MAMMALS 477 



The Retinal Vascularization 



The hyaloid system of vessels is unique m its development in 

 Mammals (Figs. 624-630). We have already seen in Fishes that this 

 system of vessels runs along the ventral part of the globe in the oj^en 

 foetal fissure, an arrangement seen in its most fully developed form in 

 the falciform process of Teleosteans. In many Amphibians this 

 arrangement is extended to constitute a superficial membrana vasculosa 

 retinae. In the Sauropsida the hyaloid vessel on entering the eye 

 atrophies except for the formation of a conus or pecten at the disc itself. 

 In ]\Iammals the hyaloid artery in embryonic life runs directly to the 

 posterior jjortion of the tunica vasculosa lentis, while a multitude of 

 vessels ramifies in the vitreous. These vessels disappear in the later 

 stages of embryonic life, the only visible remnant being a small residuum 

 of glial tissue lying on the optic disc (Bergmeister's pajDilla). Some- 

 times this condition remains in the adult mammalian eye so that the 

 retina itself is avascular ; more usually vessels grow out from the 

 hyaloid trunk and invade to a greater or less degree the substance of 

 the retina itself ; in this event the hyaloid trunk becomes the central 

 retinal artery. With the exception of the eel and a colubrid snake, ^ 

 it is only within the class of Mammals among all Vertebrates that a 

 retina directly supplied by capillaries is found. 



The mode of entrance of the central artery varies in different 

 species. It is derived from the ciliary branch of the external ophthalmic, 

 sometimes supplemented by anastomosis with the small internal 

 ophthalmic artery.^ In some species such as the rabbit a central artery 

 accompanied by a central vein enters the optic nerve and runs upwards 

 to reach the centre of the disc ; there, just before or just after emerging, 

 it divides into nasal and temporal branches (Bruns, 1882 ; Hen- 

 derson, 1903 ; Davis, 1929). In the cat the central retinal artery 

 was found by Davis and Story (1943) to be invariably occluded and 

 vestigial, the retina being supplied by the terminal posterior ciliary 

 branches of the ciliary artery. In the dog, on the other hand, there is 

 no centra] retinal artery but several posterior ciliary vessels pierce the 

 sclera around the optic nerve-head whei^e they give off retinal branches, 

 appearing at the margin of the disc as cilio-retinal arteries. In this 

 animal a central vein is sometimes present but even when it exists it 

 immediately breaks up to leave the eye with the marginal arteries to 

 enter the subarachnoid space (Wolff and Davies, 1931). Subendothelial 

 cushions were described by Moffat (1952) in the ciliary arteries of the 

 dog, the contraction of which might act by shutting off the choroidal 

 blood supply and diverting it to the retina. In the Primates including 

 man, the central retinal branch of the ophthalmic artery supplies the 

 whole retina ajjart from small anastomoses from the posterior ciliary 



1 p. 390. 2 p_ 498 



