282 COMPARATIVE ANATOMY 



hyaline cartilage and is often pigmented : ossifications are not 

 known to occur. 



Reptiles and Birds. In the Sauropsida, more especially in 

 Birds, the eye-ball is much larger relatively to the head than in 

 the Amphibia. The sclerotic here, too, is in great part cartilagin- 

 ous, and in Lizards and Chelonians is provided with a ring of 

 delicate bony sclerotic plates around the external portion. Many 

 fossil Reptiles and Amphibians possessed similar plates, as do also 

 existing Birds (Figs. 207, 208) ; in Birds horseshoe-shaped or ring- 

 shaped bony structures are also usually present 

 close to the entrance of the optic nerve. 



The eyeball of Reptiles has a more or less 

 globular form, while that of Birds, more especially 

 nocturnal Birds of prey (Owls), is more elongated 

 and tubular, an external larger segment being 

 sharply marked oft' from an internal smaller one, 

 the outer portion being bounded externally by 

 Fi<;. -207. EYK the very convex cornea and enclosing a large 

 owLacertamu- aqueous chamber: moreover, the whole eye is 







SHOWINC , , , /T7V <rno\ T 11 xl c< 



THE RING OF relatively larger (Fig. 208). In all the Saurop- 

 BONY SCLERO- sida, except Snakes, there is a complicated ciliary 

 TIC PLATES. muscle composed of striped fibres, one portion of 

 which, extending towards the cornea, is known 

 in Crampton's muscle : ciliary processes are well developed, especi- 

 ally in Birds. This muscle is also transversely striated in Reptiles, 

 and, especially in Chelonians, is always well developed, though not 

 to such an extreme degree as in Birds. 



In Reptiles (e.g. Lizards) a tapetum may be developed, but 

 an argentea and " choroid gland " are never present ; all these 

 parts are wanting in Birds. A cone-like or cushion-like structure, 

 the so-called " pecten," which is to some extent homologous with 

 the processus falciformis of Fishes, is, however, present in Lizards, 

 but is more or less reduced in other Reptiles. It varies much in 

 its development in different forms, is very vascular and some- 

 times pigmented, and extends into the vitreous chamber through 

 the proximal end of the choroid fissure. In Birds, 1 a somewhat 

 similar organ is very largely developed, and in some cases may 

 even extend as far as the capsule of the lens : it is four-cornered, 

 more or less folded, and pigmented, and consists mainly of a 

 closely-felted network of capillaries. From its form, it is better 

 described in Reptiles as a " cone '' or " cushion " and in Birds as a 

 "fan " (Fig. 208). In both Reptiles and Birds this organ appears, 

 amongst other possible functions, to be important in connection 

 with the nutrition of the contents of the eyeball and of the retina : 

 it has nothing to do with accommodation. 



The iris, regulated by striated muscle, is able to respond very 



1 In Apteryx the pecten disappears during development. 



