424 



JAMES ROLLIN SLONAKER 



Some of the layers of the retina as they are traced to the ora 

 serrata show a greater amount of thinnmg than the others. The 

 inner nuclear layer shows the greatest reduction. At the periph- 

 ery it has thinned to almost a sixth of its greatest thickness. 

 The nerve-fiber and outer molecular layers are represented by 

 approxhnately one-fourth; the inner molecular layer by about 

 one-third, and the outer nuclear, rod-and-cone, and pigment 

 layers by about one-half their greatest thickness. 



The reason for this reduction in thickness of the different 

 layers of the retina toward the ora serrata can be readily per- 



TABLE 3 



Showing the thickness in mm. of the layers of the adult sparrow retina at different 



regions 



Center of fovea 



Edge of fovea (0.- 

 326 mm. from 

 center of fovea) . . 



Area centralis (0.- 

 816 mm. from 

 center of fovea) . . 



Ora serrata 



a « 



Ph ^ 



z; 



0.0163 



0.0163 

 0.0040 



•A 



O K 



o 



0.0121 



0.0245 



0.0243 

 0.0360 



0.0108 



0.0653 



0.0652 

 0.0240 



0.0245 



0.1305 



0.1142 

 0.0240 



B 

 >J 

 O 



S K 

 (S « 



o 



0.0082 



0.0163 



0.0163 

 0.0040 



o 



0.0184 



0.0245 



0.0163 

 0.0120 



0.0489 



0.0245 



0.0204 

 0.0120 



0.0326 



0.0287 



0.0244 

 0.0120 



< 6, 



So 



0.1555 



0.3306 



0.2974 

 0.1280 



ceived when the number of cells is considered. If the number of 

 cells in equal areas be determmed it is noticed that the relative 

 number rapidly decreases toward the periphery of the retina. 

 This is shown in table 4. Comparison of the thickness of the 

 retina of the sparrow and of man shows a marked difference. 

 The greatest thickness in the sparrow is almost twice that of 

 man, the ratio being 1 to 1.8. This is due mainly to the great 

 thickness of the inner molecular and inner nuclear layers in the 

 bird. In fact, these two layers alone are almost equal to the 

 entire thickness of the human retina. When compared to other 

 vertebrates, we find that the retina of the bird is thickest of all; 



