Re: WHY DO HUMANS BLINK THEIR EYES CONSTANTLY

Dear Jay, 

Here is a fairly detalied answer to your question I found at the website 
for www.britannica.com.

Eye blinking:

In the waking hours the eyes blink fairly regularly at intervals of two to 
10 seconds, the actual rate being a characteristic of the individual. The 
function of this is to spread the lacrimal secretions over the cornea. It 
might be thought that each blink would be reflexly determined by a corneal 
stimulus--drying and irritation--but extensive studies indicate that this 
view is wrong; the normal blinking rate is apparently determined by the 
activity of a "blinking centre" in the globus pallidus of the caudate 
nucleus, a mass of nerve cells between the base and the outer substance of 
the brain. This is not to deny that the blink rate is modified by external 
stimuli.

There is a strong association between blinking and the action of the 
extraocular muscles. Eye movement is generally accompanied by a blink, and 
it is thought that this aids the eyes in changing their fixation point.

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.

I  hope this satisfies your curiosity =) 
Have fun with science!
Elsa