Tears are produced by the lachrymal glands. Our eyes are coated with a film of tear almost all the time and not just when we cry. Tears lubricate the eyes and wash off dust. They are also necessary for clear vision. The tears flow from the upper margin of the eye socket and drain at the inner corner of the eye through two small openings. From there they drain into the nose. The drainage mechanism is to some extent controlled by facial muscles.
When we yawn, our facial muscles are tightened and our eyes shut. This causes a pressure on the lacrymal duct, which in turn causes an increased amount of fluid to be forced into the eyes. It also temporarily reduces the amount of tears being drained. When the tear ducts are closed, a small amount of tears is forced out.
Why do we yawn? A yawn is a reflex caused when one’s blood contains increased amounts of carbon dioxide and becomes in need of oxygen. Yawning helps us bring more oxygen into the blood and move more carbon dioxide out of the blood. There are a number of theories to explain why humans yawn and this is the most popularly accepted.
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