The tear film is currently thought to be a mixed gel consisting of soluble mucus, fluids, and proteins that are contributed by the lacrimal gland, conjunctival goblet cells, and surface epithelium (Fig 1-3). This hydrophilic gel moves over the glycocalyx of the superficial corneal epithelial cells and is topped by a lipid layer produced by the meibomian glands.

A healthy tear film is critical for both good vision and a healthy eye. Functions of the tear film include

Maintenance of the tear film is thus critical to normal corneal function.

The conjunctiva can be broadly divided into 3 geographic zones as follows:

The plica semilunaris is a crescent-­shaped vertical fold of conjunctiva, located at the medial ­angle of the eye. The caruncle—­a fleshy, ovoid mass approximately 5 mm high and 3 mm wide—is attached to the inferomedial side of the plica semilunaris and contains goblet cells and lacrimal tissue, as well as hairs, sebaceous glands, and sweat glands. This area also contains the lacus lacrimalis (lacrimal lake), a small triangular space where tears accumulate ­after bathing the ocular surface.

The cell morphology of the conjunctival epithelium varies from stratified cuboidal over the tarsus and columnar in the fornices to squamous on the globe. Goblet cells account for up to 10% of basal cells of the conjunctival epithelium and are most numerous in the palpe­ bral conjunctiva, the inferonasal bulbar conjunctiva, and the area of the plica semilunaris.

The substantia propria of the conjunctiva consists of loose connective tissue. Conjunctiva-­ associated lymphoid tissue (CALT), which consists of lymphocytes and other leukocytes, is pre­sent, especially in the fornices. Lymphocytes interact with mucosal epithelial cells through reciprocal regulatory signals mediated by growth ­factors, cytokines, and neuropeptides.

The palpebral conjunctiva shares its blood supply with the eyelids. The bulbar con­ junctiva is supplied by the anterior ciliary arteries, which arise from muscular branches of the ophthalmic artery. ­These capillaries are fenestrated and leak fluid, producing chemo- sis (conjunctival swelling), as a response to allergies or other inflammatory events.

The sclera is the opaque, white, fibrous tissue that extends from the corneal limbus to the optic nerve, where it merges to form the dural sheath of the optic nerve. It is divided into 3 layers (from outermost inward): episclera, stroma, and lamina fusca. The scleral stroma is composed of collagen fibers, which have varied orientation, separation, and dia­meter, resulting in the sclera’s opaque appearance in contrast to the cornea. The sclera receives its vascular supply from the anterior and posterior ciliary arteries and drains through the vortex veins. The thickness of the sclera ranges from 0.3 mm to 1 mm; it is thinnest ­behind the insertions of the rectus muscles. It is innervated by the ciliary nerves of cranial nerve V₁.

For additional discussion of the sclera, see BCSC Section 2, Fundamentals and Princi­ ples of Ophthalmology, and Section 4, Ophthalmic Pathology and Intraocular Tumors.

The cornea is a transparent, avascular tissue that consists of 5 layers (Activity 1-1; Fig 1-4): epithelium, Bowman layer, stroma, Descemet membrane, and endothelium; ­these are dis­ cussed in the subsections that follow.

In adults, the cornea mea­sures approximately 11–12 mm horizontally and 10–11 mm vertically. It is 500–600 µm thick at its center, gradually increasing in thickness ­toward the periphery.

ACTIVITY 1-1  Corneal layers and corresponding confocal images.

Confocal images courtesy of Danielle Trief, MD, and David D. Verdier, MD.

For nutrition, the cornea depends on diffusion of glucose from the aqueous humor and diffusion of oxygen through the tear film. The peripheral cornea is supplied with oxygen from the limbal circulation. The density of nerve endings in the cornea is among the high­ est in the body, and the sensitivity of the cornea is 100 times that of the conjunctiva. Sen­ sory nerve fibers extend from the long ciliary nerves, penetrating the cornea in the deep peripheral stroma near the limbus and coursing anteriorly to form a subepithelial plexus.