The bulb of the eye (bulbus oculi; eyeball), or organ of sight, is contained in the cavity of the orbit, where it is protected from injury and moved by the ocular muscles. Associated with it are certain accessory structures, viz., the muscles, fasciæ, eyebrows, eyelids, conjunctiva, and lacrimal apparatus.
The bulb of the eye is imbedded in the fat of the orbit, but is separated from it by a thin membranous sac, the fascia bulbi.
It is composed of segments of two spheres of different sizes.The anterior segment is one of a small sphere; it is transparent, and forms about one-sixth of the bulb. It is more prominent than the posterior segment, which is one of a larger sphere, and is opaque, and forms about five-sixths of the bulb.
The term anterior pole is applied to the central point of the anterior curvature of the bulb, and that of posterior pole to the central point of its posterior curvature; a line joining the two poles forms the optic axis.
The axes of the two bulbs are nearly parallel, and therefore do not correspond to the axes of the orbits, which are directed forward and lateralward. The optic nerves follow the direction of the axes of the orbits, and are therefore not parallel; each enters its eyeball 3 mm. to the nasal side and a little below the level of the posterior pole. The bulb measures rather more in its transverse and antero-posterior diameters than in its vertical diameter, the former amounting to about 24 mm., the latter to about 23.5 mm.; in the female all three diameters are rather less than in the male; its antero-posterior diameter at birth is about 17.5 mm., and at puberty from 20 to 21 mm.