Page:Popular Science Monthly Volume 1.djvu/474

460 Although less directly, the optic nerve, as well as the retina, is accessible to our observation. Hence we ascribe certain scintillations, visible to us in a rapid motion of the eye, to a twist of the nerve; surgical operations, dating from a period when narcotics were not employed, have likewise shown that contact with this nervous cord produces only sensations of light, not those of pain.

Lastly, we can point out the seat of the root, or the central termination of the optic nerve, by anatomically tracing the fibrils of the visual nerve into this tract; and partly, too, by an analysis of the phenomena observable in healthy and diseased states. When the brain has been excited by a narcotic, and the irritation is transmitted to the aforesaid tract, there arise sensations of light, which, combining with ideas of luminous objects, simultaneously excited, are transformed into what we call phantasms. The same thing takes place when the blood, as in fever, heats the brain; or when that part of the organ is excited from other causes. And thus it is with our visual impressions during dreams, or even in a half-waking condition.

But all this does not constitute any relation between sensation and the objects of the external world; that is, proper sensory action, whether it be the gay visions that surround us in the intoxication caused by opium; the comic phantasmagoria that hashish conjures up; the compact shapes that belladonna brings so near us; the airy forms seen in our dreams, or the scintillations produced by pressure, they all proceed from irritation of the special sensory power, and it is indifferent to the brain whether it receives its impressions from direct vision, or only from internal influences. All those operations, therefore, which proceed from direct irritation of the nervous part of the visual organ, without the medium of the eye and of light, under the term subjective vision, are opposed to those phenomena produced by the media of eye and light, and known as objective vision.

Great as are the influences of this subjective sight for the refreshment of our brain during sleep, and powerfully as they affect the temperament of the blind, they cannot connect us with the outer world. The yellow light which floods our field of vision, on rubbing the retina, is of no use to light up external objects. Hence, when, some years ago, a man pretended to recognize a delinquent who had attacked him in the night, by the sparks of fire produced by a blow on his eye, and founded an accusation thereon, it was, of course, unjustifiable, although the authorities consulted did not declare against the impossibility of the fact.—Baron Münchausen went still further in the use to which he put those visual sparks; for, when attacked by bears in the night, he not only struck out light enough by which to prosecute the chase, but fire, too, for his guns with the same blow.

We cannot entirely overlook the question whether sensations of light can be produced with the assistance of any other mechanism in the body but that of the visual nerve. As only the part where