from certain nerve-cells in connection with the retina, to the sensory
nerve-cells of the nose, causing it to tickle; and from these, to the
cells which command the various respiratory muscles (the orbiculars
included) which expel the air in so peculiar a manner that it rushes
through the nostrils alone.
To return to our point: why are tears secreted during a screaming-fit
or other violent expiratory efforts? As a slight blow on the eyelids
causes a copious secretion of tears, it is at least possible that the
spasmodic contraction of the eyelids, by pressing strongly on the
eyeball, should in a similar manner cause some secretion. This seems
possible, although the voluntary contraction of the same muscles does
not produce any such effect. We know that a man cannot voluntarily
sneeze or cough with nearly the same force as he does automatically;
and so it is with the contraction of the orbicular muscles: Sir C. Bell
experimented on them, and found that by suddenly and forcibly closing
the eyelids in the dark, sparks of light are seen, like those caused by
tapping the eyelids with the fingers; but in sneezing the compression
is both more rapid and more forcible, and the sparks are more
brilliant. That these sparks are due to the contraction of the eyelids
is clear, because if they are held open during the act of sneezing, no
sensation of light will be experienced. In the peculiar cases referred
to by Professor Donders and Mr. 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.
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
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