THE CROCODILIAN RETINA 



615 



The Retina — The crocodilian retina is strongly 'nocturnal' in organ- 

 ization, and seems to have long ago lost any need for nutritive provisions 

 other than the chorioid. At any rate, the only traces of a former conus 

 papillaris (if indeed they are such remnants!) are a glial pad on the 

 adult disc, which contains a capillary or two but scarcely protrudes 

 toward the vitreous at all, and a superficial dusting thereof with melanin 

 granules. The following remarks apply to the alligator: 



The pigment epithelium is highly modified, in the superior half of the 

 retinal cup, forming a guanin tapetum lucidum (q.v.). Toward the 



Fig. 177 — Representative visual cells of a crocodilian, Alligator mississippiensis. xlOOO. 



a, single cone, double cone, and rod (the cones from the ventral fundus; the rod frotn the 

 region of the tapetum lucidum). 



b, single and double, partially-transmuted cones from opposite the center of the taf>etum. 



ventral border of the tapetalized area there is a horizontally-elongate 

 area centralis, from which no attempt seems to have been made to elim- 

 inate the rods, though all the visual cells have here been slenderized 

 and aggregated. 



The horizontal cells are not quite as heavy-fibered as those of turtles; 

 but the Miiller fibers are particularly numerous and conspicuous. The 

 extent of summation may be gathered from the fact that there are one 

 to one and a half rows of outer nuclei, four to five rows of inner ones, 

 and a single scattered row of ganglion-cell nuclei. 



