excitement of any one branch should spread to the nerve-cells or roots
of the other branches.
The internal parts of the eye likewise act, under certain conditions,
in a reflex manner on the lacrymal glands. The following statements
have been kindly communicated to me by Mr. Bowman; but the subject is a
very intricate one, as all the parts of the eye are so intimately
related together, and are so sensitive to various stimulants. A strong
light acting on the retina, when in a normal condition, has very little
tendency to cause lacrymation; but with unhealthy children having
small, old-standing ulcers on the cornea, the retina becomes
excessively sensitive to light, and exposure even to common daylight
causes forcible and sustained closure of the lids, and a profuse flow
of tears. When persons who ought to begin the use of convex glasses
habitually strain the waning power of accommodation, an undue secretion
of tears very often follows, and the retina is liable to become unduly
sensitive to light. In general, morbid affections of the surface of the
eye, and of the ciliary structures concerned in the accommodative act,
are prone to be accompanied with excessive secretion of tears. Hardness
of the eyeball, not rising to inflammation, but implying a want of
balance between the fluids poured out and again taken up by the
intra-ocular vessels, is not usually attended with any lacrymation.
When the balance is on the other side, and the eye becomes too soft,
there is a greater tendency to lacrymation. Finally, there are numerous
morbid states and structural alterations of the eyes, and even terrible
inflammations, which may be attended with little or no secretion of
tears.
It also deserves notice, as indirectly bearing on our subject, that the
eye and adjoining parts are subject to an extraordinary number of
reflex and associated movements, sensations, and actions, besides those
relating to the lacrymal glands. When a bright light strikes the retina
of one eye alone, the iris contracts, but the iris of the other eye
moves after a measurable interval of time. The iris likewise moves in
accommodation to near or distant vision, and when the two eyes are made
to converge.[623] Every one knows how irresistibly the eyebrows are
drawn down under an intensely bright light. The eyelids also
involuntarily wink when an object is moved near the eyes, or a sound is
suddenly heard. The well-known case of a bright light causing some
persons to sneeze is even more curious; for nerve-force here radiates
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