The lens is situated behind the pupil and is contained within a capsule of its own.
The capsule is a close-fitting, firm, transparent membrane. The anterior surface forms the posterior boundary of the cavity containing the aqueous humor, and the iris in its movement glides on it. The posterior surface is in contact with the vitreous humor.
The vitreous humor occupies four-fifths of the interior of the eyeball. It is globular in form, with a depression in front for the lodgment of the lens. It is colorless, transparent, and of a consistency like thin jelly. It is enveloped by a delicate capsule—the hyaloid membrane—which is connected in front with the suspensory ligament of the lens, and ends by joining the capsule behind the lens.
The orbital cavity, at the side of the head, is circumscribed by a bony margin; posteriorly, however, there are no bony walls, and the cavity is often confounded with the depression above and behind the orbit—the temporal fossa. A fibrous membrane completes this cavity and keeps it distinct from the temporal fossa. This membrane—the ocular sheath or periorbita—is attached posteriorly around the opening in the back part of the orbital cavity (the orbital hiatus) and anteriorly to its inner face; then it becomes prolonged beyond the margin to form the fibrous membrane of the eyelids. When complete the orbital cavity has the form of a regular hollow cone, open at its base and closed at the apex. The opening of this cone is directed forward, downward, and outward. Independently of the globe of the eye, this cavity lodges the muscles that move it, the membrana nictitans, and the lacrimal gland.
The muscles of the eye are seven in number—one retractor, four straight, and two oblique. The retractor oculi envelops the optic nerve between the brain and the ball of the eye and becomes attached upon the external face of the sclerotic tunic. When this muscle contracts, it draws the globe back into the orbit, away from the light. The superior, inferior, external, and internal recti or straight muscles are attached to the back part of the orbital sheath and spread forward in four bundles over the globe of the eye, where they are inserted by a fibrous expansion into the sclerotic at the margin of the cornea. When they act singly, they turn the globe either upward, downward, inward, or outward. The great oblique, by its action, pivots the eye inward and upward in the orbit. The small oblique turns the eye outward and downward.
The eyelids are two movable curtains, superior and inferior, which cover and protect the eye in front. They are attached to the circumference of the orbit and have a convex external face formed by the skin and a concave internal face molded on the anterior surface of the eye and are lined by the conjunctiva, which is reflected above and below on the eyeball. The border of each lid is slightly beveled on the inner side and shows the openings of the Meibomian glands. These glands secrete an unctuous fluid, which is thrown out on the border of the lids, the function of which is to facilitate their movements and enable them to retain the tears in the ocular cavity. The eyelid is composed of a fibrous inner membrane ending in a stiff arch near the border, a muscle to close the lid, another to open it, the skin externally, and the conjunctival mucous membrane internally. The border of each lid is covered and protected by long hairs to prevent floating particles of matter in the atmosphere from gaining entrance to the eye.
The membrana nictitans, which is also named the third eyelid, winking eyelid, haw, etc., is placed at the inner angle of the eye, whence it extends over the eyeball to relieve it from foreign bodies which may fall upon it. It has for its framework a fibro-cartilage, irregular in shape, thick, nearly prismatic at its base, and thin anteriorly where it is covered by the conjunctiva; behind, it is loosely attached to a fatty cushion.
The lacrimal gland is situated between the orbital process and the upper part of the eyeball. It secretes the tears destined to lubricate the anterior surface of the eye. This fluid escapes upon the organ at the outer angle of the lids and is carried between them and the eyeball toward the inner angle.
The caruncula lacrimalis is a small round body, frequently entirely or partially black, situated in the inner angle of the eye, and is designed to direct the tears toward the puncta lacrimalia.
The puncta lacrimalia are two little openings, situated one in each eyelid, a short distance from the inner corner, which admit the tears into the lacrimal ducts leading to the lacrimal canal, whence they are emptied into the nasal passages.