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."1Fig.10.Diagram by Which Helmholtz Illustrated His Theory of AccommodationR is supposed t be the resting state of the lens, in which it is adjusted for distant vision.In A the suspensory ligament is supposed tohave been relaxed through the contraction of the ciliary muscle, permitting the lens to bulge forward by virtue of its own elasticity."The question was decided," says Tscherning, "by the observation of thechanges of the images of Purkinje during accommodation, which prove thataccommodation is effected by an increase of curvature of the anterior surface ofthe crystalline lens."21The Refraction and Accommodation of the Eye and their Anomalies,authorized translation by Culver, 1886, p.151.2Physiologic Optics, authorized translation by Weiland, 1904, p.163.Marius Hans Erik Tscherning (1854 ) is a Danish ophthalmologist whofor twenty-five years was co-director and director of the ophthalmologicallaboratory of the Sorbonne.Later he became professor of ophthalmology inthe University of Copenhagen.Fig.11.Thomas Young (1773-1829)English physician and man of science who was the first to present aserious argument in support of the view that accommodation isbrought about by the agency of the lens.28Scientific Credulity 29"The greatest thinkers," says Cohn, "have mastered a host of difficulties indiscovering this arrangement, and it is only in very recent times that itsprocesses have been clearly and perfectly set forth in the works of Sanson,Helmholtz, Brucke, Hensen and Volckers."1Huxley refers to the observations of Helmholtz as the "facts of adjustment withwhich all explanations of that process must accord,"2 and Donders calls histheory the "true principle of accommodation."3Arlt, who had advanced the elongation theory and believed that no other waspossible, at first opposed the conclusions of Cramer and Helmholtz,4 but lateraccepted them.5Yet in examining the evidence for the theory we can only wonder at thescientific credulity which could base such an important department of medicalpractice as the treatment of the eye upon such a mass of contradictions.Helmholtz, while apparently convinced of the correctness of his observationsindicating a change of form in the lens during accommodation, felt himself unableto speak with certainty of the means by which the supposed change waseffected,3 and strangely enough the question is still being debated.Finding, ashe states, "absolutely nothing but the ciliary muscle to which accommodationcould be attributed,"7 Helmholtz concluded that the changes which he thought hehad observed in the curvature of the lens must be effected by the action of thismuscle; but he was unable to offer any satisfac-1The Hygiene of the Eye in Schools, English translation edited by Turnbull, 1886, p.23.HermannCohn (1838-1906) was professor of ophthalmology in the University of Breslau, and is knownchiefly for his contributions to ocular hygiene.2Lessons in Elementary Physiology, sixth edition, 1872, p.231.3On the Anomalies of Accommodation and Refraction of the Eye, p.13.4Krankheiten des Auges, 1853-56, vol.iii, D.219, et seq.5Ueber die Ursachen und die Entstehung der Kurzsichtigkeit, 1876.Vorwort.6Handbuch der physiologischen Optik, vol.i, pp.124 and 145.7Ibid, vol.i.P.144.30 Accepted Theory of Accommodationtory theory of the way it operated to produce these results, and he explicitlystated that the one he suggested possessed only the character of probabilitySome of his disciples, "more loyal than the king," as Tscherning has pointed out,"have proclaimed as certain what he himself with much reserve explained asprobable,''1 but there has been no such unanimity of acceptance in this case asin that of the observations regarding the behavior of the images reflected fromthe lens.No one except the present writer, so far as I am aware, has ventured toquestion that the ciliary muscle is the agent of accommodation; but as to themode of its operation there is generally felt to be much need for more light.Sincethe lens is not a factor in accommodation, it is not strange that no one was ableto find out how it changed its curvature.It "is" strange, however, that thesedifficulties have not in any way disturbed the universal belief that the lens doeschange.When the lens has been removed for cataract the patient usually appears tolose his power of accommodation, and not only has to wear a glass to replacethe lost part, but has to put on a stronger glass for reading.A minority of thesecases, however, after they become accustomed to the new condition, becomeable to see at the near-point without any change in their glasses.The existenceof these two classes of cases has been a great stumbling block toophthalmology.The first and more numerous appeared to support the theory ofthe agency of the lens in accommodation; but the second was hard to explainaway, and constituted at one time, as Dr.Thomas Young observed, the "grandobjection" to this idea.A number of these cases of apparent change of focus1Physiologic Optics, p.166.Herman- Ludwig Ferdinand von Helmholtz (1821-1894)whose observations regarding the behavior of images reflectedfrom the front of the lens are supposed to have demonstratedthat the curvature of this body changes during accommodation.32 Accepted Theory of Accommodationin the lensless eye having been reported to the Royal Society by competentobservers, Dr.Young, before bringing forward his theory of accommodation, tookthe trouble to examine some of them, and considered himself justified inconcluding that an error of observation had been made.While convinced,however, that in such eyes the "actual focal distance is totally unchangeable," hecharacterized his own evidence in support of this view as only "tolerablysatisfactory." At a later period Donders made some investigations from which heconcluded that "in aphakial not the slightest trace of accommodative powerremains [ Pobierz całość w formacie PDF ]