The eyelid may be divided into four layers – the skin layer contains the eyelashes, the muscular layer contains the orbicularis oculi muscle, which lowers the eyelid to close the eye; the fibrous connective tissue layer contains many modified oil glands, whose secretions keep the eyelids from sticking together and the innermost layer is composed of a portion of the lining of the eyelid, the conjunctiva. Eyelids protect eyeball from dust and another harmful external objects.
The edges of the eyelids are lined with short, thick hairs, the eyelashes. They act as strainers to prevent foreign materials from entering the eye. The eyelids of each eye contain about 200 eyelashes. Eyebrows are thickened ridges of skin over the protruding frontal bone, covered with short , flattened hairs. They protect eye from sweat, excessive sunlight, and foreign materials.

The conjunctiva
It is a thin, transparent mucous membrane that lines the eyelids and bends back over the surface of the eyeball, terminating at the transparent cornea, which is uncovered. The portion that lines the eyelid is the palpebral conjunctiva, and the portion that covers the white of the eye is the bulbar conjunctiva. Between both portions of the conjunctiva are two recesses called the conjunctival sacs. The lacrimal apparatus
It is made up of the lacrimal gland, lacrimal sac, lacrimal ducts and nasolacrimal duct. The eyeball is kept moist by secretions of the lacrimal gland, located under the upper lateral eyelid and extending inward from the outer canthus of each eye. The eye blinks every 2 to 10 seconds, with each blink lasting only 0.3 to 0.4 second. Blinking stimulates the lacrimal gland to secrete a sterile fluid that serves at least four purposes. It washes foreign particles off the eye; it kills invading bacteria with a mild antibacterial enzyme, lysozyme; it distributes water and nutrients to the cornea and lens and it gives the eyeball a clear, moist and smooth surface.
The tears
The tears are composed of salts, water, the enzyme lysozyme and mucin (organic compounds produced by mucous membranes). In addition to the steady secretion of tears, reflex tears are produced in emergencies, as when the fumes from sliced onion irritate the eyes.
Approximately 3 to 12 lacrimal ducts lead from each gland onto the superior conjunctival sac in the upper eyelid. From there tears flow down across the eye into small openings near the inner canthus called lacrimal puncta. The puncta open into the lacrimal ducts, which drain excess tears from the area of the inner canthus to the lacrimal sac, the dilated upper end of the nasolacrimal duct. .