780 



OPTIC NERVES. 



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 sensibility 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 same results 

 have been observed to occur in the human 

 subject from disease affecting the fifth nerve 

 within the cranium. It must be perfectly ob- 

 vious, however, that these facts have no bearing 

 upon the question more 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. 

 Tins 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. 



His 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 most 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 observations 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 

 health i/. 



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 them 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 

 painfully disagreeable impressions on their re- 

 spective organs, and it therefore becomes diffi- 

 cult to reconcile, with the foregoing 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 

 facts 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 sufficiently 

 brilliant to dazzle the retina, still continues to 

 be a luminous impression, and in like manner 

 sonorous vibrations and odours, though actually 

 offensive to their respective organs, are still 

 nothing more than sounds and scents. So that 

 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 cata- 

 ract, the general proposition that the optic nerve 

 in man and the higher animals enjoys little, if 

 any, tactile sensibility, seems pretty well es- 

 tablished. 



Effects of stimulants. Although the optic 

 nerve betrays little indication of pain in con- 

 sequence of injuries, nevertheless mechanical 

 and other stimulants produce upon it peculiar 

 effects : mechanical injuries and irritants arouse 

 its special sensibility instead of exciting painful 

 sensations their ordinary effects on common 

 sentient nerves. 



* Journal de Physiologic, t. iv. 



* Journal de Physiologic, t. iv. 



