644 



THE EYE IN EVOLUTION 



adaptation to increase sensitivity to light ^ ; but 

 dative elasticity is thereby also rendered possible. 



some accommo- 



Dynamic Devices 



Dynamic accommodation involves one of two expedients — a 

 movement of the lens as a whole or its deformation ; both are brought 

 about by muscular activity and in every case the essential muscles 

 involved are under the control of the oculomotor nerve. In the first 

 case the lens may either be pushed or pulled backwards or forwards ; 

 in the second it can be deformed by direct pressure or in an indirect 

 way through varying the tautness of an elastic capsule. 



An accoinmodative function has been ascribed to two other devices on inore 

 questionable grounds. It has been claimed that the columns of connective and 

 muscle tissue traversing the thickness of the choroid of Birds may pull the 

 retina backwards.^ Such an axial moveinent of the retina as an aid to accom- 

 modation is, however, by no means established. A still more questionable 

 hypothesis is that the pecten of Birds serves as an adjuvant to accommodation.^ 



(a) A movemejit of the lens as a wJwle. This mechanism is 

 characteristic of the more primitive Vertebrates. The firm spherical 

 lens of high refractive index necessary for optical purposes in an 

 aquatic environment is obviously not easily susceptible to deformation ; 

 this mechanism is therefore seen in C-yclostomes and Fishes and has 

 been retained by Amphibians ; it also occurs in snakes, the eyes of 

 which, as we have remarked,* are essentially primitive in most of their 

 characteristics. It is to be noted, however, that in the last an entirely 



novel and distinct technique has been evolved 

 bearing no relation to the ichthyopsidan plan. 

 If the lens is moved backwards the eye 

 becomes hypermetropic and vision is ad- 

 justed for distant objects (negative accom- 

 modation) ; this is characteristic of 

 Cyclostomes and Teleosteans. If the lens is 

 moved forwards the eye is rendered myopic 

 and accommodation is attained for near 

 vision ; this is seen in Selachians and 

 Amphibians ; the same direction of move- 

 ment is also seen in snakes. 



(i) A backward ynoveynent of the lens 

 induced by corneal jyressure. This mechanism 

 is seen only in the most primitive of Verte- 

 brates — the CYCLOSTOMES ; to these it is 

 unique and an intra-ocular accommodative 



Fig 



1 7 1 . — Accommodation 

 IN Cyclostomes. 



N, the eye adjusted for 

 near vision ; D, the eye 

 adjusted for distance vision. 

 The cornealis muscle, CM, 

 pulls the cornea backwards 

 which in turn pushes the 

 lens nearer the retina. 



pp. 305, 364. 

 p. 416. 



p. 404. 

 p. 383. 



