EYE OF THE ENGLISH SPARROW 423 



From experiments on man we must conclude that distinct 

 vision is confined to the area in the bottom of the fovea and that 

 the clearness of detail grows rapidly less toward the ora serrata. 

 Applying these facts to the sparrow, we find that the surface 

 which would function in sharp vision is an ellipse .016 mm. long 

 and .008 mm. wide. This is equivalent to an area of .0001 sq. 

 mm. Rays of light passing through the focal center of the lens 

 to the margins of this area would form an angle of .23 degrees in 

 a horizontal plane and .115 degrees in a vertical plane. These 

 angles would give as a visual field at a distance of one meter an 

 ellipse 4 mm. broad and 2 mm. high. This is equivalent to 

 6.28 sq. mm. Since there are between thirty-five and forty 

 cones in this foveal area involved, it follows that each square 

 millmieter, one meter from the sparrow, is capable of stimulat- 

 ing approximately six cones. The distance at which the spar- 

 row usually selects its food may be placed at 10 cm. At this 

 distance each square millimeter would stimulate the equivalent 

 of sixty cones. Taking into consideration the one to one rela- 

 tionship advanced by Cajal, we see that the mechanism for dis- 

 tinct vision in the fovea has reached a high degree of perfection. 



There is a great variation in the thickness of the retina at 

 different places. It is thickest in the area centralis at the mar- 

 gin of the foveal depression, where it measures .3306 mm. From 

 this elliptical region it grows gradually less and less toward the 

 ora serrata, where it is thinnest and measures .1280 mm. The 

 retina at the ora serrata is thus reduced to almost one-third its 

 maximum thickness at the area centralis. 



Table 3 shows that this reduction in thickness is due to a 

 gradual thinning of all the layers of the retina. An apparent 

 exception to this general statement is seen in the greater thick- 

 ness of the ganglion cell layer at the ora serrata than at the area 

 centralis. As a matter of fact, the number of cells involved in 

 these two regions is as 1 to 50. At the ora serrata this layer is 

 only one cell deep and the cells are far apart, while at the edge 

 of the fovea they are arranged close together and it is five cells 

 deep. The structure which makes this layer so thick at the ora 

 serrata is largely supporting tissue and modified retina. 



