EYE OF THE ENGLISH SPARROW 427 



The rods are relatively few in number in the sparrow. They 

 are much more slender than the cones (fig. 36, R). The relative 

 lengths of the inner and outer segments is about the same as in 

 the cones. Their diameter is ahnost uniform throughout their 

 length and varies from one-third to one-half the diameter of the 

 cones. The fact that the rods are more numerous than the 

 cones in night birds indicates that they are capable of function- 

 ing in a light too dim for the proper functioning of the cones. 



The rods and cones in the sparrow eye are approximately 

 parallel to the rays of light that would pass to them through the 

 center of the lens and not parallel to the radii from the center of 

 the eye. Therefore, toward the periphery of the retina the 

 rods and cones change from an arrangement perpendicular to 

 the retina to a more and more oblique position. This is espe- 

 cially noticeable near the ora serrata where the rods and cones 

 form an angle of about 28° with a perpendicular through the 

 retina. 



This arrangement enables rays of light reaching these dif- 

 ferent portions of the retina to pass through the rod and cone 

 layer parallel to them. In other words, a ray of light coming 

 from a given object would stimulate but one of the sensitive 

 elements and not pass diagonally through several. This is an 

 adaptation for distinct vision, even in the peripheral portions of 

 the retina. 



The pign:ent cells of the sparrow are thicker than those of 

 man. The pigmented processes are also longer than those of 

 mammals (Ellenberger '06). In a tangential section they 

 appear as rather regular hexagonal bodies arranged in a honey- 

 comb-like manner (fig. 37 B). In birds killed in the light the 

 cell bodies contain little pigment. This fact was demonstrated 

 long ago in other vertebrates by Angelucci ('78). The nucleus 

 is rather conspicuous and is surrounded by a slightly pigmented 

 cytoplasm. The average diameter is .0145 mm. The arrange- 

 ment is so definite that each cell is in contact with and sur- 

 rounded by six adjacent ones. A section vertical to the retina 

 of a bird killed in the light shows that the pigment of the cells 

 has migrated in long streamer-like processes between the rods 



