io4o VISION. 



paratus for exerting uniformly circular traction on the zonula, failed to 

 make satisfactory observations on the peripheral parts of the lens. 



Tscherning neglects in his theory the circular fibres of the ciliary 

 muscle. In Schon's theory, 1 on the other hand, they play an im- 

 portant part. Schon supposes that the zonula becomes more tense 

 during accommodation, being compressed by the ring fibres, assisted by 

 contraction of the meridional fibres. He regards the lens capsule, 

 zonula, and choroid, as together forming an elastic and continuous 

 covering of the globular eyeball, and the result of circular pressure 

 round one pole of this globe would cause protrusion of the part within 

 the ring of pressure, namely, the anterior lens surface. The posterior 

 surface of the lens would be prevented from altering in form by the 

 increased pressure in the vitreous, which Schon supposes to be present. 

 While Schon's theory is satisfactory in supplying the circular fibres 

 with a function, it goes too far, for it leaves the other parts of the 

 muscle the only parts existing in most animals without any direct 

 function in accommodation. 2 The theories of both Schon and Tscherning 

 involve increased tension of the suspensory ligament during accommo- 

 dation. Schon supposes that the observation of Hensen and Voelckers, 

 showing relaxation of the zonula, arises from the folding upon itself of 

 the posterior fold of the ligament, and does not concern the more import- 

 ant fold going to the anterior surface of the capsule. Both theories 

 must fall, if Hess has given a correct explanation of his observations. 



Before the changes in the lens were described by Helmholtz, one of 

 the favourite theories was, that accommodation for near objects de- 

 pended on increase in length of the antero-posterior diameter of the 

 eyeball, brought about by the external pressure of the contraction of the 

 extra-ocular muscles associated with convergence, together with internal 

 traction by the ciliary muscle. On the discovery of the lens changes, 

 the theory was discarded, but it has recently been supposed to be a 

 subsidiary factor, and to be the explanation of the apparent accommoda- 

 tion which has been observed in cases of aphakia, and also when the 

 ciliary apparatus is supposed to be completely paralysed by atropin. 

 Knapp's observations show that in normal conditions the changes in 

 the lens are sufficient wholly to account for the range of accommoda- 

 tion. Any change in the shape of the eyeball would be accompanied 

 by change in the curvature of the cornea ; all agree that the latter does 

 not occur in normal eyes, and Silex 3 has found no change in a boy who 

 seemed to have some power of accommodation after removal of the lens. 



Accommodation in birds, reptiles, amphibia, fishes, and cephalo- 

 pods. The comparative physiology of accommodation has been very thoroughly 

 investigated by Beer. In birds 4 he has found that the cornea consists of two 

 distinct lamellae, and that Crampton's muscle produces retraction of the inner 

 or posterior lamella. In some birds (owls, hawks, buzzards) he found with the 

 ophthalmometer that the peripheral parts of the cornea were flattened, while 

 the curvature of the central part increased. The lens showed increased 

 curvature on stimulation of the ciliary muscle. On dividing the ligamentum 

 pectinatum, the lens was found to have increased its curvature, and stimulation 

 of the muscle then had no influence on the curvature. There was no 



1 Areh.f. d. ges. Physiol., Bonn, 1895, Bd. lix. S. 427. 



3 Schon supposes that the meridional fibres assist in maintaining and regulating intra- 

 ocular pressure. 



3 Arch. Ophlh., N. Y., 1889, vol. xviii. p. 274. 



4 Areh.f. d. ges. Physiol., Bonn, 1893, Bd. liii. S. 175. 



