THE PERCEPTION OF FORM 



647 



the depth of focus of the eyes of Fishes is so great as not to demand 

 much from accommodation. 



In Holosteans the lens is slung on a dorsal suspensory ligament and an 

 ectodermal lenticular muscle is present apparently analogoiis to the campanula ; 

 its action, however, is unknowTi. 



(iii) A forward jnovement of the lens, rendering the eye more myopic 

 to accommodate for near objects is brought about by two separate 

 mechanisms in the vertebrate jih^'him — bj- the direct action of a sj^ecial 

 muscle in Selachians (and possibly Holocephalians) and Amphibians, 

 and indirectly by increasing the pressure in the vitreous cavity as a 

 result of contraction of the sphincter of the 

 pupil, a mechanism seen in snakes. 



In SELACHIANS the lens is suspended as 

 in Teleosteans by a dorsal suspensory liga- 

 ment, and is said to be swung forwards by 

 the action of the smooth (ectodermal) muscle 

 fibres in the ventral ciliary papilla ^ (Fig. 

 777). Among Selachians, Franz (1905-31) 

 demonstrated such a movement in the rays, 

 Raja asterias, and Torpedo, doubtfully in the 

 dogfish, Scyllium, but not in the dogfish, 

 Mustelus ; in some species he found a wide 

 range of accommodation (15 to 20 D). It is 

 to be remembered, however, that neither 

 Beer (1894) nor v. Hess (1912) obtained any 

 such response to electrical stimulation, while 



Verrier (1930) and Rochon-Duvigneaud (1943) found that the muscular 

 fibres in the ciliary papilla were scanty or absent ; these workers 

 therefore concluded that the accommodation of the Selachians which 

 they investigated was minimal or lacking. 



In those amphibians which have accommodation, the amplitude 

 is much poorer^ — never more than 4 or 5 D (Beer, 1898) — an amount 

 quite useless in maintaining good visual acuity in both an aerial and 

 an aquatic environment. Our knowledge of this subject, however, is 

 again meagre and somewhat conflicting. Beer (1898) advanced the 

 theory that accommodation for near vision was attained through the 

 contraction of the ciliary muscles compressing the vitreous body which 

 in turn thrust forwards the lens. His experimental techniques and 

 conclusions, however, have been challenged, particularly by v. Hess 

 (1912) on the basis of his findings with direct electrical stimulation. It 

 is true that in the small eyes of Amphibians with their short focal 

 distance, the length of the receptor elements would allow the image 



1 p. 285. 



Fig. 777. — Accommodation 

 IN Selachians. 



The ej-e in the normal 

 condition of rest {D, distance 

 vision). The eye in accom- 

 modation for near vision (N) 

 with the lens moved for- 

 wards. 



