BIRDS 



409 



but more spherical, although never completely so, in nocturnal and 

 aquatic types (Figs. 501-3). It is always soft and readily deformed ; 

 apart from its capsule it has no consistency (Rabl, 1898), and according 

 to Kajikawa (1923), the soft mouldability is retained all through life 

 into old age. In some aquatic species, particularly the cormorant, it 

 comj)ares in softness only with the lens of turtles. The system of 

 sutures is simple, comprising a single line in some species, a star-shape 



Figs. 501 to 503. — The Lenses of Birds. 



Fig. 501.— The pigeon. Fig. 502.— The owl. Fig. 503.— The bullfinch. 



Note the relatively flat anterior surface (to the right in each ca.se). 



in others. The annular pad is usually well formed, sometimes enor- 

 mous in diurnal predators with a high degree of accommodation, as in 

 the hawk, wherein it occupies half the area of a cross-section of the 

 lens (Fig. 504), smaller in nocturnal species (Fig. 505), still smaller in 

 aquatic forms wherein the sphincter of the iris rather than the ciliary 

 muscle is especially active in accommodation (as in the Anseriformes 

 such as ducks, geese, swans, etc. ; the Ciconiiformes, such as herons, 

 storks, spoonbills; and the cormorant), and very small indeed or even 

 vestigial in running birds (Palaeognathae, particularly the kiwi, 

 Ajyteryx) ; in the Australian goose, Cereojisis, a terrestrial bird which 

 hardly ever leaves the ground, the pad is practically non-existent. 



Figs. 504 and 505. — The Lenses and Annular Pads of Birds. 



Fig. 504. — The lens of a diurnal predator 

 (a hawk). Showing a very large annular 

 pad. 



Fig. 505. — The lens of a nocturnal bird 

 (an owl). Showing a small annular 



