EYE OF THE ENGLISH SPARROW 429 



Since the pigment of these cells is capable of such great migra- 

 tion the dirnension of the cell in this radial direction depends 

 upon the condition or the amount of light entering the eye. 

 They may vary in length from .0071 mm. in the dark to .0300 

 mm. m bright light. These pigment processes are arranged 

 parallel to the rods and cones in all portions of the retina. 



The combined thickness of the rod and cone and pigment 

 layers of the sparrow is greater than that of man, the ratio 

 being about 4 to 3. 



The entrance of the optic nerve in birds is unique and requires 

 a more detailed description than was given under the discussion 

 of the nerve supply. The external appearance of the optic 

 nerve as it enters the eye of the sparrow is seen in figure 23. I is 

 elliptical in shape and pierces the sclera about .7 mm. behind 

 and .81 mm. below the center of the eyeball as indicated by the 

 fovea. A cross-section of the nerve a short distance from the 

 eye is circular. Toward the eye it becomes more and more 

 flattened until at its entrance into the eye its diameters are 

 about as 1 to 2. The greater diameter is arranged obliquely in 

 a downward and forward direction. Immediately after pierc- 

 ing the sclera the flattening continues until it spreads out over a 

 wide area. This is especially noticeable after the removal of 

 the sclera (fig. 16). The main part of this expansion lies just 

 inside the scleral coat and displaces the chorioid and retina in 

 its course. In its downward and forward course it becomes 

 smaller and smaller by the distribution of the fibers over the 

 retina. It finally terminates a short distance from the ora ser- 

 rata (figs. 14 and 15, plate 3, and 16 to 21, plate 4). The total 

 distance from the central entrance to its peripheral termination 

 is approximately 2.7 mm. The fundus view shows but little of 

 the nerve entrance (fig. 1, plate 1). The pecten covers all of 

 the optic disc except a small oval part at the central end. When 

 the pecten is removed the optic disc appears as an elongated 

 area, widest at the central end and gradually tapering in its 

 downward and forward course to a point at its termination 

 near the ora serrata. 



THE JOURNAL OF MORPHOLOGY, VOL. 31, NO. 3 



