180 ADAPTATIONS TO DIURNAL ACTIVITY 



eye and brain (Fig. 105e, p. 261). In the birds, the stripe-like disc is con- 

 cealed under the base of the 'pecten', a pleated fin of pigmented, highly 

 vascular tissue which arises embryologically from the lips of the embry- 

 onic fissure and projects lens-ward through the vitreous (Fig. 80, p. 188). 

 The birds thus have only one narrow scotoma where they might have 

 had two if they had located the pecten elsewhere. 



The squirrels exhibit the most remarkable of all modifications of the 

 disc (Fig. 74). It is a stripe, oriented horizontally to interfere minimally 

 with the perception of vertical contours which are so important to an 

 arboreal animal. It has been moved far above the optic axis whereas in 

 other vertebrates it is almost invariably located close below the axis or 

 even on it, in the center of the fundus. Since it is the lower part of the 



Fig. 74 — The optic disc in various members of the squirrel family. 



(In schematized views of the fundal portions of left eyes, the anterior segments being cut 

 away; the drawings are not to the same scale). 



a, prairie-dog, Cynomys ludoyidanus (inhabits open spaces, very bright light), b, wood- 

 chuck, Marmota monax rufescens (inhabits less bright places), c, gray squirrel, Sciurus 

 carolinensis leucotis (inhabits dense woods), d, flying squirrel, Glaucomys v. volans (noc- 

 turnal, with a nearly pure-rod retina). 



retina which looks upward, vision of the sky, where the squirrel's chief 

 enemies soar, is thus left unimpeded. The stripe-like disc is so slender 

 that it bites out only tiny bits of vertical lines; and a tiny head or eye 

 movement, up or down, will move any horizontal line off the disc and 

 onto functional retina. Where the number of optic nerve fibers varies 

 from species to species, the stripe varies in length (but not in width), 

 in sympathy with the species' preference for bright light — from 7%% 

 of the diameter of the eye in the sun-worshipping ground squirrel 

 down to 30% in heavy-timber tree squirrels and even less in the Eu- 

 ropean squirrel (where also it widens somewhat) , which seeks the darkest 

 woodlands. The palm squirrel and the nocturnal flying-squirrels, as 

 might be expected, have perfectly orthodox small, circular discs located 

 just below the center of the fundus. 



