EYE 



213 



The eyes of Proteus and of the Gymnophiona, as already mentioned, 

 always lie more or less deeply beneath the skin ; they are very small, and 

 are much degenerated. In Proteus the crystalline lens and iris are both 

 wanting, and the vitreous humour is only slightly developed. 



Reptiles and Birds. — In these also, the sclerotic is in great 

 part cartilaginous, and in Lizards and Chelonians it is provided 

 with a ring of delicate bony sclerotic plates around 

 the external portion (Fig. 170). Many fossil Eep- 

 tiles and Amphibians possessed similar plates, as 

 do also existing Birds (Fig. 171) ; in Birds horse- 

 shoe- or ring-shaped bony structures are also 

 usually present close to the entrance of the optic 

 nerve. 



The eyeball of Reptiles has a globular form 

 (Fig. 170), while that of Birds, more especially 

 nocturnal Birds of prey (Owls), is more elongated 

 and tubular, an external larger segment being 

 sharply marked off from an internal smaller 

 one : moreover the whole eye is relatively larger. 



(Fig. 171). The 



Fig. 170. — Eye 

 OP Lacerta niu- 

 ralis, SHOWING 

 THE Ring of 

 Bony Sclero- 

 tic Plates. 



Fig. 171. — Eye of an Owl. 



Rt, retina ; Ch, Choroid ; Sc, sclerotic, 

 with its bony ring at t : CM, ciliary 

 muscle ; Co, cornea ; VN, point of 

 jvmction between sclerotic and cornea ; 

 Jr, iris ; VK, anterior chamber ; L, 

 lens ; Cr, vitreous humour ; P, pecten ; 

 Op, OS, optic nerve and sheath. The 

 dotted line passing across the broadest 

 portion of the circumference of the eye 

 divides the latter into an inner and 

 an outer segment. 



outer portion is bounded ex- 

 ternally by the very convex 

 cornea and encloses a large 

 anterior chamber as well as a 

 complicated ciliary muscle com- 

 posed of striated fibres. This 

 muscle is also transversely stri- 

 ated in Reptiles, in which — 

 especially in Chelonians, it is 

 always well developed, though 

 . not to such an extreme degree 

 as in Birds. 



In Reptiles (Lizards, for in- 

 stance) a tapetum may be 

 developed, but an argentea and 

 choroid gland are never present ; 

 all these parts are wanting in 

 Birds. A structure which is 

 homologous with the processus 

 falciformis of Fishes is, how- 

 ever, present in most Reptiles 

 and in Birds. Absent in 

 Hatteria and the Chelonia, 

 this so-called ^^ecten is largely 

 developed in Birds i (Fig. 171), 

 and may extend from the point 

 of entrance of the optic nerve 



to the capsule of the lens, but 

 In Apteryx the pecten disappears during development. 



