REPTILES 



357 



It, 



fi!i»VN[*»l'--»'?^^« 



of the lens (Figs. 423-4). The musculature is complicated and is 

 divided into 3 sj^stems. The ciliary muscle (of Briicke) is well developed, 

 the fibres running meridionally from their origin from the inner layers of 

 the cornea, not to the choroid as does the tensor choroidese of Fishes 

 and Ampliibians (or the ciliary muscle of Mammals), but to the 

 orbiculus ciliaris. where its anchorage is continued by a tenacular 

 LIGAMENT running from the orbiculus into the sclera. These fibres are 

 particularly marked anteriorl}^ those 

 arising from the cornea being to some 

 extent isolated to form the iniuscle 

 OF CRAiNiPTON, a muscular bundle 

 more fully developed in Birds. ^ The 

 meridional ciliary fibres are some- 

 times augmented by circumferential 

 fibres arising dorsally and extend- 

 ing round in the temporal half of 

 the globe ; and m most species by 

 an inferior transverse muscle. This 

 muscle arises ventrally from the con- 

 nective tissue between the ciliary 

 body and the sclera and passes 

 tlu'ough an open portion of the foetal 

 cleft to be inserted into the zonular 

 fibres and thus indirectly to the lens. 

 It would seem analogous to the pro- 

 tractor lent is muscle of Ampliibians 

 and probably moves the lens nasally 

 during accommodation, presumabh^ 

 to increase convergence {Seps, 

 Lacerta — Leplat , 1921). 



The iris is relatively thin at the 

 periphery, but thick toAvards the 

 pupillary margm w^here it forms a well-marked ramp. The two 

 posterior ectodermal layers are deeply f)igmented and from the anterior 

 are derived the striated fibres of the jDupillary musculature. The 

 circumferential spMncter fibres are well developed. The dilatator 

 fibres form a thin layer next the ej)itheliuni, their ordinarily radial 

 direction assummg complex configurations in those species wherein the 

 pupil is slit-shaj^ed. The mesodermal portion of the iris is usually 

 highly coloured as if in an attempt to make the eye consi^icuous, some- 

 times with red, yellow and melanin jDigments, sometimes, as in the 

 chameleon, having a brilhant metallic sheen owing to a layer of guanine- 



Fig. 422. — The Posterior Segment of 

 THE Eye of the Lizaed. 



Showing the retma, r, with its pig- 

 mentary epitheHum, p, choi'oid, ch, 

 scleral cartilage, s, and the fibrous sclera, 

 sc ( X 320) (Norman Ashton). 



1 p. 405. 



