Surface Anatomy

The eye itself is covered externally by the eyelids, which are movable folds protecting the eye from injury and excessive light. The lids serve to swab the eye and spread a film of tears over the cornea, thereby preventing evaporation from the surface of the eye. The upper eyelid extends to the eyebrow, which separates it from the forehead, whereas the lower lid usually passes without any line of demarcation into the skin of the cheek. The upper lid is the more mobile of the two, and when the lid is open it covers about 1mm of the cornea. A muscle that elevates the lid, the levator palpebrae superioris, is always active, keeping the eyelid open. During sleep the eyelid closes by relaxation of this muscle. The lower lid lies at the lower border of the cornea when the eye is open, and rises slightly when it shuts.

Normally, when the eyes are open there is a triangular space visible on either side of the cornea. These triangular spaces are formed by the junction of the upper and lower lids, and are called the canthi of the eye.

(Fig. 2)  Surface Anatomy  

Most eyes are practically the same size; therefore, when we speak of the eyes appearing large or small, we usually refer not to the actual size but portion of the eyeball visible on external examination, which in turn depends on the size of the palpebral fissure. In the medial canthus there are two fleshy mounds; the deeper one, called the plica-semilunaris, and the superficial one, called the caruncle. The caruncle is modified skin containing sweat and oil glands.