AMPHIBIANS 



341 



form an incomplete ring romid the circumference of the ora ; thence 

 branches run posteriorly to form a membrana vasculosa retina 

 lying in the vitreous on the surface of the retina, a form of vasculariza- 

 tion analogous to that seen in many Fishes i (Plate III ; Figs. 405-6) 

 (Hyrtl, 1801 : Cuignet, 1860 : Hirschberg, 1882). The capillaries of 

 this system form a close net at the posterior jDole but are few in the peri- 

 phery and are associated Avith the veins rather than the arteries. They 

 are collected by three large venous trunks, a ventral, a nasal and a tem- 

 poral, and combine to form a hyaloid vein which eventually leaves the 

 globe alongside the entering artery and drains into the ophthalmic vein. 

 It is interesting that the arteries of the anterior segment are plentifully 



Fig. 405. — Injected Membrana Vasculosa Retinae of an Adult Frog. 

 There is a capillary-free zone around the artery (A) ( X 161) (I. C. Michaelson). 



provided with pad-like valves (Grynfeltt, 1907) while in the hyaloid 

 vessels of the vascular membrane of the retina contractile cells are 

 unusually prominent (Rouget, 1873 ; Mayer, 1902) ; it is possible, 

 therefore, that there may be a switch-mechanism from one circulation 

 to the other as illumination and activity vary. 



Ophthalmoscopically the retina of the frog appears as a somewhat 

 mottled slatey-grey background over which the semi-opacpie nerve 

 fibres radiate in immense numbers uniformly from the oj^tic disc to 

 the periphery in Rana, for a relatively short distance in Bufo and 

 HyJa ; in these latter the remainder of the fundus is covered with 

 orange or golden sago-like grains. In the Pvanida? and Bufonidse the 

 optic disc is long and narrow, resembling in its appearance a white 

 caterpillar lying vertically ; in the Hylidse it is circular ; sometimes it 

 is covered by a dark grey or even black pigment (the giant toad, Bufo 



1 p. -im. 



Bufo 



Hyla 



