A CCO M MO DA TION. 1 04 1 



appreciable alteration in the curvature of the posterior surface of the lens. 

 Beer concluded that in far vision the anterior surface of the lens is compressed 

 by traction exerted by the ciliary processes (which in the bird are directly 

 attached to the lens capsule). This traction is kept up by the tension of the 

 ligamentum pectinatum ; when the ciliary muscle contracts, it draws back the 

 inner lamella of the cornea, and the tension of the ligamentum pectinatum 

 being relaxed, the anterior lens surface is allowed to become more convex. 

 This mechanism has the same principle as that formulated by Helmholtz for 

 the human eye, but differs greatly in its details. In some birds the same 

 mechanism supplements the increased refraction of the lens by increasing the 

 curvature of the central part of the cornea. 



In reptiles (except snakes), Beer 1 found that the mechanism of accommo- 

 dation closely resembles that of birds. In most snakes, on the other hand, 

 no change in the form of the lens was observed, but a forward movement of 

 the whole lens was found to take place. This movement was brought about by 

 means of a striped muscle composed of circular fibres, which exerts pressure 

 on the vitreous during contraction, and forces forward the most movable part 

 of the wall of the vitreous chamber, namely, the lens. When the vitreous 

 chamber has been opened, contraction of the muscle causes retraction of the 

 lens. Among the snakes examined, only one was found (Tropidonotus tessel- 

 latus) in which the anterior surface of the lens became more convex, but this 

 change was also probably accompanied by a bodily movement of the lens for- 

 wards. Most of the amphibia examined 2 were found to accommodate in the 

 same way as snakes, by means of a forward movement of the lens. Some were 

 found to be approximately emmetropic in air, others in water ; but in no case 

 was the amount of accommodation present sufficient to enable an animal to see 

 distinctly in both media. In some snakes and amphibia, the power of accom- 

 modation seemed to be absent. 



In fishes, Beer found the mechanism to be of an absolutely different nature. 3 

 It had previously been found that the fish is myopic in water, and Beer con- 

 firmed this, using the methods of skiascopy and direct ophthalmoscopy, the 

 myopia varying from about 3 to 12 D in water in most fishes, and from 40 to 90 

 D in air. With the ophthalmoscope Beer found a decrease of the myopia on 

 electrical stimulation of the eye. With the phakoscope and ophthalmometer 

 he found no change in the curvature of the lens, but distinct movement of the 

 lens backwards and slightly to the temporal side during stimulation. He found 

 that this movement was due to the campanula, which could be seen to contract ; 

 while retraction of the lens did not occur after cutting the campanula or its 

 tendon. The lateral movement of the lens was due to the position of the 

 campanula on the temporal side, and through the space between the pupil and 

 the lens Beer was able to observe a corresponding lateral movement of the 

 retinal image. This mechanism seemed to be limited to the Teleostean fishes ; 

 no movement of the lens could be detected in the Elasmobranchs examined. 



In Teleostean fishes, the eye when at rest is shown to be adjusted for near 

 vision, and accommodation for far vision is brought about by movement of 

 the lens backwards. Beer 4 has found that this is also true of the cephalopod 

 eye, though the mechanism of retraction is different. The determination of 

 the refraction was very difficult, but Beer satisfied himself by skiascopy that 

 myopia existed at rest, varying from 2 to 10 D, being greatest in the octopods. 

 On electrical stimulation the myopia decreased ; no alteration in the curva- 

 ture of the lens could be detected ; and, by means of a needle in the lens, 

 graphic tracings, showing retraction of the lens during stimulation, were 

 obtained. The retraction was found to be due to the ring muscle first 



*& 



' Arch./, d. gcs. Physiol, Bonn, 1898, Bd. Lxix. S. 507. 



2 Ibid., 1S98, Bd. lxxiii. S. 501. 3 Ibid., 1894, Bd. lviii. S. 523. 



4 Ibid., 1897, Bd. lxvii. S. 541. 



vol. ii. — 66 



