Anatomy of the Cat

by Jacob Reighard; H. S. Jennings

M. levator palpebræ superioris.—This is a small thin muscle which takes origin on the wall of the optic foramen close to the rectus superior, passes over the outer surface of the rectus superior and beneath the lachrymal gland, and forms a thin tendon which is inserted into the margin of the upper eyelid. It is innervated by the oculomotor.

5. The Eyeball.—The eyeball is approximately spherical in form, but the cranial surface has a slightly greater curvature than the caudal portion, so that the eye appears slightly pointed at the cranial (free) surface. The large cylindrical optic nerve ([Fig. 167], a) enters the eyeball at nearly the centre of the caudal half of the eye. On the outer surface is seen the transparent cornea (c), covering the free surface of the eye, and the opaque sclerotic (b), covering the remainder of the ball. Through the cornea can be seen the yellow curtain-like iris, with an opening in its centre, the pupil.

Fig. 167.—Diagram of a Section of the Eye.

a, optic nerve; b, sclerotic (b′, thickened “white zone”); c, cornea; d, choroid; e, iris; f, ciliary body; g, retina; h, crystalline lens; i, zonula ciliaris; j, anterior chamber of eye; k, vitreous humor.

Coats of the Eye ([Fig. 167]).—The outer coat of the eyeball is formed by the sclerotic (or sclera) (b) and the cornea (c). The sclerotic is the whitish opaque portion covering the caudal three-fourths of the eye. It forms a tough sac serving as a protection for the structures within. The cornea (c) is the circular transparent portion of the outer coat, covering the free surface of the eye; it is a direct continuation of the sclerotic. Just before passing into the cornea the sclerotic is much thickened, forming a broad white zone (b′) about the eyeball near its equator. To this zone, at its caudal margin, are attached the rectus muscles of the eyeball. The sclerotic has many pigment-cells on its inner surface, giving it a brownish appearance and forming the lamina fusca.

The cornea (c) is transparent and more strongly curved than the sclerotic. It is thicker than most of the sclerotic coat, but not so thick as the white zone of the latter.

Within the sclerotic is the second coat of the eye, formed by the choroid (d) and the iris (e). The choroid (d) is a vascular layer, and contains much pigment, giving it a dark color. It lines the inner surface of the sclerotic, but does not cover the inner surface of the cornea. At the white zone of the sclerotic it is thrown into a large number (about seventy) of meridional folds, the plicæ ciliares or ciliary folds, which together constitute the so-called ciliary body (corpus ciliare) (f). This forms a circular zone on the inner surface of the white zone of the sclerotic, and is continuous with the iris (e). The choroid coat is furnished in certain parts of its area with a layer of cells containing crystals, which give it a brilliant color, forming the tapetum. The color of the tapetum is a metallic yellowish blue or green; it is this layer which causes the cat’s eye to “shine” in the dark.

The iris (e) is a continuation of the choroid. It projects inward from the white zone of the sclerotic, forming a circular curtain lying some distance behind the cornea, and perforated by an opening, the pupil. The iris is usually yellow in color on its outer surface, darkly pigmented within. The pupil varies normally in size and form according to the amount of light to which the eye is subjected. In a cat killed with chloroform the pupil is very large and circular; in the living animal it is elliptical with the long axis dorsoventral, or when much contracted it is a mere dorsoventral slit.

The inner coat of the eye is formed by the retina (g). This is the part of the eye which is sensitive to light; it is formed by an expansion of the optic nerve (a). In a preserved eye it is usually soft and opaque, and may be seen to line the caudal half of the inner surface, extending apparently to the ciliary body. Here it seems to end as a free margin, the ora serrata; it really becomes thin and passes onto the surface of the ciliary body, forming the ciliary portion of the retina, and onto the inner surface of the iris, where it forms the uvea. Near the centre of the retina the entrance of the optic nerve is marked by a small round spot, the blind spot.