicker than the rod and rests upon the limiting membrane; the outer portion tapers to a fine point, which is known as the cone-style. The cones, as a rule, are somewhat shorter than the rods. The proportion of rods to cones varies in different parts of the retina, though there are on the average about fourteen rods to one cone. In the macula lutea, where vision is most acute, the rods are almost entirely absent, cones alone being present. All the retinal elements at this point are changed. The nerve fiber layer is absent, the axis cylinders radiating in such a manner as to leave the spot free from their covering. The remaining layers are all thinned and the stroma reduced to a minimum. The optic nerve after passing forward from the brain penetrates in succession the sclerotic, choroid, and retina; the nerve fibers then spread out over the anterior surface of the retina and become connected with the large ganglionic cells, the third layer of the retina.

The number of optic nerve fibers in the retina is estimated to be about 800,000, and for each fiber there are about seven cones, one hundred rods, and seven pigment cells. The points of the rods and cones are directed toward the choroid, or away from the entering light, and dip into the pigmentary layer. They, with the pigment layer, are the elements intermediating the change of the ethereal vibrations into nerve force; out of these nerve vibrations the brain fashions the sensations of light, form, and color. The vitreous humor, which supports the retina, is the largest of the refracting media; it is globular in form and constitutes about four-fifths of the ball; it is hollowed out anteriorly for the reception of the crystalline lens,

The outer surface of the vitreous is covered by a delicate, transparent membrane, termed the hyaloid membrane, which serves to maintain its globular form.

The aqueous humor found in the anterior chamber of the eye is a clear alkaline fluid, having a specific gravity of 1.003-1.009. It is secreted most probably by the blood-vessels of the iris and ciliary processes. It passes from the interior of the eye, through the canal of Schlemm and the meshes at the base of the iris, into the anterior circular vein.

The crystalline lens, enclosed within its capsule, is a transparent bi-convex body, situated just behind the iris and resting in the depression in the anterior part of the vitreous. The two convexities are not quite alike, the curvature of the posterior surface being slightly greater than that of the anterior. The lens measures about one-third of an inch in the transverse diameter and one-fifth of an inch in the antero-posterior diameter.

The suspensory ligament, by which the lens is held in position, is a firm, transparent membrane, united to the ciliary processes. A short distance beyond its origin, it splits into two layers, the anterior of which is inserted into the capsule of the lens and blends with it; the posterior, passing inward behind the lens, becomes united to its capsule. The anterior layer presents a series of foldings, Zone of Zinn, which are inserted into the intervals of the folds of the ciliary processes. The triangular space between the two layers is the canal of Petit.

Blood-vessels and Nerves. The structures composing the eyeball are supplied with blood by the long and short ciliary arteries, branches of the ophthalmic; they pierce the sclerotic at various points and are ultimately distributed to all tissues within the ball.

The nerve supply comes largely from the ophthalmic or ciliary ganglion. This is a small body, situated in the posterior part of the orbit; it receives motor fibers from a branch of the motor-oculi, or third nerve; a sensory branch from the ophthalmic division of the fifth nerve and fibers from the cavernous plexus of the sympathetic. From the anterior border of the ganglion proceed the ciliary nerves, which, entering the eyeball, endow its structures with motion and sensation.

The Eyeball a Living Camera Obscura.-The eyeball may be compared in a general way to a camera obscura. The anatomical arrangement of its structures reveal many points of similarity. The sclerotic and choroid may be compared with the walls of the chamber; the combined refractive media, cornea, aqueous humor, lens, and vitreous humor, to the lens for focusing the rays of light; the retina to the sensitive plate receiving the