The globe of the eye is situated in the anterior portion of the orbit, beyond which it extends, and its axis, which is on the plane of that of the orbit, is directed inwards towards the centre of the base of the cranium. The eye is fixed in the orbit by an aponeurotic capsule, the optic nerve, and by six muscles which turn it in every direction. A mucous membrane, the conjunctiva, so named because it unites the eye to the lids, spreads over the anterior portion of the globe, as is proved by the injection of its vessels in some ophthalmic affections, and then folds back on itself, and lines the internal surface of the eyelids. According to some anatomists, it is not the conjunctiva, but only an expansion of its epithelium, which covers the cornea.
An elliptical muscle extends in front of the orbit, which is formed of concentric fascicles, and which presents a transverse chink closed during contraction, and open in the shape of an almond when its fibres are relaxed. This is the orbicular muscle of the eyelids. Its ocular surface is covered by the conjunctiva, its external face by the skin, its opening is circumscribed by the edge of the lids, which are made firm by the tarsal cartilages. The upper lid is larger than the lower, and is raised by a special muscle, the contraction of which alternates with that of the orbicularis, which is its antagonist The points where the eyelids are united by their commissures are called the angles of the eye. At the internal or greater angle of the eye, the conjunctiva forms a fold, the semi-lunar fold, which is in fact a rudimentary representative of the third eyelid (membrana nictitans) of certain animals. Inside of this fold is the lachrymal caruncle, a small glandular body of a rose colour, which is covered by the conjunctiva. The edges of the lids are ornamented with a fringe of silky hairs which protect the eye, and add greatly to its beauty. The greater or less extent of the opening of the lids makes the eye appear larger or smaller; the conformation of the palpebral muscles and the tarsal cartilages gives to the eye an elongated and languishing form as in the East, or round and bold as among the Occidentals; but the dimensions and form of the globe are the same in all countries and in all individuals.
The upper lid, which is attached to the arch of the orbit, is surmounted by the eyebrow, which is designed to protect the eye like a visor, and its movements play an important part in the expression of the face.
This is composed of, firstly, the lachrymal gland, which lies in a depression of the orbital arch, and of little glands of the same nature, which form a granular layer in the substance of the upper lid; secondly, of the lachrymal canals, by which the tears are poured out upon the conjunctiva, a little above the border of the upper lid; thirdly, the lachrymal ducts; which are destined to receive the tears after they have bathed the eye, and of which the orifices or lachrymal points are seen near the internal commissure of the lids; fourthly, the lachrymal sac, in which the lachrymal ducts terminate, and which empties the tears into the nasal canal.
The tears by running over the surface of the conjunctiva render it supple and facilitate the movements of the globe and eyelids by lessening the friction. They serve the same purpose in the eye as the synovia does in the articulations. The influence of moral or physical causes increases their secretion, and the lachrymal ducts do not suffice to carry them off when they run over the lids.