The eyeball, lying in the orbital cavity, is largely protected from injury by the prominent eyebrow and the bones forming the cavity. The eyebrow possesses a considerable range of movement, and when struck is frequently divided by the underlying bone from within outwards, a clean-cut wound being thus produced. The well-defined supra-orbital margin is easily made out by palpation, as well as the supra-orbital notch at the junction of its inner and middle thirds, which transmits the supra-orbital vessels and nerve. Penetrating wounds in the orbital region are frequently serious, as the bones in this region being very thin, the brain, cavernous sinus, or even the internal carotid artery, may be damaged. Ar ter io-venous aneurysm may follow the wounding of the two latter structures. The upper eyelid covers about three-quarters of the anterior surface of the eyeball, and opening and closing of the eye is chiefly due to its movements. The tarsal cartilage of the upper eyelid is the larger of the two, and, commencing close to the palpebral fissure, extends upwards for rather more than 1/4 inch. Beyond its stiff upper margin the eyelid is soft, and this fact is illustrated in the usual method of everting the eyelid. On eversión of the lids the large Meibomian glands, whose ducts open along their margins, may be seen showing through the conjunctiva. Tarsal cysts are produced by blocking of these ducts, while a stye, or hordeolum, is a small boil originating in a hair follicle or sebaceous gland. Marginal blepharitis is an extensive inflammation of the palpebral margin, which frequently results in an inturning of hair follicles, the lashes thus growing toward and touching the conjunctiva, causing what is known as trichiasis. The skin of the eyelids is very lax, and advantage is taken of this in performing numerous plastic operations on the lids. This laxity, however, also favours the occurrence of marked deformity by traction of tubercular and other cicatrices, giving rise frequently to an eversión of the lower lid, known as ectropion. Entropion, or incurling of the lid. is generally the result of prolonged conjunctivitis, and most frequently affects the upper lid. Rodent ulcer frequently affects the eyelid, where the fact that it does not cause contraction is well illustrated, no contraction deformity resulting from its presence. The laxness of the tissues of the eyelids is also illustrated by the frequent occurrence of oedema from inflammatory conditions and Bright's disease ; by occasional emphysema in fracture of the nasal bone, with tearing of the nasal mucous membrane ; and by the readiness with which effusions of blood occur into them from a blow, producing a 1 black eye.' This condition requires to be distinguished from effusion of blood resulting from fracture of the orbital plate of the frontal bone. In black eye the effusion into the eyelids and under the conjunctiva occurs within a few hours of the injury, is general and diffuse, is generally of a chocolate colour, and the subconjunctival portion is limited to the anterior segment of the eyeball. The effusion due to fracture seldom appears before some forty-eight hours have elapsed, is patchy, of a blue colour over the eyelids, of which the lower is generally first affected (owing to gravity), and bright red over the conjunctival portion, the effusion here not being limited to the anterior segment, but extending backwards beyond the visible portion of the eyeball. As the occipito-frontalis muscle sends an expansion into the upper eyelids, effusions of pus or blood may extend from the scalp into the upper lid.

The eyelids are closed by the facial nerve acting on the orbicularis palpebrarum. Thus, in a complete facial paralysis due to destruction of the nerve trunk the patient cannot close the eye on the affected side (lagophthalmos) ; the eyeball appears slightly prominent (proptosis), owing to want of the restraining action of the orbicularis, and the conjunctiva tends sooner or later to become inflamed from undue exposure. The lower eyelid tends to droop outwards, allowing the tears, the secretion of which is increased by the irritation, to flow on to the cheek (epiphora). Blepharospasm, or persistent contraction of the muscle, is seen in photophobia from inflammation of the cornea and uveal tract, and sometimes in cases of otitis, trigeminal neuralgia, carious teeth, etc. The eyelids are opened by the third nerve acting on the levator palpebra? superioris. In paralysis of the third nerve there is therefore a marked drooping of the upper eyelid (ptosis), which can only be slightly raised voluntarily by an exaggerated action of the frontalis muscle, which at the same time wrinkles the forehead. A slight ptosis of the upper eyelid may also be produced by paralysis of the cervical sympathetic, which then cannot act on the unstriped fibres, and a pseudo ptosis may be caused by great swelling of the eyelid, or lipoma of the skin fold overhanging the upper eyelid (blepharo-chalasis), or to some congenital defect in the muscles. The fifth nerve supplies sensation to the eyelid, the first division supplying four twigs to the upper, and the infra-orbital of the second division supplying the lower. The blood-supply is derived from the ophthalmic of the internal carotid, which sends a small twig to the inner side of each eyelid, while by its lachrymal branch it sends a small twig to the outer side of each. Some of the lymphatics drain to the preauricular glands.

While the eyelids meet externally at an acute angle to form the outer canthus, internally they first diverge from one another (lacus lachrymalis) before forming the inner canthus. At the point of divergence on each lid there is a slight conical elevation, with a small aperture at its summit, the punctum lachrymale, which communicates with the lachrymal canaliculus, and conveys the tears to the lachrymal sac. The small space left by the divergence of the lids or lacus lachrymalis is occupied by the lachrymal caruncle. The tarsal cartilages of the upper and lower eyelids are connected internally and externally by the tarsal ligaments, which are Y-shaped, the stem of the Y being attached to the malar bone externally.