The Eyeball, or globe of the eye, is a device for focusing light upon a sensitized nervous surface which it incloses and protects. In shape it is nearly spherical, being about [pg 374]an inch in diameter from right to left and nine tenths of an inch both in its vertical diameter and from front to back. It has the appearance of having been formed by the union of two spherical segments of different size. The smaller segment, which forms about one sixth of the whole, is set upon the larger and forms the projecting transparent portion in front. The walls of the eyeballs are made up of three separate layers, or coats—an outer coat, a middle coat, and an inner coat (Fig. 159).
Fig. 159—Diagram of the eyeball in position. 1. Yellow spot. 2. Blind spot. 3. Retina. 4. Choroid coat. 5. Sclerotic coat. 6. Crystalline lens. 7. Suspensory ligament. 8. Ciliary processes and ciliary muscle. 9. Iris containing the pupil. 10. Cornea. 11. Lymph duct. 12. Conjunctiva. 13. Inferior and superior recti muscles. 14. Optic nerve. 15. Elevator muscle of eyelid. 16. Bone. A. Posterior chamber containing the vitreous humor. B. Anterior chamber containing the aqueous humor.
The Outer Coat surrounds the entire globe of the eye and consists of two parts—the sclerotic coat and the cornea. The sclerotic coat covers the greater portion of the larger spherical segment and is recognized in front as "the white of the eye." It is composed mainly of fibrous connective tissue and is dense, opaque, and tough. It preserves the form of the eyeball and protects the portions within. It is pierced at the back by a small opening which admits the[pg 375] optic nerve, and in front it becomes changed into the peculiar tissue that makes up the cornea.
The cornea forms the transparent covering over the lesser spherical segment of the eyeball, shading into the sclerotic coat at its edges. It has a complex structure, consisting in the main of a transparent form of connective tissue. It serves the purpose of admitting light into the eyeball.
The Middle Coat consists of three connected portions—the choroid coat, the ciliary processes, and the iris. These surround the larger spherical segment. All three parts are rich in blood vessels, containing the blood supply to the greater portion of the eyeball.
The choroid coat lies immediately beneath the sclerotic coat at all places except a small margin toward the front of the eyeball. It is composed chiefly of blood vessels and a delicate form of connective tissue that holds them in place. It contains numerous pigment cells which give it a dark appearance and serve to absorb surplus light. Near where the sclerotic coat joins the cornea, the choroid coat separates from the outer wall and, by folding, forms many slight projections into the interior space. These are known as the ciliary processes. The effect of these folds is to collect a large number of capillaries into a small space and to give this part of the eyeball an extra supply of blood. Between the ciliary processes and the sclerotic coat is a small muscle, containing both circular and longitudinal fibers, called the ciliary muscle.
The iris is a continuation of the choroid coat across the front of the eyeball. It forms a dividing curtain between the two spherical segments and gives the color to the eye. At its center is a circular opening, called the pupil, which admits light to the back of the eyeball. By varying the[pg 376] size of the pupil, the iris is able to regulate the amount of light which passes through and it employs for this purpose two sets of muscular fibers. One set of fibers forms a thin band which encircles the pupil and serves as a sphincter to diminish the opening. Opposing this are radiating fibers which are attached between the inner and outer margins of the iris. By their contraction the size of the opening is increased. Both sets of fibers act reflexively and are stimulated by variations in the light falling upon the retina.
Fig. 160—Diagram showing main nervous elements in the retina. Light waves stimulate the rods and cones at back surface of the retina, starting impulses which excite the ganglion cells at the front surface. Fibers from the ganglion cells pass into the optic nerve.