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.With this glassit was found that he could read the twenty line of the test card at twenty feet, andread diamond type easily at fourteen inches.Subsequently the right lens wasremoved, after which no accommodation was observed in this eye.Two yearslater,1Nov., 1893, p.932. 36 Accepted Theory of AccommodationMarch 16, 1895, he was seen by Dr.Davis, who found that the left eye now hadan accommodative range of from ten to eighteen inches.In this case no changewas observed in the cornea.The results of the Donders tests were similar tothose of the earlier case, and under scopolamine the eye accommodated asbefore, but not quite so easily.No accommodation was observed in the right eye.These and similar cases have been the cause of great embarrassment tothose who feel called upon to reconcile them with the accepted theories.With theretinoscope the lensless eye can be seen to accommodate; but the theory ofHelmholtz has dominated the ophthalmological mind so strongly that even theevidence of objective tests was not believed.The apparent act ofaccommodation was said not to be real, and many theories, very curious andunscientific, have been advanced to account for it.Davis is of the opinion that"the slight change in the curvature of the cornea, and its slight advancementobserved in some cases, may, in those cases, account for some of theaccommodative power present, but it is such a small factor that it may beeliminated entirely, since in some of the most marked cases of accommodation inaphakial eyes no such changes have been observed."The voluntary production of astigmatism is another stumbling block to thesupporters of the accepted theories, as it involves a change in the shape of thecornea, and such a change is not compatible with the idea of an ''inextensible''1eyeball.It seems to have given them less trouble, however, than theaccommodation of the lensless1Inasmuch as the eye is inextensible, it cannot adapt itself for the perception of objects situated at differentdistances by increasing the length of its axis, but only by increasing the refractive power of its lens. DeSchweinitz: Diseases of the Eye, eighth edition, 1916, pp.35-36. Voluntary Production of Astigmatism 37eye, because fewer of these cases have been observed and still fewer havebeen allowed to get into the literature.Some interesting facts regarding one havefortunately been given by Davis, who investigated it in connection with thecorneal changes noted in the lensless eye.The case was that of a housesurgeon at the Manhattan Eye and Ear Hospital, Dr.C.H.Johnson.Ordinarilythis gentleman had half a diopter of astigmatism in each eye; but he could, at will,increase this to two diopters in the right eye and one and a half in the left.He didthis many times, in the presence of a number of members of the hospital staff,and also did it when the upper lids were held up, showing that the pressure of thelids had nothing to do with the phenomenon.Later he went to Louisville, and hereDr.J.M.Ray, at the suggestion of Dr.Davis, tested his ability to produceastigmatism under the influence of scopolamine (four instillations, 1/5 per centsolution).While the eyes were under the influence of the drug the astigmatismstill seemed to increase, according to the evidence of the ophthalmometer, toone and a half diopters in the right eye and one in the left.From these facts, theinfluence of the lids and of the ciliary muscle having been eliminated, Dr.Davisconcluded that the change in the cornea was "brought about mainly by theexternal muscles." What explanation others offer for such phenomena I do notknow. CHAPTER IVTHE TRUTH ABOUT ACCOMMODATION AS DEMONSTRATED BYEXPERIMENTS ON THE EYE MUSCLES OF FISH, CATS, DOGS, RABBITSAND OTHER ANIMALSTHE function of the muscles on the outside of the eyeball, apart from that ofturning the globe in its socket, has been a matter of much dispute; but after thesupposed demonstration by Helmholtz that accommodation depends upon achange in the curvature of the lens, the possibility of their being concerned in theadjustment of the eye for vision at different distances, or in the production oferrors of refraction, was dismissed as no longer worthy of serious consideration."Before physiologists were acquainted with the changes in the dioptic system,''1says Donders, "they often attached importance to the external muscles in theproduction of accommodation.Now that we know that accommodation dependson a change of form in the lens this opinion seems scarcely to need refutation."He states positively that "many instances occur where the accommodation iswholly destroyed by paralysis, without the external muscles being the leastimpeded in their action," and also that "some cases are on record of paralysis ofall or nearly all of the muscles of the eye, and of deficiency of the same, withoutdiminution of the power of accommodation."2If Donders had not considered the question settled, he1The refractive system.2On the Anomalies of Accommodation and Refraction of the Eye, p.22.38 The External Muscles of the Eyeball 39might have inquired more carefully into these cases, and if he had, he might havebeen less dogmatic in his statements; for, as has been pointed out in thepreceding chapter, there are plenty of indications that the contrary is the case.Inmy own experiments upon the extrinsic eye muscles of fish, rabbits, cats, dogsand other animals, the demonstration seemed to be complete that in the eyes ofthese animals accommodation depends wholly upon the action of the extrinsicmuscles and not at all upon the agency of the lens.By the manipulation of thesemuscles I was able to produce or prevent accommodation at will, to producemyopia, hypermetropia and astigmatism, or to prevent these conditions [ Pobierz całość w formacie PDF ]

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