Optic nerve

“ The apparatus of rods, cones, and other structures are absolutely essential to enable light to give that stimulus to the optic nerve which, conveyed to the brain, is converted into visual sensations. So if this apparatus were absent in our organs of vision, in vain would the optic nerve proper be spread out over the interior of the eyeball: we should be no more able to see with such eyes than we are able to see with our hands.
We now invite the reader’s careful consideration to the diagram, Fig. 236, which is a section of the retina through the yellow spot. ”

“ In many cases the effect of prolonged gazing upon the brilliant surface will simply produce a loss of sight, the optic nerve will be temporarily paralyzed and will as little respond to stimulation from within as from without; in other cases, however, the nerve will be only deadened as regards external impressions, while retaining sufficient activity to react against a stimulus177 from the brain centres. It is almost invariably stated that, prior to the appearance of the desired visions, the crystal seems to disappear and a mist rises before the gazer’s eye. ”

“ Having established this, we have a basis for an understanding of the complete process of vision. We see that it is by no means solely the nerve part of the eye which is responsible for vision, as the spectator-physiology was bound to imagine. The very fact that the place where the optic nerve enters the eye is blind indicates that the function of mediating sight cannot be ascribed to the nerve alone. What we call 'seeing' is far more the result of an interplay between the retina carrying the nerves, and the choroid carrying the blood-vessels. ”

“ For reasons stated above, the collection of complete histories necessary to afford actual demonstration of the truth of these propositions is difficult; and all the more so as there is reason to believe that in those cases in which the early stress of the disease falls upon the nervous apparatus of the eyeball the spinal symptoms are apt to be slight in degree as well as delayed in development. This is certainly the case when the ocular disturbance takes the form of optic-nerve atrophy. ”

“ The optic nerve, or nerve of sight, which pierces the tunics of the eye at the back and near the centre, spreads out and forms an expanded tunic of nerve-structure which enwraps the vitreous body as far as its most forward edge, where the colored iris descends in front of it. Enwrapping again this nerve-tunic or retina is a vestment, chiefly made of blood-vessels, connected by fine tissue and thickly coated with black pigment, having its own optical uses. ”

“ Thus we see that external light produces no other effects in the optic nerve than other agents of an entirely different nature. In one respect only does light differ from the other causes which are capable of exciting this nerve: namely, that the retina, being placed at the back of the firm globe of the eye, and further protected by the bony orbit, is almost entirely withdrawn from other exciting agents, and is thus only exceptionally affected by them, while it is continually receiving the rays of light which stream in upon it through the transparent media of the eye. ”

“ About twenty-five years ago a patient came to the New York Eye Infirmary with well-marked atrophy of the optic nerve. According to all that we know of the laws of pathology he should have been totally blind; yet his vision was normal. ”

“ The optic nerve is the channel by which the mind peruses the hand-writing of Nature on the retina, and through which it transfers to that material tablet its decisions and its creations. The eye is consequently the principal seat of the supernatural. ”

“ The optic nerve, the retina and the visual centers of the brain are as passive as the finger-nail. They have nothing whatever in their structure that makes it possible for them to do anything, and when they are the subject of effort from outside sources their efficiency is always impaired. ”

“ On the other hand, when a limb is not used, as by Eastern fanatics, or when the nerve supplying it with nervous power is effectually destroyed, the muscles wither. So again, when the eye is destroyed the optic nerve becomes atrophied, sometimes even in the course of a few months. ”

“ This wonderful organ, writes Brewster, may be considered as being the sentinel that guards the passage between the world of matter and that of mind, and as the medium through which they interchange all their communications. The optic nerve perceives the objects written on the retina by the hand of nature, and conveys them to the brain in all their integrity of form and colour. ”

“ A traumatic cataract is one which follows an injury (trauma) to the lens, the opacity being due largely to the formation of connective tissue in the pupil, and, in advance of the event, I should have pronounced the cure of such a condition impossible, although I had previously demonstrated that when patients practice central fixation connective tissue is absorbed in the optic nerve, retina and cornea. In the retina and optic nerve the circulation can be seen to improve as the connective tissue disappears, and I can only assume that this is the cause of its disappearance. ”

“ On the other hand, the optic nerve, by reason of the peculiar structures in connection with the ends of its fibres, the rods and cones of the retina, is incomparably more sensitive to rays of light than any other nervous apparatus of the body, since the rest can only be affected by rays which are concentrated enough to produce noticeable elevation of temperature. ”

“ The peculiar appearances known as spectres in optics are certain illusions of vision in which an object is apparently presented to the view which does not really exist. In such cases either the brain, the retina, or the optic nerve are unnaturally excited, and made sensitive to an appearance that, physically speaking, does not exist. There is such a close connexion between the senses and the mind, that we continually, and without knowing it, transfer to the physical world that which belongs to the domain of thought. ”

“ You ask how men have arrived at the knowledge of these things, seeing the mechanism in question is almost entirely removed from any direct investigation. First, then, the umbellar expansion of the optic nerve, the retina, enables us to make experiments; this retina being in such close contact with the eye, the optical part of the visual organ, that it is accessible to every sort of mechanical irritation. ”

“ An optic nerve and retina would thus result. One might corroborate this reasoning by pointing to what happens in cases of squint. The squinting eye gives double images which are so inconvenient that the mind is forced to abstract its attention from them. This resolute refusal to attend to the sensations of one eye soon makes it totally blind. It would seem, indeed, that the attention positively suppressed the function of the retina, for the presence of cataract which keeps the image from It altogether, results in no such paralysis. ”