BIRDS 401 



living creatures. This somewhat sweeping statement apphes to all 

 birds with remarkably few exceptions, such as the shy, nocturnal 

 kiwi, Apteryx, the eye of w'hich, a small myopic organ, is the poorest 

 among birds, for the dominant sense is smell rather than vision — a 

 unique j)henomenon in this class. Interestingly, its nostrils are placed 

 near the tip instead of the base of its long, exploring beak (Fig. 484). 



Built on the same general plan as the eyes of their ancestors, the 

 Reptiles, the eyes of Birds are remarkably standardized throughout 

 the entire class, showing few variations among themselves. The 

 general features of the avian eye are as follow^s : 



The large size of the eye ^ and its flattened, globular or tubular shajie 

 with a nasal eccentricity of the cornea and lens to assist binocular vision. 



The deep concavity in the ciliary region to maintain which the sclera 

 is supported by scleral ossicles, the non-spherical shajic of the globe being 

 further supported by a posterior cartilaginous cup. 



The presence of muscular elements in the choroid, ectodermal striated 

 muscles in the iris, and a complex and tvell-developed ciliary musculature 

 which bulges the lens forwards in accommodation. 



A lens ivith a well-defined annular pad. 



An elaborate vascularized glial j)ccten supiilementing the choroid in 

 supp>lying nourishment to the retina. 



A thick ayid remarkably icell-formed retina with precise layering and 

 quite unusually dense pachiyig of the visual elements, duplex in type with 

 rods and single and double cones containing oil-droplets, and p>rovided 

 with one or sometimes two fovece. 



THE GLOBE OF THE AViAX EYE with few exceptions is relatively 

 and absolutely large although, being entirely covered by the lids 

 apart from the relatively small cornea, its external appearance 

 gives the opposite impression (Fig. 494). The two ej'es of a bird, 

 however, often out^^'eigh the brain, and some hawks or owls, despite 

 their comparatively small size, have eyes larger than those of man. The 

 shape is peculiar and distinctive : the cornea is small and globular, 

 the posterior segment almost hemispherical with the horizontal 

 diameter often slightly greater than the vertical, but the intermediate 

 region between the tw'o varies (Figs. 488 and 490). This is the region 

 strengthened by the ring of scleral ossicles and its conformation 

 determines the shape of the eye (Figs. 487 to 491). Most commonly 

 it resembles a flat disc in which the cornea is set centrally while the 

 peripheral border joins with the hemispherical posterior segment of the 

 globe ; the result is a flat eye with a short ant ero -posterior axis, a 



^ The general rule illaller's ratio, 1768) (p. 450) that the size of the eye is inversely 

 proportional to the .size of the body is here overshadowed by the complementary 

 generalization (Leuckart's ratio, 1876) that the size of the eye varies directly with 

 swiftness of movemeut. 



S.O.— VOL. I. 26 



