THE LIVING BIRD 35 



other nervous elements of the retina and it seems 

 certain that the perception of color is a function of 

 the cones alone. The rods build up images without 

 color and are, phylogenetically speaking, the older 

 element. In birds the cones are relatively limited, 

 a fact which suggests a limitation of color vision. 

 Experiments, although they present certain difficul- 

 ties, seem to support this inference. At all events, 

 it appears to be well established that birds are 

 practically blind to blues and violets but as fully 

 appreciative of the yellows as man himself. 



Two points of particular interest must be men- 

 tioned in connection with birds' eyes. One is the 

 universal presence of a structure known as the 

 pecten in the posterior chamber of the eye-ball. 

 Both its constitution and function have been vari- 

 ously interpreted. It is generally assumed to be 

 associated with a bird's exceptional powers of rapid 

 accomodation and is unknown in other eyes save 

 those of certain reptiles. The solving of the prob- 

 lem should offer an attractive, as well as profitable, 

 field to the anatomical investigator. 



The second point is the presence of more than one 

 fovea in the eyes of many birds. The mammalian 

 (as well as avian) eye is so constructed that the rods 

 and cones lie behind the other constituent parts of 

 the retina and through them the light rays must 



