In the sense of vision, one consciously perceives the various objects being looked upon. In addition to this, there are a number of protective reactions to visual input - the visual reflexes.
When an unexpected visual stimulus occurs within the visual field, the individual's response will often include movement and other types of reaction. This is a part of the startle reflex.
When there is a change in the amount of light entering the eyeball, the size of the pupil will change. This is the pupillary reflex. The muscles of the iris automatically constrict or dilate to control the amount of light entering the eyeball.
In the blink reflex, the eyelids automatically move over the exterior surface of the eyeball. This reflex results in the automatic washing of the exterior surface of the eyeball with the lacrimal fluids. It also helps to keep the surface moist.
The eyeball is suspended in the orbit and faces outward. Helping to fill the orbit are a number of structures associated with the eyeball; these are the adnexa. Among these other structures is the lacrimal apparatus.
The lacrimal gland is located in the upper outer corner in front. Via small ducts, it secretes the lacrimal fluid into the space between the external surface of the eyeball and the upper eyelid.
The inner surface of the eyelids and the outer surface of the eyeball are covered by a continuous membrane known as the conjunctiva. The lacrimal fluid keeps the conjunctiva transparent. Also, with the blink reflex, the lacrimal fluid washes away any foreign particles that may be on the surface of the conjunctiva.
The free margins of the upper and lower eyelids have special oil glands. The oily secretion of these glands helps prevent the lacrimal fluid from escaping.
With the movement of the eyeball and the eyelids, the lacrimal fluid is gradually moved across the exterior surface of the eyeball to the medial inferior corner. Here, the lacrimal fluid is collected into a lacrimal sac, which drains into the nasal chamber by way of the nasolacrimal duct.
Thus, the continuous production of lacrimal fluid is conserved by being recycled within the body.