THE RETINA. 739 



network in all the layers, holes being left for the nervous portions (Sg). The inner segments 

 of the rods and cones are also surrounded by a sustentacular substance. As the retina passes 

 forward to the ora serrata, it becomes thinner and thinner, gradually becoming richer in con- 

 nective-tissue elements and poorer in nerve elements, until, in the ciliary part, only the cylin- 

 drical cells remain (fig. 519). 



[Macula Lutea and Fovea Centralis. There are no rods in the fovea, while the cones are 

 longer and narrower than in the other parts of the retina (fig. 521). The other layers also are 

 thinner, especially at the macula lutea, but they become thicker towards the margins of the 

 fovea, where the ganglionic layer consists of several rows of bipolar cells. The yellow tint is 

 due to pigment lying between the layers composing the yellow spot.] 



The blood-vessels of the retina lie in the inner layers near the inner granular layer. Only 

 near the entrance of the optic nerve are they connected by fine branches with the choroidal 

 vessels ; they are surrounded by perivascular lymph-spaces. The greatest number of capillaries 

 runs in the layers external to the inner granular layer {Hesse). The fovea centralis is devoid 

 of blood-vessels (Nettleship, Becker). Except in mammals, the eel (Denissenko), and some 

 tortoises (H. Miiller), the retina receives no blood-vessels. Destruction of the retina is followed 

 by blindness. 



[Retinal Epithelium. The single layer of pigmentary cells containing granules of melanin 

 sends processes downwards, like the hairs of a brush, between the rods and cones ( 398). 

 Kuhne has shown that the nature and amount of light influence the condition of these processes 



Fig. 521. 



Section of the fovea centralis, a, cones ; b and g, int. and ext. limit, memb. ; e, ext, and e, 

 nuclear layer ; d, fibres ; /, nerve-cells. 



(fig. 563). The protoplasm of these cells in a frog kept for several hours in the dark, is 

 retracted, and the pigment-granules lie chiefly in the body of the cell and in the processes near 

 the cell. In a frog kept in bright daylight, the processes loaded with pigment penetrate down- 

 wards between the rods and cones as far as the external limiting membrane.] 



Each rod and cone consists of an outer and an inner segment. During life, the outer 

 segment contains a reddish pigment or the visual purple (Boll). 



Visual purple [or rhodopsin] may be preserved by keeping the eye in darkness ; but it is 

 soon bleached by daylight, while it is again restored when the eye is placed in darkness. It 

 can be extracted from the retina by means of a 2*5 per cent, solution of the bile acids, especially 

 from eyes that have been kept in 10 per cent, solution of common salt (Ayres). The rods are 

 0*04 to 0*06 mm. high and 0*0016 to 0*0018 mm. broad, and exhibit longitudinal striation, 

 produced by the presence of fine grooves ; a fine fibril runs in their interior (Bitter) . The 

 external segment occasionally cleaves transversely into a number of fine transparent discs. [It 

 is a very resistant structure, and in this respect resembles neuro-keratin.] Krause found an 

 ellipsoidal body, the "rod ellipsoid," at the junction of the inner and outer segments of the 

 rods. The cones are devoid of visual purple, but their outer segment is striated longitudinally, 

 and it also readily breaks across into thin discs. Only cones are present in the macula lutea. 

 In the neighbourhood of the yellow spot, each cone is surrounded by a ring of rods. The cones 

 become less numerous towards the periphery of the retina. In nocturnal animals, such as the 

 owl and bat, there are either no cones or imperfect ones. The retinae of birds contain many 

 cones, that of the tortoise only cones. The rods and cones rest on the sieve-like perforated 

 external limiting membrane (Le). Both send processes through the membrane, the cones to 

 the larger and higher-placed nuclei, the rods to the nuclei, with transverse markings in the 

 external nuclear layer. [The cones are particularly large in some fishes, e.g., the cod, while 



