The Conjunctiva, Tenon’s Capsule, The Sclera And Episclera

A teaching microscope aids in the large scale viewing of the different eye parts.

The conjunctiva, when examined and viwed with the use of a teaching microscope, is the thin, transparent mucous mem¬brane that covers the posterior surface of the lids (the palpebral conjunctiva) and the anterior surface of the sclera (the bulbar conjunctiva). It is continuous with the skin at the lid margin (a mucocutaneous junction) and with the corneal epithelium at the limbus. The palpebral conjunctiva lines the posterior sur¬face of the lids and is firmly attached to the tarsus. At the superior and inferior margins of the tarsus, the con¬junctiva is reflected posteriorly (at the superior and in¬ferior fornices) and covers the episcleral tissue to be¬come the bulbar conjunctiva.

The bulbar conjunctiva is loosely attached to the orbital septum in the fornices and is folded many times. This allows the eye to move and enlarges the secretory conjunctival surface. (The ducts of the lacrimal gland open into the superior temporal fornix.) Except at the limbus (where Tenon’s capsule and the conjunctiva are fused for about 3 mm), the bulbar conjunctiva is loosely attached to Tenon’s capsule and the underlying sclera, as seen in a teaching microscope.

A soft, movable, thickened fold of bulbar conjunc¬tiva (the semilunar fold) is located at the inner canthus and corresponds to the nictitating membrane of some lower animals. A small, fleshy, epidermoid structure (the caruncle) is attached superficially to the inner por¬tion of the semilunar fold and is a transition zone con¬taining both cutaneous and mucous membrane ele¬ments.

Histology When examined under a teaching microscope, the conjunctival epithelium consists of two to five layers of stratified columnar epithelial cells, superficial and basal. Conjunctival epithelium near the limbus, over the caruncle, and near the mucocutaneous junc¬tions at the lid margins consists of stratified squamous epithelial cells. The superficial epithelial cells contain round or oval mucus-secreting goblet cells. The mucus, as it forms, pushes aside the goblet cell nucleus and is necessary for proper dispersion of the precorneal tear film. The basal epithelial cells stain more deeply than the superficial cells and near the limbus may contain pigment.

The conjunctival stroma, as evident in the teaching microscope, is divided into an adenoid (superficial) layer and a fibrous (deep) layer. The ade¬noid layer contains lymphoid tissue and in some areas may contain “follicle-like” structures without germinal centers. The adenoid layer does not develop until after the first 2 or 3 months of life. This explains why inclu¬sion conjunctivitis of the newborn is papillary in nature rather than follicular and why it later becomes follicular. The fibrous layer is composed of connective tissue that attaches to the tarsal plate. This explains the ap¬pearance of the papillary reaction in inflammations of the conjunctiva. The fibrous layer is loosely arranged over the globe.

The accessory lacrimal glands (glands of Krause and Wolfring), which resemble the lacrimal gland in structure and function, are located in the stroma. Most of the glands of Krause are in the upper fornix, the re¬maining few in the lower fornix. The glands of Wolf¬ring lie at the superior margin of the upper tarsus.

Blood Supply, Lymphatics, & Nerve Supply The conjunctival arteries are derived from the anterior ciliary and palpebral arteries. The two arteries anastomose freely and-along with the numerous conjuncti¬val veins that generally follow the arterial pattern¬form a considerable conjunctival vascular network. The conjunctival lymphatics are arranged in superficial and deep layers and join with the lymphatics of the eyelids to form a rich lymphatic plexus. The conjunctiva re¬ceives its nerve supply from the first (ophthalmic) divi¬sion of the fifth nerve. It possesses a relatively small number of pain fibers.

TENON’S CAPSULE (Fascia Bulbi) Tenon’s capsule is a fibrous membrane that envelops the globe from the limbos to the optic nerve. Adjacent to the limbos, the conjunctiva, Tenon’s capsule, and the episclera are fused together. More posteriorly, the inner surface of Tenon’s capsule lies against the sclera, and its outer aspect is in contact with orbital fat and other structures within the extraocular muscle cone. At the point where Tenon’s capsule is pierced by tendons of the extraocular muscles in their passage to their at¬tachments to the globe, it sends a tubular reflection around each of these muscles. These fascial reflections become continuous with the fascia of the muscles, the fused fasciae sending expansions to the surrounding structures and to the orbital bones. The fascial expan¬sions are quite tough and limit the action of the ex¬traocular muscles and are therefore known as check lig¬aments. They regulate the direction of action of the extraocular muscles and act as their functional mechan¬ical origins. The lower segment of Tenon’s capsule is thick and fuses with the fascia of the inferior rectus and the inferior oblique muscles to form the suspensory lig¬ament of the eyeball (Lockwood’s ligament), upon which the globe rests as seen in a teaching microscope.

THE SCLERA & EPISCLERA The sclera is the fibrous outer protective coating of the eye consisting almost entirely of collagen. When seen under a teaching microscope, it is dense and white and continuous with the cornea anteriorly and the dural sheath of the optic nerve poste¬riorly. Across the posterior scleral foramen are bands of collagen and elastic tissue, forming the lamina cribrosa, between, which pass the axon bundles of the optic nerve. A thin layer of fine elastic tissue, the epi¬sclera, which contains numerous blood vessels that nourish the sclera, covers the outer surface of the anterior sclera. The brown pigment layer on the inner surface of the sclera is the lamina fusca, which forms the outer layer of the suprachoroidal space.

At the insertion of the rectus muscles, the sclera is about 0.3 mm thick; elsewhere it is about 0.6 mm thick. Around the optic nerve, the long and short posterior ciliary arteries and the long and short ciliary nerves penetrate the sclera. The long posterior ciliary arteries and long ciliary nerves pass from the optic nerve to the ciliary body in a shallow groove on the inner surface of the sclera at the 3 and 9 o’clock meridians. Slightly posterior to the equator, are the four vortex veins draining the choroid exit through the sclera, usually one in each quadrant. About 4 mm pos¬terior to the limbus, slightly anterior to the insertion of the respective rectus muscle, the four anterior ciliary ar¬teries and veins penetrate the sclera. The nerve supply to the sclera is from the ciliary nerves.

Histologically using a teaching microscope, the sclera consists of many dense bands of parallel and interlacing collagen bundles, each of which is 10-16 µm thick and 100-140 µm wide. The histologic structure of the sclera is remarkably sim¬ilar to that of the cornea. The reason for the trans¬parency of the cornea and the opacity of the sclera is the relative deturgescence of the cornea. Read more on this subject