ACCOMMODATION IN LAMPREYS 



259 



through the elasticity of the sclera and vitreous and the equaUty of 

 intra-ocular pressure throughout the globe. This method is at the mercy 

 of quick, great changes of hydrostatic pressure and so would not work 

 well in a fish which makes rapid changes in the depth at which it swims. 

 Despite the manipulation of the eyeball by the muscle or by water pres- 

 sure, the lens cannot become dislocated, for it projects well through the 

 pupil and is firmly held fore and aft by contact with the cornea and the 

 vitreous (Fig. 103, ay). There is no zonule or suspensory ligament, nor 

 any need of one, for the spherical lens is trapped like one of the balls 

 in a ball bearing, the pupil comparing with the aperture in the ball cage 

 and the cornea and vitreous acting like the outer and inner races. 



Fig. lO'l — The elasmobranch eye and its mechanism of accommodation — Carcharodon 

 carcharius. x 1 1/3 . Combined from figures of Franz. 



The eye is represented in full accommodation; were the protraaor lentis muscle relaxed, the 

 lens would be withdrawn from the cornea, a, horizontal seaion. b, vertical seaion. 



c- cornea; ch- chorioid; c{- ciliary folds, from which gelatinous zonule passes to lens equator; 



ext- external rectus; i- iris; inf- inferior rectus; int- internal rectus; /- lens; op- optic pedicel; 



p- papilla bearing protractor lentis muscle; r- retina; s- suspensory 'ligament' (a thickening in 



the zonule); sc- scleral cartilage; sf- fibrous portion of sclera; so- superior oblique; 



sup- superior rectus. 



The musculus cornealis does not interfere with the rotation of the 

 eyeball, as it would do if inserted into the true cornea instead of into the 

 primary spectacle. There is obviously some reason to think that the con- 

 version of the spectacle into a conjunctiva (see Chapter 11, section D) 

 had to wait for the evolution of intra-ocular muscles of accommodation. 

 It has been claimed that the lamprey's oculomotor muscles (which are 

 very thin and much blended where they cling to the globe, and reach far 

 forward so as to form a smooth jacket) accomplish accommodation for 

 near objects by contracting in unison and thus elongating the eyeball. 

 There is as yet no adequate experimental basis for this belief, and 



