In such cases it occasionally happens that little or no difference in the size of the two pupils can be detected so long as both eyes remain exposed to the light, the iris of the dis eased organ contracting and dilating sitnulta neously with that of its healthy neighbour ; but as soon as the lids of the sound eye are closed, the pupil of the amaurotic eye becomes dilated, and the most intense light admitted into this diseased organ takes no effect on the iris, now become perfectly motionless.
The explanation of these phenomena is found in the preceding proposition ; so long as the healthy eye continues exposed to light, the impression falling on a sound retina excites reflex motion in the pupils of both eyes, as well the amaurotic as the sound; but when the light is excluded from the healthy retina, the influence of that agent upon the diseased retina of the other eye has uo longer the power to excite reflex motions.
The exercise of these excito-motory proper ties of the optic nerve is generally accompanied with excitement of its special sensibility ; thus a person is in general conscious of the lumi nous impression, or, in other words, he sees the light which causes contraction of his pupil; but the reflex phpomena may be manifested by the iris, altho,igh the incident impression pass unnoticed by the individual.
In certain cases of general insensibility (as, for example, concussion occasionally) the pupil contracts upon the admission of light while the patient remains perfectly unconscious, and something similar seems to occur at times even in health; for the iris varies its dimensions ,with each successive change in the volume and . intensity of the light, although from inatten tion we do not perceive these trifling changes. Some degree of attention appears requisite in order that weak or transitory impressions should arouse the special sensibility of the optic nerves, whereas attention is not a condition essential to the production of reflex phenomena.
Mayo's experiments here again admiL of application; he found that in decapitated pi geons in which the optic nerves were subse quently divided, irritation of the cerebral ex tremity of the cut nerve produced contraction of the pupil, although under the circumstances the animal could not have been conscious of such irritation.
Radiated or sympathetic sensations.
The optic nerve participates in a class of ob scure sensations to which a brief allusion may be here permitted ; these are called radiated or sympathetic sensations; they occur occasion ally in health, though they are more frequently symptomatic of disease or irritation elsewhere situated, and as they are likewise manifested by other nerves it would be erroneous to con sider them peculiar to the optic. The pheno
mena to which allusion is made are for the most part transitory affections of sensitive nerves which do not seem to depend on ainy direct impression made upon the nerve af fected, but rather to be produced by causes which act on other (generally distant) parts of the system.
The following will serve as examples of the affections under considemtion.
A discordant sound, such as that produced by setting a saw or scmtching glass, gives rise to shuddering, or a sensation as if water were dropping over the surface. Tickling the soles of the feet occasions general sensations of the most disagreeable nature ; and the impression of a strong light on the eye is often followed by a sense of irritation in the nose, with violent sneezitig.
Similar affections of the optic nerve will readily occur to the reader's recollection ; thus various forms and degrees of temporary insen sibility or excitement of the retina, which are known to depend on gastricdisturbance, belong to this category, and many other such instances might be adduced.
Facts, such'as the foregoing, have long been familiar to physiologists; but to account for them seems still to be a matter of difficulty. The supposition that the connections of the sympathetic with the nerves affected explain the problem is far from satisfactory ; the most plausible theory is that which supposes the primary irritation to be propagated in a centripetal direction along the nerves of the part to the cerebro-spinal centres, and thence reflected upon the roots of those nerves in which the sensations are developed, in some what the same way that excito-rnotory impres sions on nerves come to produce reflex motions; the difference in the two cases amounting to this, that in the one the primary impression, reacts upon motor nerves, giving rise to reflex motions, in the other on sensitive nerves causing thereby reflex sensations.
lliough this may be the true explanation, it is nevertheless not perfectly satisfactory, for it does not sliew why the reflex irritation should be prone to fall on one sensitive nerve in pre ference to another, yet the optic is known in such cases to suffer more frequently than the auditory or olfactory.