Eyelids, Conjunctiva, and Lacrimal Gland
The anterior portion of the eyeball is protected by the Eyelids, the Conjunctiva, and the fluid produced by the Lacrimal Gland.
- Each eyelid consists of an outer cutaneous portion lined by stratified squamous epidermis overlying a loose connective tissue, dermis, and skeletal muscle (orbicularis oculi muscle), and an inner conjunctival portion lined by a thin mucus membrane (the conjunctiva). The tarsal plate is responsible for the rigidity of the eyelids. The eyelid contains four other major types of glands (besides sweat glands and sebaceous glands in skin):
- Tarsal glands (Meibomian glands). The sebaceous secretion of the tarsal glands produces an oily layer on the surface of the tear film that retards the evaporation of the normal tear layer.
- Sebaceous glands of eyelashes (glands of Zeis). Small, modified sebaceous glands that are connected with and empty their secretion into the follicles of the eyelashes.
- Apocrine glands of eyelashes (glands of Moll). Modified apocrine sweat glands with unbranched sinuous tubules that begin as a simple spiral. Secrete lipid that adds to the superficial layer of the tear film. All glands of the human eyelid are innervated by neurons of the autonomic nervous system, and their secretion is synchronized with the lacrimal glands.
- Accessory lacrimal glands (glands of Wolfring and glands of Krause).
- The conjunctiva is made of stratified columnar epithelial cells with mucus-secreting goblet cells. At the corneal rim, the conjunctival epithelium becomes stratified squamous and is continuous with the corneal epithelium. A lamina propria with capillaries supports the lining epithelium.
- The lacrimal gland is a tubuloacinar serous gland with myoepithelial cells. It is organized into separate lobes with 12 to 15 independent excretory ducts. Tears are produced by the lacrimal glands and to a lesser degree by the accessory lacrimal glands. Blinking produces gentle compression of the lacrimal glands and the release of fluid. Tears keep the surface of the conjunctiva and cornea moist and rinse off dust particles. Tears protect the corneal epithelium and contain antibacterial and UV-protective agents.
- The thin film of tears covering the corneal surface is not homogeneous but a mixture of products secreted by the lacrimal glands, the accessory lacrimal glands, the goblet cells of the conjunctiva, and the tarsal glands of the eyelid. The tear film contains proteins including tear albumins, lactoferrin (antibacterial action by chelation of iron), lysozyme (antibacterial enzyme that degrades bacterial cell walls), immunoglobulin A (neutralizes infectious agents), and mucins.
- Lacrimal glands receive neural input from parasympathetic nerve fibers (predominate), inducing tear secretion, and sympathetic nerve fibers.