646 THE EYE IN EVOLUTION 



providing accommodation for distant objects and giving the 

 normally myopic eye a fairly high degree of hypermetropia ; Franz 

 (1934) claimed that the extraordinarily wide range of accommodation 

 from + 20 to — 20 D is thus rendered available. 



(ii) A backward {and sideways) movement of the lens induced by 

 direct muscular action as an accommodative mechanism for distant 

 vision is unique to teleostean fishes among which it is of general 

 occurrence, although it is absent or ineffective in very small-eyed 

 forms which have a relatively large lens. The classical view put 

 forward by Beer (1894) and confirmed by most authors (Franz, 1905-31 ; 

 Header, 1936 ; Rochon-Duvigneaud, 1943) is the following (Fig. 772). 

 As in the lamprey, the spherical lens normally approximates the cornea, 

 suspended naso-dorsally by a zonular ligament on which it can swing 

 pendulum-like backwards and forwards. A backward movement is 

 brought about by the retractor lentis muscle (the campanula of Haller), 

 a small ectodermal muscle situated naso- vent rally derived from the 

 ectoderm at the borders of the falciform process. ^ It is to be noted 

 that in the act of accommodation the lens moves as much temporally 

 (towards the tail) as backward into the eye, if not more so, thus moving 

 the image sideways across the retina ; by this inovement the image 

 will leave the temporal fovea (when one is present) which is used in the 

 state of relaxation for convergence upon near objects (Figs. 773-6). 



While this is the most generally accepted explanation of teleostean 

 accommodation, an entirely different view has been put forward by 

 Bourguignon and Verrier (1930). Beer ( 1 894) had found that on electric 

 stimulation the campanular muscle contracted and pulled the lens 

 backwards. The former authors failed to substantiate this ; on the 

 contrary, on electrical stimulation they foimd in a number of species 

 (the roach, the tench, the goldfish, the barbel and the chub) that a 

 deformation of the globe was produced by the tensor choroideae muscle 

 which encircles the eye at the corneo-scleral junction, resulting in a 

 lengthening of the antero -posterior axis. If the average teleostean eye 

 is hypermetropic ^ a retraction of the lens would, of course, increase 

 the optical error and have the reverse of an accommodating effect ; 

 the myopia induced by an increase in the antero -posterior axis would 

 be effective in accommodating for near vision. On the other hand, if, 

 as Beer claimed, the normal refraction is myopic, a retraction of the 

 lens would ensure good distance vision at the expenditure of muscular 

 effort. Further experimental exploration of this mechanism in 

 teleostean fish is required to clear up the position but it is to be 

 remembered that with the spherical lens and its dense central core, 



* p. 302. Seen also in a well-develoiaed form in tubular ej^es with their enormous 

 lenses (Fig. 380). 

 - p. 638. 



