longitudinal folds of skin, the one above, the other below, which close like curtains over the eye. Beneath the external layer of skin in the lids is fatty tissue and then the orbicularis palpebrarum muscle by means of which they are closed. They are kept in shape by the tarsal plates or cartilages, in whose ocular surface are embedded the Meibomian glands, whose secretion prevents the free edges of the lids from sticking together. Along these edges grows a double or triple row of stiff hairs, the eye-lashes, which curve outward so as not to interfere with each other and also to prevent the entrance into the eye of foreign bodies. Lining the inner surface of the lids and reflected thence over the anterior surface of the sclerotic coat of the eye is a mucous membrane, the conjunctiva, which is thick, opaque, and vascular on the lids but thin and transparent on the eye-ball. The angles between the lids are known as the internal and the external canthus.
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Fig. 23.—The external ocular muscles. (Pyle.)
Muscles and Nerves.—The eyeball is held in position by the ocular muscles, the conjunctiva, and the lids, while surrounding it, yet allowing free movement, is a thin membranous sac, the tunica vaginalis oculi. The superior and inferior recti muscles at the upper and lower edges of the ball turn the eye up and down; the internal and external recti at the inner and outer
