THE AVIAN RETINA 659 



pecten but no one does anything about it. The above discussion commits 

 this same crime, but offers a reasonable working hypothesis which, it is 

 hoped, will receive a proper test at the hands of experimental biologists. 



The Retina — The precision and elaboration of retinal layering reaches 

 its peak in the birds. Scarcely a cell is out of place — i.e., in a layer 

 inappropriate to its type; and the inner nuclear and inner plexiform 

 layers are more clearly differentiated into sub-layers than in other verte- 

 brates with the possible exception of the prairie-dog. The fovea of the 

 birds is the most perfect of all foveae, and many birds have more than 

 one in each retina. 



The cells of the pigment epithelium are of the usual sauropsidan type, 

 with numbers of fine processes, each containing a chain of bacilloid 

 fuscin granules and extending as far as the inner segments of the visual 

 cells. The latter are so slender and so tightly packed, and the ratio of 

 conductive to sensory cells is so high, that all three of the nuclear layers 

 and the inner plexiform as well (but not the outer) are relatively thick. 

 The whole retina (whether diurnal or nocturnal) is thereby thickened — 

 one and one-half to two times as thick as in vertebrates in general, and 

 equalled only in some of the teleost fishes (compare Fig. 193a with Fig. 

 19, p. 43; note also Fig. 72, p. 177). Some sample nuclear-layer counts, 

 made in the general fundus (away from the influence of any fovea 

 present) , follow : Ro^,s ^p . 



Species: Outer Inner Ganglion 



nuclei nuclei cells 



Week-old chick iGallusdomesticus) 2.5 18 2.5 



Domestic pigeon (Co/m 772 ^d /m'd) 3 15 2 



Robin (Turdus migratorius) 3 28 3 



English 'sparrow^ {Passer domesticus) 3 12 2 



Flicker (Coldptes auratus) 2.5 18 2 



Marshhawk {Circus hudsonius) 4 20 3 



Red-tailed hawk {Buteo borealis) 3 17 2 



The inner nuclear layer contains the bodies of many amacrine cells of 

 several types, as well as a greater number of bipolars. The nuclei of the 

 Muller fibers are much elongated in the direction of the retina's thick- 

 ness, and form a single compact layer, within the inner nuclear layer, 

 about one-half to three-fifths of the way through its thickness from the 

 outer to the inner side. Outwardly from this line of Muller nuclei (to- 

 ward the outer nuclear layer) are the bodies of the bipolars. Inwardly 

 (toward the ganglion layer) lie those of the amacrines. 



