THE AVIAN EYE 



643 



Horizontal ellipsoidality is slight at most — the ratio of the horizontal 

 diameter to the vertical is usually 1:1 and never greater than 1.2:1, 

 But nasad asymmetry is universal: the cornea and lens are not only 

 tilted toward the beak, but shifted in that direction as well (Fig. 190; 

 Fig. 71, p. 173). This effort to secure a maximal overlapping of the 

 monocular visual fields is just as great, or greater, in elongated eyes 

 whose retinae and visual fields are restricted by their tubularity. 



The sclera always contains a hyaline-cartilage cup, which extends for- 

 ward to the back edge of the ossicular ring, where it is usually locally 



Fig. 190 — Bird eyes, showing charaaeristic shapes, xl. After Soemmering. 



(Each drawing shows the ventral half of the left eyeball; the nasal side is to the right; 

 the plane of the ora terminalis retinae has been placed horizontally to bring out the nasad 

 asymmetry which is present to some degree in the eyes of all birds). 



a, commonest, 'flat' type (in a swan, Cygnus olor). b, 'globose' type (in an eagle, Aquila 

 chrysaetos). c, 'tubular' type (in an owl, Bubo bubo). 



thickened. External to the cartilage is a dense fibrous layer, often as thick 

 as (or thicker than) the cartilage itself, particularly in the fundus and 

 particularly in large eyes. Surrounding the optic nerve there is often 

 (most often in small eyes*) a plate of bone, the 'Gemminger's ossicle'. 

 This may be horseshoe- or washer-shaped, or may be represented by sev- 

 eral separate pieces. It is set in the cartilaginous cup as if formed from 

 a portion of the latter's substance; but its mode of development is un- 

 known, as is also its physiological value. The anterior scleral ossicles 



*And supposed to be conspicuously developed in the woodpeckers; but it is lacking in the 

 flicker. 



