ACCOMMODATION. 1039 



ing; the anterior surface of the lens. The longitudinal fibres of 

 the ciliary muscle are supposed to have their more fixed attachment 

 in front, and on contraction to pull the choroid forward, relaxing the 

 ligament, and releasing the pressure on the lens. The circular fibres are 

 supposed to prevent distortion of the ciliary processes by making them 

 follow the retreating lens. 



The observations of Hensen and Voelckers on animals are all in 

 favour of this theory, and especially the occurrence of movement of the 

 choroid forwards. The recent observations of Hess seem to afford con- 

 vincing evidence in favour of the main feature of the theory, namely, 

 slackening of the suspensory ligament during accommodation. Recently 

 the theory has been attacked in several quarters ; one objection made 

 by Schon is based on the supposed difference of pressure in the aqueous 

 and vitreous humours in the eye adjusted for near vision. Schon points 

 out that the ability of the contracting ciliary muscle to pull the choroid 

 forwards depends on the intra-ocular pressure, and that it is impossible 

 that both rise of pressure in the vitreous and slackening of the sus- 

 pensory ligament should take place from the same cause. As we have 

 seen, however, there is no satisfactory evidence that the pressure in the 

 vitreous becomes higher than that in the aqueous humour. 



The greatest difficulty of the Helmholtz theory lies in the role 

 assigned to the circular fibres. It has been found by Iwanoff 1 that 

 the circular fibres are hypertrophied in cases of hypermetropia, in which 

 accommodation has been over-used, and are atrophied in myopia, in 

 which less than the normal effort is required. This change is said not 

 to be constant, but such changes are only to be expected in those cases 

 in which the refractive anomaly has been left uncorrected. If this 

 observation is accepted, the circular fibres must be of more importance 

 than is assigned to them by the theory. On the other hand, the cir- 

 cular fibres are absent or almost absent in nearly all mammals in whom 

 the mechanism -of accommodation is probably of essentially the same 

 nature as in man. Schon has objected that numerous pathological 

 conditions associated with hypermetropia and over-action of accom- 

 modation are not intelligible if the process of accommodation is one of 

 relaxation of tension. 



When accommodation was found to depend on a change in the form 

 of the lens, Cramer supposed that the increased curvature of the central 

 part of the lens was brought about by means of the pressure of the iris 

 on the peripheral parts. This was soon disproved by the observation 

 that accommodation took place quite normally when the iris was absent, 

 but this flattening of the periphery of the lens by pressure and bulging 

 of the centre is the essential feature of the two recent theories of 

 Tscherning and Schon. 



Tscherning 2 supposes that the posterior attachment of the meridional 

 fibres of the ciliary muscle is more fixed than the anterior, and that 

 when these fibres contract they pull on the suspensory ligament, 

 rendering it more tense, thereby flattening the peripheral parts of the 

 lens, while the centre becomes more convex. He states that when 

 traction is made on the suspensory ligament in the extirpated eye, the 

 change in shape of the lens corresponds to that taking place during 

 accommodation. Crzellitzer, however, who has devised a special ap- 



1 Arch./. Ophth., 1869, Bd. xv. Abth. 3, S. 284. 

 -Arch, dephysiol. norm, etpath., Paris, 1894, t< 



tome vi. p. 40. 



