608 



THE EYE IN EVOLUTION 



Didelphys 



animal's near-point (10 to 20 feet in emmetropic animals with little 

 accommodation) which gives rise to the striking " eye -shine " in 

 suitable optical conditions so familiar in the cat family. ^ The peculiar 

 hue often associated with a beautiful iridescent effect is due to inter- 

 ference phenomena depending on the size and stratification of the 

 reflective elements of the tapetum and irregularities on its surface 

 (Briicke, 1845 ; Hess, 1912 ; Roggenbau, 1928), an effect which may 

 perhaps be heightened by fluorescence (Hosoya, 1929).^ 



To be effective in this way the tapetum must lie behind the 

 receptor elements. Two sites have been utilized, either the pigmentary 

 epithelium of the retina or the choroid, and in both cases the pigment 

 normally found in the former must be eliminated or reduced to 

 insigniflcant proportions to allow the light to traverse it. Both the 

 utilization of diffusely reflected light and the elimination of pigment, 

 of course, militate against acuity of vision in bright illumination ; it 

 is, therefore, interesting and significant that although a tapetum 

 frequently occurs with an area centralis, it is never found in an eye 

 with a true fovea. Whereas in the purely nocturnal eye a static 

 tapetum is thus an effective visual aid, an occlusible tapetum wherein 

 the mirror can be used in dim light and obscured in bright light 

 is a much more efficient and plastic mechanism for the arhythmic 

 eye.^ 



(i) RETINAL TAPETA. The pigmentary epithelium of the retina is 

 converted into a mirror by a packing of the cells of this layer with 

 guanine (in Teleosts and Crocodilians) or some related substance (in 

 Mammals). Guanine is a purine related to uric acid and, either in the 

 pure form or as the calcium salt, is deposited as highly reflecting 

 crystals. We have already studied its effect as a silver-coated mirror 

 in the scales of fishes in determining the colour of their integument ; 

 deposited in the retinal epithelium a similar reflection of incident light 

 is attained. In a tapetum of the static (non-occlusible) type the 

 epithelial cells are filled with reflecting crystals and the fuscin is reduced 

 to a minimal amount. Such a tapetum is rare, being found among 

 some abyssal Teleosts {Evermanella, etc.), in Crocodilians, and, among 

 Mammals, in the larger bats (Megachiroptera) (Fig. 581) and (occupy- 

 ing the entire upper half of the fundus) in the Virginian opossum, 

 Didelphys virginiana. Some reflecting crystals are also seen in the 

 retinal pigmentary epithelium of the dog which probably aid the 

 reflection of light by the underlying choroidal tapetum. 



(ii) CHOROIDAL TAPETA (of the non-occlusiblc type) may be of 

 three types : 



^ For this reason the ancient Egyptians worshipped the cat. the eyes of which 

 magically reflected the light of the Sun-god even at night. 

 ^ Cf., the diffractive coloration of the integument, p. 89. 

 3 p. 612. 



