478 



THE EYE IN EVOLUTION 



arteries through the circle of Zinn (Wybar, 1956), but in the lower 

 Mammals the tendency is for the posterior ciliary arteries to assume 

 greater imf)ortance. It is to be remembered, however, that the 

 appearance of arteries emerging from the optic nerve-head onto the 

 retina in a marginal position around the disc, a formation suggestive of 



Figs. 631 to 634. — Types of Placental Retinal Vascularization 

 (See also Plates XIV and XV.) 



iio. bol. — The FuNDL-i wi jiiL Dot.. 

 The lightly coloured area is the tapetum. 

 There is a venous circle at the disc. 



Fig. G3_'. — The Fundus of the Cat. 

 The lightly coloured area is the tapetum. 



Fig. 633. — The Fundus of the Rabbit. 



The ves.sels are confined to the leashes of 

 opaque nerve fibres. 



Fig. 634. — The Fundus of the Horse. 



The lightly coloured area above the disc 

 is the tapetum. 



a ciliary origin, does not jDreclude their derivation from a central artery 

 that has broken up into retinal branches in the substance of the 

 nerve. 



Among the Placentals almost every possible variety of retinal 

 vascularization occurs, ranging from a complete absence of vessels, in 

 whicl ise the retina is nourished entirely from the choriocapillaris, to 

 an elu' ate system covering the entire retina in which the capillaries 



