THE AVIAN RETINA 661 



number of parallel threads, which terminate in an infinity of tiny trump- 

 ets to compose the tile-work of the internal limiting membrane. 



The diurnal majority of birds have great numbers of single and double 

 cones (their oil-droplets of divers colors, as in turtles) , and relatively few 

 rods. The rods may be restricted to the periphery or may even be entirely 

 lacking in some instances. In nocturnal birds the rods predominate, 

 though there may be large numbers of cones as well, some of them with 

 pallid, though definitely pigmented, oil-droplets. The rod and cone 

 nuclei and foot-pieces are of the same, 'cone', type in diurnal birds, but 

 are differentiated in many or all nocturnal birds, as they are in other 

 duplex vertebrates whose rods are numerous and very slender (teleosts, 

 mammals) . The rods of all birds contain rhodopsin. 



These visual-cell types are pictured in Figure 193b (in their plump, 

 easily-studied peripheral versions — compare Fig. 22e, p. 54) . The avian 

 cones are the same elements, phylogenetically, as their opposite numbers 

 in the reptiles and the lower mammals (see Plate I). The bird rod has 

 a paraboloid like those of chelonian and crocodilian rods, though it may 

 be difficult to make out in the slenderized rods of the fundus, where it 

 appears to form a long, slender tube. The rod is clearly comparable with 

 the rod of the turtle, that of the alligator, and the cone of Sphenodon 

 (see Figs. 176b, 177a, 179; pp. 612, 615, 621) ; but it has become a rod 

 independently in the birds (or perhaps in their immediate ancestors — 

 see Plate I), and is fully differentiated in the morphological sense only 

 in nocturnal birds (v.s.). 



The proverbial resolving power of the bird eye is based partly upon 

 its large size and the relatively large image cast upon the retina, partly 

 upon the dense concentration of the cones and the high ratio of optic- 

 nerve fibers to visual cells. In the little white wagtail (Motacilla alba), 

 outside the foveal region, Franz found approximately 120,000 visual 

 cells and 100,000 ganglion cells per square millimeter of retina (compare 

 the human fovea: 200,000—200,000). In an owl (Bubo bubo), with its 

 relatively great summation, the corresponding figures were 56,000 and 

 3,600 (compare the overall summation-ratio of the human retina: ca. 

 125:1). In the fovea, even such birds as little Passer domesticus have 

 400,000 or more cones per square millimeter — and each cone presumably 

 has its own bipolar and ganglion cells. The grand champion of all 

 foveae is perhaps that of a hawk (Buteo buteo) , in which Rochon-Du- 

 vigneaud found 1 ,000,000 cones per square millimeter. Even outside the 



