404 



THE EYE IN EVOLUTION 



The cormorant 

 Phalucrocorar. 



532 species without any apparent logical distribution ; no satisfactory theory 

 for its 2:)resence has been put forward. 



Tlie cornea is usually small, tliin and liighly arched but becomes 

 large and prominently globular in predators, particularly those of 

 nocturnal habit ; in diving birds it is relatively flat and tliick. In 

 these a zone around the limbus becomes thickened and opaque, resem- 

 bling the sclera, while the scleral ossicles are particularly heavy to stiffen 

 the globe against the shock of immersion (as in the cormorant, 

 Phalacrocorax). In structure it conforms to the usual vertebrate plan. 



The anterior chamber of certain owls {Strix (Syrnium) aluco) contains a 

 slimy, highly visc<ius, mucinovis substance of a mucopolysaccharide (hyaluronic 



mm^ 





^y^yy^/ fi!^Lmmiwi.^a»,i ^mm v^,T\p^<m^^^ 



Fig. 498. — The Posteriok Segment of the Globe of the Chicken. 

 r, retina ; cJi. fhoroid ; s, scleral cartilage ; sc, sclei'a ( X 80) (Norman A.shton). 



acid) nature ; it is most concentrated (or more highly polymerized) close to 

 the cornea ami is perhaps secreted by the corneal endothelium (Abelsdorff and 

 Wessely, 1909 ; Barany et al., 1957). It .should be noted that the anterior 

 chamber of the owl's eye is relatively enorinous and it may be that this material 

 allows the fluid in the anterior joart to remain almost stagnant to decrease 

 the turnover that would be necessary were the exceptionally large amount of 

 aqueous to be renewed at the average rate. 



The uveal tract has several peculiarities. The choroid is tliick, 

 particularly posteriorly, often especially so in the region of the macular 

 area (Fig. 498). The lamina fusca lies directly on the scleral cartilage. 

 Immediately external to the choriocapillaris there lies a stratum of 

 feeding arteries, outside which is a thick layer of venous sinusoidal 

 spaces traversed by radial cords of smooth (the heron, Ardea) or 

 striated (the cross-bill, Loxia) muscle fibres and connective tissue of 

 ry variable distiibution. These muscular cords, originally described 

 Wittich n855), Pagenstecher (I860) and H. Midler (1861). 



