Because, under the emotions of the mind, the circulation of blood in the brain, and in its nearest branches, becomes considerably quickened. The eyes receive a larger amount of blood, and the secretion of the lachrymal glands being increased, the fluid overflows, and tears are formed. The use of tears is probably to keep the eyes cool during the excitement of the brain. They are formed also during laughing, but less frequently.
"If the whole body were an eye, where were hearing? if the whole were hearing, where were smelling?"—Corinthians xii.
978. Why do we feel inconvenienced by sudden light?
Because an excess of light enters the eye before the iris has had time to adjust the pupil to the amount of light to be received.
979. Why if we look upon a very bright light, and then turn away, are we unable to see?
Because the iris has so reduced the pupil while we were looking at the bright light, that immediately upon turning to a darker object, the pupil is too small to admit sufficient rays to enable us to see.