482 THE EYE IN EVOLUTION 



superficial. In most cases the superficial net is formed by the end- 

 branches of the arterioles which do not reach the deeper net ; the two 

 nets, however, intercommunicate freely by perpendicular or oblique 

 capillary vessels, while the latter drains into the retinal veins and in all 

 cases there is a zone free from capillaries around the arteries (His, 

 1880 ; Bruns, 1882). In some species of Rodents, however, members 

 of the family Gliridse (dormice) such as Glis and Eliomys, and the 

 flying squirrel {Pteromys), the capillaries penetrate more deeply, 

 reaching to the outer nuclear layer to supply the bodies of the visual 

 cells and are not reflected until thej^ approach the external limiting 

 membrane (Kolnier, 1929 ; Rochon-Duvigneaud, 1943) ; in these 

 animals it is interesting that the choroid is unusually thin. 



It may be useful at this point to summarize the vascularization of the 

 vertebrate retina. The retina is avascular, novirished indirectly from the choroid 

 in Cyclostomes, Selachians, the coelacanth, Chondrosteans, Urodeles, Sphenodon, 

 Chelonians, Monotremes, Marsupials (except Macropodida?, Petaurus and 

 Didelphys), as well as anangiotic Placentals. This source may be supplemented 

 by a specific structure — a falciform process in most Teleosts (except eels, Cypri- 

 noids and goby-fish) and Holosteans ; a conus occurs in lizards and the kiwi 

 (rudimentary in C'rocodilians and the Macrojaodidte) ; a pecten in Birds (except 

 the kiwi). 



Direct vascularization occurs by means of a membrana vasculosa retinae 

 in a few Teleosteans (certain eels, Cyprinoids and goby-fishes), Dipnoi, 

 Polypterini, Anurans and Ophidians : in the eel and in Tarbophis the vessels 

 penetrate into the retinal substance. Retinal vessels occur only in some 

 Marsupials [Petaurus and Didelphys) and most Placentals. 



The 'placental retina is of the ordinary vertebrate type with none 

 of the specific peculiarities so frequently evident in other species (Fig. 

 639).^ In its general architecture it does not show the same density or 

 purity of lamination as is seen in Birds ; these features are most fully 

 developed in some of the more active diurnal Rodents (the squirrel, 

 Sciurus ; the prairie-dog, Cynom.ys). The visual elements in most 

 species are duplex, the rods outnumbering the cones ; the cones are 

 always single and are of simple construction, without oil-droplets or 

 paraboloids (Figs. 266-7). In some of the lowest nocturnal forms rods 

 alone are present (among Insectivores in the hedgehog and the 

 shrew ; in the Chiroptera ; among Xenarthra in the armadillo ; 

 and among Primates in the small nocturnal lemuroids, such as the 

 galago and the loris, and in Tarsius and Nyctipithecus). The noc- 

 turnal Rodents have frequently been said to have a pure-rod retina, 



1 For descriptive anatomy, see H. Muller (1856), SchuUze (1866-71), Schiefferdecker 

 (1886), Dogiel (1888), Chievitz (1891), Cajal (1894), Krause (1895), Greeff (1900), Ziirn 

 (1902), Detwiler (1924-49), Woollard (1925-27), Uyama (1934) (cat), Kolmer and 

 Lauber (1936) (all classes), Parry (1953) (dog), Vonwiller (1954) (ox), and others. For 

 the rih ;a-structure of the rods of the guinea-pig, see Sjostrand (1949-53), of the rabbit, 

 see d. ■;obertis (1956), of tlie synapses of the visual cells see de Robertis and Franchi 



(1956; 



