Conjunctiva

The conjunctiva (Latin: tunica conjunctivae) is a thin mucous membrane covering the inner surface of the eyelids and the sclera. The conjunctiva helps to lubricate the eye.

Structure of the conjunctiva

The conjunctival epithelium changes into the epidermis of the skin at the eyelid's margin and into the corneal epithelium at the limbus. A conjunctival sac is formed by these continuations. The conjunctiva can be divided into three parts or regions: the palpebral conjunctiva, conjunctival fornices, and bulbar conjunctiva.

The palpebral conjunctiva is the part that attaches to the posterior surfaces of the tarsal plates and lines the inner surface of the eyelids. By being reflected onto the anterior surface of the eyeball is covers part of the sclera. This part of the conjunctiva is highly vascularized and thus gives a reddish colour to it.

The conjunctival fornices are the regions between the palpebral and bulbar conjunctiva. The conjunctiva of the superior and inferior fornices is lightly attached to the fascial expansion of the superior levator palpebrae and recti muscles’ sheaths. When these muscles contract, the conjunctiva is able to move alongside the eyeball and eyelids. The lateral part of the superior fornix is where the lacrimal gland’s ducts open. Both fornices are located at different distances from the limbus: the superior – 10 mm from the limbus, while the inferior – 8 mm. The fornices are present only on the lateral side, while on the medial side, they are absent and replaced by the lacrimal caruncule and plica semilunaris.

The bulbar conjunctiva is in direct contact with the eyeball. It is translucent. The bulbar conjunctiva is lightly attached to the sclera, and the bulbar fascia covering the recti muscles’ tendons via connective tissue. Near the cornea, the bulbar conjunctiva more tightly attaches to the sclera and fascia. The line called the conjunctival limbus is where the conjunctiva emerges with the cornea.

The conjunctiva is covered by stratified columnar epithelium. This epithelium changes to stratified squamous, nonkeratinized epithelium at the limbus region. The conjunctiva houses the accessory lacrimal glands within its connective tissue. The conjunctival submucosa consists of fine connective tissue that ends at the edges of the cornea.

Vasculature and innervation of the conjunctiva

Blood supply and venous drainage

The conjunctiva receives its blood supply from the two palpebral arches and anterior ciliary arteries. The palpebral arches can be divided into large marginal and small peripheral arches.

The larger marginal arch branches supply the palpebral conjunctiva. The small peripheral arch supplies the superior and inferior conjunctival fornices. Some branches from the small arch travel below the bulbar conjunctiva and from the posterior conjunctival arteries that supply the bulbar conjunctiva. These arteries anastomose with the anterior conjunctival arteries from the anterior ciliary arteries. In some people, the peripheral arch may be absent, so the marginal arch, together with the anterior ciliary arteries, supply the conjunctiva.

Venous drainage happens via veins running along the arteries and drain into the palpebral veins or directly into the superior and inferior ophthalmic veins.

Lymphatic drainage

The lymphatic vessels in the conjunctiva are in the form of the superficial and deep plexuses in the submucosa. The lymph from the lateral side drains into the superficial parotid nods, while from the medial side – the submandibular nodes.

Innervation

The bulbar conjunctiva receives its innervation from the long ciliary nerves originating from the nasociliary nerve from the ophthalmic division of the trigeminal nerve (CN V). The superior palpebral conjunctiva and the superior fornix are innervated by the frontal and lacrimal branches of the ophthalmic division of the trigeminal nerve. Inferior palpebral conjunctiva and inferior fornix receive innervation from the lacrimal branch of the ophthalmic division of the trigeminal nerve laterally and the infraorbital nerve originating from the maxillary division of the trigeminal nerve medially.