REPTILES 



377 



1927) ; it contracts to a vertical slit which becomes narrowed to a 

 stenopoeic slit when the animal basks in the sun. The contraction 

 time is short, the dilatation time long (Laurens, 1923). The lens is 

 ellipsoidal in shape and the annular pad small ; accommodation is 

 slow and its range relatively small. 



In the alligator the retinal epithelium is modified in the upper half 

 of the fundus to form a tapetum which shines with a bright pinkish- 

 orange glow ; in a dark-adapted eye the red shimmer of rhodopsin 



Fig. 456. — The Visual Cells of Crocodilians. 



The visual cells of the American alligator, Alligator mississippiensis. 

 Reading from the left, the elements are : a single cone and a double cone from 

 the ventral fundus ; a rod ; a single cone and a double cone from the periphery 

 of the fundus opposite the centre of the tapetum lucidum ( X 1,000) (Gordon 

 Walls). 



can be seen ophthalmoscopically against the bright background rapidly 

 fading on exposure to light, a phenomenon which provided the first 

 demonstration of visual purjile in the living eye (AbelsdorfiF, 1898). 

 The retinal epithelium in the tapetal area is heavily packed with 

 guanine crystals and does not contain sufficient fuscin in the cell- 

 bodies or in their processes to occlude the mirror effect of the tapetum 

 (Kopsch, 1892 ; Laurens and Detwiler, 1921). 



The visual cells resemble those of the Chelonians except that oil- 

 droplets are lacking from the cones (Fig. 456). The rods, however, 

 greatly outnumber the cones (12 to 1 in the periphery, Verrier, 1933) 

 and in the tapetal area the cones, both single and double, tend to 

 assume a slender, more rod-like shape, forming, in Walls's (1934) view, 

 a transition stage ]iet\\een the two visual elements. Near the ventral 

 border of the tapetum there is a horizontally oval area centralis, 



