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The Expression of the Emotions in Man and Animals by Charles Darwin
page 176 of 382 (46%)
Another cause may come conjointly into play.
We have seen that the internal parts of the eye, under certain
conditions act in a reflex manner on the lacrymal glands.
We know that during violent expiratory efforts the pressure
of the arterial blood within the vessels of the eye is increased,
and that the return of the venous blood is impeded.
It seems, therefore, not improbable that the distension
of the ocular vessels, thus induced, might act by reflection
on the lacrymal glands--the effects due to the spasmodic pressure
of the eyelids on the surface of the eye being thus increased.

In considering how far this view is probable, we should bear
in mind that the eyes of infants have been acted on in this
double manner during numberless generations, whenever they
have screamed; and on the principle of nerve-force readily
passing along accustomed channels, even a moderate compression
of the eyeballs and a moderate distension of the ocular vessels
would ultimately come, through habit, to act on the glands.
We have an analogous case in the orbicular muscles being almost
always contracted in some slight degree, even during a gentle
crying-fit, when there can be no distension of the vessels
and no uncomfortable sensation excited within the eyes.

Moreover, when complex actions or movements have long been performed
in strict association together, and these are from any cause at first
voluntarily and afterwards habitually checked, then if the proper exciting
conditions occur, any part of the action or movement which is least under
the control of the will, will often still be involuntarily performed.
The secretion by a gland is remarkably free from the influence of
the will; therefore, when with the advancing age of the individual,
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