480 



THE EYE IN EVOLUTION 



(c) PAURANGiOTic {nocvpog, small) (Plate XV ; Fig. 634). The 

 vessels are very minute and extend only a short distance from the disc. 

 This occurs in Perissodactyla (horse, tapir, rhinoceros which has only 

 capillaries around the disc), the elephant, the Hyracoidea, the Sirenia 

 {Manatus, Halicore) and among the Rodents in the guinea-pig {Cavia 

 porcinus) (Fig. 636). 



(d) ANANGiOTic (a, privative) (Plate XV). The retina is without 

 vessels. This group comprises the more primitive Mammals and 

 includes most of the Chiroptera (Ijats), the Xenarthra (sloths and 

 armadillos), and certain Rodents (the porcupine, Hysfrix, the chin- 

 chilla, the beaver, Castor, and otliers). Many of these anangiotic 



Fig. 636. — Retinal Vessels of the Horse. 



A, the general arrangement of tlie retinal vessels. B, a portion of the 

 vascularized retina of the horse showing the peripheral loops, the T-shaped 

 loops between the branches of the main vessel. There are many fine vessels 

 in the optic nerve-head. Specimen injected with Indian ink (after L. Bruns). 



Fig. 637. — Retinal \ essels of the Young Rabbit. 

 •d with Indian ink, mounted in glycerine ( X 16) (I. C. Michaelson) 



