642 BIRDS 



age amphibian or reptile. The two eyes of a bird often outweigh its brain, 

 and there is often barely room enough for them in the head. The largest 

 land-vertebrate eye is that of the ostrich, 50mm. in diameter. Hawks and 

 owls, a fraction of the size of a man, have eyeballs as large as ours and 

 larger. 



Such eyeballs are necessarily a tight fit for their orbits. There is no 

 room for a muscle cone like that of a shark or a man. The reptilian re- 

 tractor bulbi muscle has been discarded — leaving behind it, however, 

 its derivative, the bursalis (see Figs. 142b, 143c and f; pp. 420-1). The 

 oculorotatory muscles are ribbon-like, and plastered snugly against the 

 globe (Figs. 70, 107d; pp. 172, 270). They never extend forward beyond 

 the limits of the convex posterior portion of the eye; hence, where the 

 latter is tubular (Fig. 190c), the muscles are relatively short (and, in 

 owls and some eagles, functionless) . 



The shapes of avian eyes fall into three rough categories : flat, globose, 

 and tubular (Fig. 190). In all, however, there is a prominent concave 

 region which coincides with the zone occupied by the ciliary body and 

 the ring of scleral ossicles which creates and supports the concavity. In 

 the 'flat' eyes exhibited by a very great majority of birds, the axis is much 

 the shortest of the three diameters, equalling but seven- or eight-tenths 

 of the vertical. The shape of these eyes is thus the same as in the lizards. 

 In those diurnal birds which need high resolution at great distances (i.e., 

 wing-feeding insectivorous forms, predators in general, and such types 

 as the crow) , the ratio goes as high as unity, yielding the 'globose' form 

 of eyeball. In most owls (and some eagles) the axis is as long as the 

 other diameters or even a bit longer, and at the same time the concave 

 zone is so broad that the eye is rendered 'tubular'. In these instances the 

 retinal area is relatively small; but the retinal image may be either small 

 (where the lens is closer to the retina and rotund — owls) or large (where 

 the lens is farther forward and flatter — eagles). Accompanying the in- 

 crease in relative axial length one sees invariably a proportionate broad- 

 ening of the curvature of the posterior segment, so that the junction of 

 this region with the concave zone becomes more and more conspicuous. 

 Except where the lens recedes into the eye and sharpens its curvature 

 (as in owls, Podargus, etc.), the phylogenetic increase in the axial length 

 of the bird eye can always be described as adaptive toward the securing 

 of higher visual acuity, through an increased 'throw' of the image from 

 optical center to retina and the consequent broadening of the image at 

 the visual-cell level. 



