If, in addition to these considerations, it be recollected that blindness occurs only as an occasional consequence of injuries to the frontal nerves, and that loss of vision is found to ensue very rarely from the irritations to which other branches of the fifth are so peculiarly liable, the importance of such cases in determining the question must be still further lessened. The observations of Dr. Jacob on this sub ject appear to the writer so apposite that he is induced to insert them. This gentle man writes, " Blindness does not seem to have followed any of the operations formerly SO Much practised of dividing the branches of this nerve, and in some of the worst cases of that form of neuralgia called tic douloureux, vision is not impaired. Moreover, thousands of children suffer from dentition and thousands of adults from tooth-ache, yet none of these become blind in cohsequence."1 The coincidence of loss of sight with injuries or irritations affetting branches of the fifth nerve, admits of explanation on other principles without assuming the fifth to be essential to vision ; the hypothesis that in such cases reflex irritation becomes propagated from the parts pri marily affected through the nervous centres to the optic nerve, seems in the present state of physiological science Sufficiently plausible ; for while it applies to cases of amaurosis resulting from abnormal conditions of other peripheral branches of the nerve as well as its ophthalmic division, it also affords a solution of the still more obscure dependence of the same disease on irritations in remote. organs.
The experiments of Magendie, confirmed as they have been by pathological observations, justify the inference that the fifth nerve endows the eye with its general sensibility, and also exerts some influence over the nutrition of that organ.
This distinguished physiologist divided the fifth nerve within the cranium of an animal and found the tactile sensihility of the surface of the eye completely abolished by the expe riment; opacity, ulceration, and sloughing of the cornea, followed by expulsion of the hu mours, and total destruction of the eye subse quently ensued; and nearly the saine results have been observed to occur in the human subject from disease affecting the fifth nerve within the cranium. It rnust be perfectly ob vious, however, that these facts have no bearing upon the question rnore immediately under discussion.
An impartial review of this highly interesting question leads to the conviction that notwith standing the great plausibility of the arguments by which the contrary view has been sustained, there is as yet no evidence that in man any other nerve than the optic enjoys special sensi bility to light.
Ordinary tactile sensibility.—Although the optic nerves are endowed with such acute sensibility to the influence of light, they would seem to possess little ordinary tactile sensibi lity,—a circumstance the more surprising, as it is difficult to imagine any impressions more delicate than those of light; and where nerves evince such exquisite susceptibility of excite ment from the stimulus of that imponderable agent, an equal obedience to those rougher stimulants which produce such marked effects on the common sentient nerves might at least be expected.
Magendie infers from his experiments on living rabbits that the retina in them is not susceptible of pain from mechanical irritation ; so much so, that puncturing or tearing that nervous expansion appeared to him to cause these animals no sort of suffering.* Precisely the same results followed from injuries inflicted by him on the retina in fish and reptiles, although in birds, cats, and dogs similar expe riments seemed to create some uneasiness.
This physiologist asserts that the human retina also is devoid of ordinary tactile sensibility, for in operating for .cataract he has proved that the membrane in question exhibits little if any susceptibility of pain.
Ilis experiments were likewise extended to the optic nerves; and in the course of his in vestigations frequent opportunities were afforded for testing the comparative sensibility of the second and fifth pairs. In all the mammalia, whether the injury was inflicted in front of or behind the chiasma, the second pair seemed quite insensible to mechanical irritation ; and whenever the slightest disturbance affected the fifth nerve, the animal, by its cries and struggles, immediately manifested the must acute suffer ings.
The phenomena noticed in extirpation of the human eye are favourable to the same views, for division of the optic nerve in this operation is not attended with the agonizing torture which an equal amount of injury to a nerve of the same dimensions endowed with common sensi bility would unavoidably produce. This fact should have some weight with physiologists in their attempts to form a just estimate of the properties of the nerve under consideration, although the results of such obserrations are inconclusive; for in many cases of extirpation of the organ the optic nerve is itself diseased, and under such circumstances it would be unfair to argue from the known effects of injuries on a diseased structure, to the probable effects of injuries on the same structure when healthy.
The optic nerve is not singular in its insensi bility to pain from mechanical irritation, for experiments on other nerves of special sense countenance the belief that some of the (11 labour under the same disability. Magendie laid bare the olfactory of a dog, and the animal did not manifest the slightest pain when the nerve was compressed, pinched, or even torn ; and when the auditory of a rabbit was subjected to similar rough treatment at his hands, the animal afforded no indication of suffering.* The specific stimulants of the organs of sense act however at times so intensely' as to produce pailfully disagreeable impressions on their re spective organs, and it therefore becomes diffi cult to reconcile, with the foreg,oing statements, facts such-as the following, which apparently favour the opinion that the optic as well as ' other nerves of special sense possess common sensibility. " An intense light dazzles the eye so as to become actually insupportable. A harsh or discordant sound produces a most distress ing impression on the organ of hearing ; and certain odours are disgusting and intolerable to the pituitary membrane." In estimating the weight to which these latter: filets are entitled, it should be recollected that' the above sensations still preserve their specific? characters, no matter how intensely disagreeable they become : thus light, although sufficientlyJ brilliant to dazzle the retina, still continues tO, be a luminous impression and in like mantle'? sonorous vibrations and odours, though actually offensive to their respective organs, are stil! nothing more than sounds and scents. So tlia; on the whole, however questionable may be the! propriety of such experimental zeal as would induce a French physiologist to test the sensi bility of the human retina in operations for cal ie ract, the general proposition that the optic tient in man and the higher animals enjoys little, i any, tactile sensibility, seems pretty well es tablished.