Efects of stimulants.—Although the opti nerve betrays little indication of pain in con sequence of injuries, nevertheless mechanics anti other stimulants produce iiprin it peculif effects : mechanical injuries and irritants arou.: its special sensibility instead of exciting pain0 I sensations—their ordinary effects on commc sentient nerves.
For example : firm pressure applied to the globe of the eye when the lids are closed and light excluded, gives rise to the sensation of luminous spectra which present different colours. Concussion of the eye-ball is often followed by the same results : division of the optic nerve in extirpation of the org-an of vision generally causes the patient to perceive a great light ; and an electric current transmitted through the optic nerve, or its immediate vicinity, seems to produce a flash of light. The second pair, in having their special sensibility excited by such varied stimulants, merely conform to the laws by which other nerves of special sense are go verned ; for electricity applied to these nerves severally may be made in each case to elicit the peculiar sensibility of the nerve which hap pens to be the subject of the experiment : in the optic nerve it produces the sensation of a flash of light ; in the auditory it excites aloud sound ; in the gustatory it gives rise to a peculiar taste; in the olfactory it developes a particular smell ; and in the common sentient nerves it causes painful sensations. In like manner a blow may occasion the optic nerve to flash fire, the auditory nerve to hear sounds, the common sensitive nerves to feel pain; and more examples niight be added to this catalogue.
• The nerves of special sense seem in general to be endowed with but one determinate sort of sensibility ; and though this is commonly excited by a specific stimulus only, it may be elicited occasionally by other means.
Ercito-motory properties.—The optic nerve is one of those paths through which incident impressions are propagated so as to excite reflex motions. The impression of light on the retina is instantaneously followed by con traction of the pupil, a phenomenon indicative of reflex motion developed in the iris; and the sudden closure of the eye-lids under the , influence of a strong light or a threatened blow • is also a familiar example of reflex motion I produced by impressions upon the terminal expansion of the optic nerve.
Lambert, Fontana, and Caldarti, have de monstrated that the optic nerve is the channel through which the incident impression travels 1 in order to excite reflex motion in the iris. I In their experiments, rays of light transmitted through a hole in a sheet of paper, and by this contrivance conveyed through the pupil di , redly to the retina, produced immediate mo tion of the iris ; but when the light was al lowed to impinge upon the iris alone without , reaching the retina, no contraction of the pupil 1_ ensued.
, Mayo's experiments on pigeons taken in • 1 connection with the foregoing facts appear par ticularly instructive, proving as they do that in the bird, irritation propagated along the optic 4 nerve in a centripetal direction may excite reflex • motion in the iris. When the optic nerves were divided within the cranial cavity of a living i pigeon by Mayo, the pupils became fully dilated and were no longer obedient to luniinous im pressions even when dazzling light was admitted into the eyes. When a pigeon was decapitated by the same experimentalist, and its optic nerves subsequently divided within the cranial cavity, irritation of that portion cif the divided nerves which continued in connection with the eye pro duced no effect on the iris ; but contraction of the pupil immediately ensued when the other extremity of the nerves, viz. that which re tained its connection with the brain, was irri tated.
In general the reflex motion is developed in the iris of the same eye on which the impres sion is incident, or in other words light falling on the right retina produces in general con traction of the right pupil and not of the left, and vice versa ; but it sometimes happens that an impression propagated along one optic nerve (for instance the right) may cause the reflex phenomena to appear in the iris of the other eye (viz. the /0).
Certain forms of amaurosis in which the dis ease affects but one eye while the other con tinues healthy will serve for illustration.