300 



THE EYE IN EVOLUTION 



meshed net is spread in which the capillaries are associated with the 

 AT^eins leaving a zone free of small vessels around the arteries. 



An exception of more than usual interest is seen in the ee\,Anguilla. 

 This fish is unique in having no demonstrable choroid, for the large 

 cells of the retinal pigment epithelium lie directly on the sclera, and 

 as if in compensation the vessels of the membrana vasculosa vascularize 

 the retina directly (Fig. 334) (W. Krause, 1876 ; Virchow, 1882 ; 

 Denissenko, 1882 ; Michaelson, 1954). The vessels of this membrane 



Figs. 332 and 333. — The Membrana Vasculosa Retinae of Teleosteans. 



Fig. 332. — In the goby fish, Gobius 

 poecilichthys. The vessels emanate 

 from the central artery of the retina 

 and run over this tissue within the 

 vitreous. The division of the prin- 

 cipal vessel into the annular vein is 

 seen on the nasal side, at V (after 

 Karsten). 



Fig. 333. — Sketch of injected retina of the 

 roach, Eutihis, .showing the concentra- 

 tion of capillaries around the vein (to the 

 left) while the peri-arterial zone (to the 

 right) is relatively free from capillaries 

 ( X 23) (I. C. Michaelson). 



derive from a large central artery entering the eye, as is usual, through 

 the optic disc and its branches form an arterial network in the vitreous 

 lying on the surface of the retina and extending to the periphery of 

 the fundus where they form capillary loops. From this arterial network 

 numerous branches pass from the vitreous through the internal 

 limiting membrane into the retina : Virchow (1882) estimated that 

 there were 9,600 of them. In the substance of the retina they divide 

 into two strata of capillaries, one in the inner and one in the outer 

 nuclear layer, and from these retinal capillary nets blood is drained 

 by large veins which combine to form four main vessels and eventually 

 join to fdrni a central vein in the optic nerve head (Figs. 334 and 335). 

 The abs; iice of a choroid in this fish is unique and the direct vasculariza- 

 tion oft inner retinal layers constitutes the only known exception 



