476 



Comparative Morphology of Chordates 



than eyes of amphibians. The external supporting layer (sclerotic) of 

 the eyeball usually contains considerable cartilage. In lizards and 

 turtles ossification of the cartilage forms a circular series of small 

 sclerotic plates surrounding the exposed outer face of the eyeball 



Fig. 368. Sections of larval (Bornj and adult 

 (Schimkewitsch) nasal organs of Lacerta. Bone 

 black, (c) Embryonic concha; (ds) dental shelf; (e) 

 eye; (g) Jacobson's glands; (j) organ of Jacobson; 

 (/) lacrimal groove; (n) main nasal cavity; (of) duct 

 of organ of Jacobson; (I) tooth-germ. (Courtesy, 

 Kingsley: "Comparative Anatomy of Vertebrates," 

 Philadelphia, The Blakiston Company.) 



(Fig. 369). Reptiles have a more refined method for "accommodating" 

 or focusing the eyes for objects at varying distances. In teleost fishes 

 the resting eye is focused for near objects. For seeing distant objects 

 the lens is pulled back toward the retina by a muscle (retractor lentis) 

 inside the eyeball. In amphibians the rest- 

 ing eye is set for distant objects. For near 

 sight, the lens is moved away from the 

 retina. In reptiles the lens is quite elastic. 

 By action of the ciliary muscles situated in 

 the adjacent region of the wall of the eyeball 

 (see p. 201), the degree of convexity of the 

 lens may be altered. For distant vision the 

 lens becomes less convex; for near vision 

 the convexity is increased. In birds and 

 mammals accommodation is effected in a 

 similar way, although differing in details 

 (see Fig. 510). Eyelids are better developed 

 than in most amphibians. There are ordi- 

 narily upper and lower lids, both freely 

 movable, and under them a semitransparent nictitating mem- 

 brane which may be pulled across the eye from front to rear. The 

 chameleon, however, has a single circular lid with a central hole in it. 

 In most lizards of the gecko family and in all snakes, eyelids are quite 



Fig. 369. Circular series 

 of bony sclerotic plates in 

 the outer wall of the eyeball 

 of a lizard. (Courtesy, Wied- 

 ersheim: " Grundriss der ver- 

 gleichenden Anatomie der 

 Wirbeltiere," Jena, Gustav 

 Fischer.) 



