278 ADAPTATIONS TO SPACE AND MOTION 



mammals. There is often a tenacular ligament (tl in Figs. 109, 112) 

 running from the sclera to the orbiculus just in front of the ora termi- 

 nalis, which prevents the chorioid's being drawn forward. Thus when 

 the ciliary muscle contracts, it can have but two possible actions — 

 and only the first of these unless the sclerocorneal sulcus happens to be 

 quite well marked: (a) a stretching of the orbiculus and a heaving of the 

 corona of ciliary processes forward and toward the axis of the eye, thus 

 pressing them against the lens. Any actual forward movement of the lens 

 is checked partly by the weak suspensory-ligament fibers, partly by the 

 'pectinate ligament' running from cornea to iris root across the angle 

 of the anterior chamber (Fig. 109, pi). The force of contraction of die 

 ciliary muscle is thus largely diverted to the accomplishment of an 

 actual squeezing of the lens, (b) A traction backward and axiad upon 



Fig. Ill— Transversalis muscle of a lizard, Lacerta serpa, as seen in a frontal section 

 through the region (mid-ventral) of the embryonic fissure. After Lasker. 



c- connective tissue of ciliary body; /- unclosed portion of embryonic fissure; m- transversalis 

 muscle; r, r- pars ciliaris retina; t- tendon of muscle, which inserts on lens. 



the limbus corneas, deepening the sclero-corneal sulcus and sharpening 

 the curvature of the cornea. 



How much of the latter action {b) ever occurs in reptiles is a question. 

 It would be helpful in accommodating the eye for near objects; but most 

 of the accommodation is certainly brought about by the sharpening of 

 the anterior curvature of the lens. The posterior surface of the lens abuts 

 upon the relatively unyielding vitreous, and the periphery of the anterior 

 surface against the similarly firm iris root. Hence most of the defor- 

 mation of the lens is confined to a central area on the anterior surface — 

 just as it is, by an utterly different mechanism, in the human eye (see 

 Fig. 109, p. 275; cj. pp. 32-4 and Fig. 14, p. 31). 



It used to be thought that a deformation of the sclero-corneal region, 

 impressing the vitreous and causing the latter to push the lens forward, 

 was the chief or only factor in sauropsidan accommodation. Ingenious 

 experiments made a few years ago by Hess, however, have shown that the 



