OPTIC NERVES. 



781 



For example : firm pressure applied to the 

 globe of the eye when the lids are closed and 

 light excluded, gives rise to the sensation of 

 luminous spectra which present different colours. 

 Concussion of the eye-ball is often followed by 

 the same results : division of the optic nerve in 

 extirpation of the organ of vision generally 

 causes the patient to perceive a great light ; 

 and an electric current transmitted through the 

 optic nerve, or its immediate vicinity, seems to 

 produce a flash of light. The second pair, in 

 having their special sensibility excited by such 

 varied stimulants, merely conform to the laws 

 by which other nerves of special sense are go- 

 verned ; for electricity applied to these nerves 

 severally may be made in each case to elicit 

 the peculiar sensibility of the nerve which hap- 

 pens to be the subject of the experiment : in the 

 optic nerve it produces the sensation of a flash 

 of light ; in the auditory it excites a loud sound ; 

 in the gustatory it gives rise to a peculiar taste; 

 in the olfactory it developes a particular smell ; 

 and in the common sentient nerves it causes 

 painful sensations. In like manner a blow 

 may occasion the optic nerve to flash fire, the 

 auditory nerve to hear sounds, the common 

 sensitive nerves to feel pain; and more examples 

 might be added to this catalogue. 



The nerves of special sense seem in general 

 to be endowed with but one determinate sort 

 of sensibility ; and though this is commonly 

 excited by a specific stimulus only, it may be 

 elicited occasionally by other means. 



Excito-motory properties. The optic nerve 

 is one of those paths through which incident 

 impressions are propagated so as to excite 

 reflex motions. The impression of light on 

 the retina is instantaneously followed by con- 

 traction of the pupil, a phenomenon indicative 

 of reflex motion developed in the iris ; and 

 the sudden closure of the eye-lids under the 

 influence of a strong light or a threatened blow 

 is also a familiar example of reflex motion 

 produced by impressions upon the terminal 

 expansion of the optic nerve. 



Lambert, Fontana, and Caldani, have de- 

 monstrated that the optic nerve is the channel 

 through which the incident impression travels 

 in order to excite reflex motion in the iris. 

 In their experiments, rays of light transmitted 

 through a hole in a sheet of paper, and by this 

 contrivance conveyed through the pupil di- 

 rectly to the retina, produced immediate mo- 

 tion of the iris ; but when the light was al- 

 lowed to impinge upon the iris alone without 

 reaching the retina, no contraction of the pupil 

 ensued. 



Mayo's experiments on pigeons taken in 

 connection with the foregoing facts appear par- 

 ticularly instructive, proving as they do that in 

 the bird, irritation propagated along the optic 

 nerve in a centripetal direction may excite reflex 

 motion in the ins. V hen the optic nerves were 

 divided within the cranial cavity of a living 

 pigeon by Mayo, the pupils became fully dilated 

 and were no longer obedient to luminous im- 

 pressions even when dazzling light was admitted 

 into the eyes. When a pigeon was decapitated 

 by the same experimentalist, and its optic nerves 



subsequently divided within the cranial cavity, 

 irritation of that portion of the divided nerves 

 which continued in connection with the eye pro- 

 duced no effect on the iris; but contraction of 

 the pupil immediately ensued when the other 

 extremity of the nerves, viz. that which re- 

 tained its connection with the\ bruin, was irri- 

 tated. 



In general the reflex motion is developed in 

 the iris of the same eye on which the impres- 

 sion is incident, or in other words light falling 

 on the right retina produces in general con- 

 traction of the right pupil and not of the left, 

 and vice versa ; but it sometimes happens 

 that an impression propagated along one optic 

 nerve (for instance the rig/it) may cause the 

 reflex phenomena to appear in the iris of the 

 other eye (viz. the left). 



Certain forms of amaurosis in which the dis- 

 ease affects but one eye while the other con- 

 tinues healthy will serve for illustration. 



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 simulta- 

 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 no 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 phenomena may be manifested 

 by the iris, although 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 admit of 

 application; he found that in decapitated pi- 

 geons in which the optic nerves were subse- 

 quently divided, irritation of the cerebral ex- 



