particles of dust not being removed, in consequence of the eye and
eyelid becoming immovable.[622] The secretion of tears from the
irritation of any foreign body in the eye is a reflex action; that is,
the body irritates a peripheral nerve which sends an impression to
certain sensory nerve-cells; these transmit an influence to other
cells, and these again to the lacrymal glands. The influence
transmitted to these glands causes, as there is good reason to believe,
the relaxation of the muscular coats of the smaller arteries; this
allows more blood to permeate the glandular tissue, and this induces a
free secretion of tears. When the small arteries of the face, including
those of the retina, are relaxed under very different circumstances,
namely, during an intense blush, the lacrymal glands are sometimes
affected in a like manner, for the eyes become suffused with tears.
It is difficult to conjecture how many reflex actions have originated,
but, in relation to the present case of the affection of the lacrymal
glands through irritation of the surface of the eye, it may be worth
remarking that, as soon as some primordial form became semi-terrestrial
in its habits, and was liable to get particles of dust into its eyes,
if these were not washed out they would cause much irritation; and on
the principle of the radiation of nerve-force to adjoining nerve-cells,
the lacrymal glands would be stimulated to secretion. As this would
often recur, and as nerve-force readily passes along accustomed
channels, a slight irritation would ultimately suffice to cause a free
secretion of tears.
As soon as by this, or by some other means, a reflex action of this
nature had been established and rendered easy, other stimulants applied
to the surface of the eye such as a cold wind, slow inflammatory
action, or a blow on the eyelids would cause a copious secretion of
tears, as we know to be the case. The glands are also excited into
action through the irritation of adjoining parts. Thus when the
nostrils are irritated by pungent vapours, though the eyelids may be
kept firmly closed, tears are copiously secreted; and this likewise
follows from a blow on the nose, for instance from a boxing-glove. A
stinging switch on the face produces, as I have seen, the same effect.
In these latter cases the secretion of tears is an incidental result,
and of no direct service. As all these parts of the face, including the
lacrymal glands, are supplied with branches of the same nerve, namely,
the fifth, it is in some degree intelligible that the effects of the
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