EYE. 



187 



fossil Reptiles and Amphibians possessed similar plates, as do 

 also existing Birds (Figs. 154, 155, f) ; in Birds horse-shoe- or 

 ring-shaped bony structures are also usually present close to the 

 entrance of the optic nerve. 



FIG. 154. EYE OF Lacerta muralis, SHOWING THE RING OF BONY SCLEROTIC 



PLATES. 



While the eyeball of Reptiles has a globular form (Fig. 154), 

 that of Birds, and especially nocturnal Birds of prey (Owls), is 

 more elongated and tubular, an external larger segment being 

 sharply marked off from an internal smaller one (Fig. 155). The 



FIG. 155. EYE OF AN OWL. 



Et, retina ; Oh, choroid ; Sc, sclerotic, with its bony ring at t ; CM, ciliary muscle ; 

 Go, cornea ; VN, point of junction between sclerotic and cornea ; Ir, iris ; VK, 

 anterior chamber ; L, lens ; Cv, 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. 



former is bounded externally by the very convex cornea (Co), and 

 encloses a large anterior chamber ( VK}, as well as a complicated 

 ciliary muscle (Crampton's muscle) composed of striped fibres. 

 This muscle is also transversely striated in Reptiles, and in them 



