The Sense of Sight / All About the Eyes

submitted by Dr. Gary Farr Last Updated June, 14, 2002

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Page: 11

The work of the auxiliary structures

The protective mechanisms

Secretion of tears

The exposed surface of the globe (eyeball) is kept moist by the tears secreted by the lacrimal apparatus, together with the mucous and oily secretions of the other secretory organs and cells of the lids and conjunctiva; these have been described earlier. The secretion produces what has been called the precorneal film, which consists of an inner layer of mucus, a middle layer of lacrimal secretion, and an outer oily film that reduces the rate of evaporation of the underlying watery layer. The normal daily (24-hour) rate of secretion has been estimated at about 0.75 to 1.1 grams (0.03–0.04 ounce avoirdupois); secretion tends to decrease with age. Chemical analysis of the tears reveals a typical body fluid with a salt concentration similar to that of blood plasma. An interesting component is lysozyme, an enzyme that has bactericidal action by virtue of its power of dissolving away the outer coats of many bacteria.

Tears are secreted reflexly in response to a variety of stimuli—e.g., irritative stimuli to the cornea, conjunctiva, nasal mucosa; hot or peppery stimuli applied to the mouth and tongue; or bright lights. In addition, tear flow occurs in association with vomiting, coughing, and yawning. The secretion associated with emotional upset is called psychical weeping. Severing of the sensory root of the trigeminal ( fifth cranial) nerve prevents all reflex weeping, leaving psychical weeping unaffected; similarly, the application of cocaine to the surface of the eye, which paralyzes the sensory nerve endings, inhibits reflex weeping, even when the eye is exposed to potent tear gases. The afferent (sensory) pathway in the reflex is thus by way of the fifth cranial, or the trigeminal nerve. The motor innervation is by way of the autonomic (involuntary) division; the parasympathetic supply derived from the facial nerve (the seventh cranial nerve) seems to have the dominant motor influence. Thus, drugs that mimic the parasympathetic, such as acetylcholine, provoke secretion, and secretion may be blocked by such typical anticholinergic drugs as atropine. Innervation of the lacrimal gland is not always complete at birth, so that the newborn infant is generally said to cry without weeping. Because absence of reflex tearing fails to produce any serious drying of the cornea, and surgical destruction of the main lacrimal gland is often without serious consequences, it seems likely that the subsidiary secretion from the accessory lacrimal glands is adequate to keep the cornea moist. The reflex secretion that produces abundant tears may be regarded as an emergency response.

A drainage mechanism for tears is necessary only during copious secretion. The mechanism, described as the lacrimal pump, consists of alternately negative and positive pressure in the lacrimal sac caused by the contraction of the orbicularis muscle during blinking.

Movements of the eyes

Because only a small portion of the retina, the fovea, is actually employed for distinct vision, it is vitally important that the motor apparatus governing the direction of gaze be extremely precise in its operation, and rapid. Thus, the gaze must shift swiftly and accurately during the process of reading. Again, if the gaze must remain fixed on a single small object—e.g., a golf ball—the eyes must keep adjusting their gaze to compensate for the continuous small movements of the head and to maintain the image exactly on the fovea. The extraocular muscles that carry out these movements are under voluntary control; thus, the direction of regard can be changed deliberately. Most of the actual movements of the eyes are carried out without awareness, however, in response to movements of the objects in the environment, or in response to movements of the head or the rest of the body, and so on. In examining the mechanisms of the eye movements, then, one must resolve them into a number of reflex responses to changes in the environment or the individual, remembering, of course, that there is an overriding voluntary control.

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Additional information regarding conditions of the eyes can be found here.