THE PERCEPTION OF FORM 651 



but into the ciliary body itself. In most Reptiles it is a simple strip running 

 from the corneal margin to the base-plate of the orbicular zone of the ciliary 

 body, but in lizards and in Birds (except diving birds) it is divided into two — 

 the anterior part, Crampton's muscle, strung like a bow-string running between 

 the periphery of the cornea and the sclera, presumably deforms the cornea ; 

 the posterior section, Briicke's muscle, thrusts the ciliary body axially on 

 contraction (Fig. 499). In some Birds, Briicke's muscle is still further sub- 

 divided, its anterior portion being known as Miiller's muscle.^ 



A further muscle is seen in Clielonians and lizards — and in ( ?) the pigeon, 

 Columba : the transversalis muscle, a strip of striated muscle originating 

 ventrally in the connective tissue between the ciliary body and the sclera and 

 inserted into the zonular fibres. Its action is to pull the lens nasally thus helping 

 binocvilar vision on accommodation and convergence. In a sense it seems 

 comparable to the ventral protractor lentis of Amphibians although not homo- 

 logous with it, and is concerned with binocular vision rather than with 

 accommodation. Such a nasal movement is also aided, particularly in Birds, 

 by the asymmetry of the ciliary body and the anterior segment of the globe. 



CHELONiANS have the softest and most readily mouldable lenses 

 amongst all Vertebrates ; accommodation in these animals is effected 

 by the formation of an anterior lenticonus by the action of the powerful 

 sphincter pupilla? (Beer. 1898 ; v. Pflugk, 1908 ; v. Hess, 1909-12) ; in 

 these reptiles the annular pad is therefore small and in tortoises and 

 terrapins the ciliary musculature relatively weak (Figs. 783-5). Sea- 

 turtles have little use for accommodation but the undoubted prowess 

 of terrestrial forms in catching insects demonstrates that their range 

 of accommodation must be good. 



The accommodation of crocodilians has not been thoroughly 

 explored ; Abelsdorjff (1898), however, concluded that it extended to 

 a range of 8 D. Similarly S-pheiiodon with its weak ciliary muscle can 

 only accommodate little — if at all ; the greatest effect would seem 

 probably, as in turtles, to come from the deforming effect of the 

 sphincter of the iris on the anterior surface of the lens. 



LIZARDS, on the other hand, have good accommodation, their 

 excellent mechanism being aided in some cases (nocturnal geckos) by 

 the stenopoeic contracted pupil. Electrical stimulation has been found 

 to increase the refraction considerably — by 15 D in Iguana, 10 D in 

 Lacerta (v. Hess, 1909-12). 



In BIRDS the accommodative mechanism is superb, the most 

 efficient, indeed, amongst Vertebrates, and in these, as in turtles, the 

 formation of an anterior lenticonus is aided considerably by the 

 powerful contraction of the sphincter muscle of the pupil which acts 

 as a " compressor lentis " (v. Hess, 1910-12). In the owl, Bubo, the 

 range is small (probably some 4 D), in the nocturnal predators, 2 to 3 D 

 and exceptionally 4 D, in the average passerine bird some 8 to 12 D, and 

 in the predatory birds (hawks, eagles, etc.) still greater. The highest 



1 p. 406. 



