610 REPTILES 



closely that of the amphibian canals (see Fig. 173, p. 595). This early 

 divergence (i.e., as one passes forward from behind) of the uvea and 

 sclera — characteristic also of other reptiles and of birds (see Figs. 109, 

 112; pp. 275, 280) — helps to approximate the ciliary body to the pe- 

 riphery of the lens. The 40-60 ciliary processes (Fig. 110, p. 277) have 

 their crests firmly fused to the lens capsule (except in some or all marine 

 forms) . They send continuations a little way onto the iris. 



The ciliary muscle fibers are mostly meridional in orientation. They 

 originate from the inner layers of the substantia propria of the cornea 

 and run close to the sclera to terminate in the connective tissue of the 

 flat posterior zone (orbiculus) of the ciliary body. The muscle as a whole 

 is small in land forms and terrapins, in which the sphincter iridis does 

 most of the work of accommodation. But in marine turtles, which have 

 not much needed to employ the iris muscle for deforming the lens (since 

 they are limited to aquatic vision, and need no tremendous range of 

 accommodation), the ciliary muscle is massive; and this is probably a 

 primitive situation. The transversalis muscle (see pp. 269, 279, 299) 

 originates in the connective tissue between the ciliary body and sclera, 

 ventrally, and passes through a portion of the (otherwise healed) em- 

 bryonic fissure of the pars caeca retinae to pull on a group of zonule fibers 

 which serve as its tendon. Its relationships are thus much like those of 

 the amphibian's ventral protractor lentis, with which it is conceivably 

 homologous. 



The iris is not sharply demarcated from the ciliary body, since the 

 base-plate of the latter is largely separated from the sclera and makes 

 no sharp angle with the iris at the latter 's periphery. Both retinal layers 

 are pigmented, and it is doubtful whether the anterior layer ever gives 

 rise to a dilatator comparable with that of the mammals. Radial muscle 

 fibers may be seen even contiguous with the anterior retinal layer, but 

 these are nucleated and are probably only re-oriented derivatives of the 

 massive sphincter muscle, which occupies the whole breadth of the iris 

 from pupil to root. 



The lens is the softest, most pliable in the vertebrates. It is flattest in 

 the tortoises (equatorial-axial diametral ratio 1.6 in Testudo grceca) , 

 less flat ica. 1.3) in terrapins, and virtually spherical in sea-turtles — 

 where of course it need not be prepared to deform as much as in the 

 other types of chelonians, but needs a strong curvature when at rest 

 owing to the optical absence of the cornea. The 'ringwulst' is weakly 

 developed in chelonians. The primary vitreous of the embryo is not 



