Bowman, we have seen that some weeks after the eye has been
very slightly injured, spasmodic contractions of the eyelids ensue,
and these are accompanied by a profuse flow of tears.
In the act of yawning, the tears are apparently due solely
to the spasmodic contraction of the muscles round the eyes.
Notwithstanding these latter cases, it seems hardly credible
that the pressure of the eyelids on the surface of the eye,
although effected spasmodically and therefore with much greater
force than can be done voluntarily, should be sufficient to cause
by reflex action the secretion of tears in the many cases
in which this occurs during violent expiratory efforts.
[23] See, on these several points, Prof. Donders `On the Anomalies
of Accommodation and Refraction of the Eye,' 1864, p. 573.
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.
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