4«S 



THE BIRDS 



xvi. 29- 



stroboscopic action increases the number of on-and-off effects pro- 

 duced by a small object in the visual field, increasing contrast and 

 allowing detection of its movement. This, however, is only one of the 

 numerous suggestions about the function of the pecten, and the only 

 real support for the idea is that the body is large and much pleated in 

 predatory birds, which detect minute movements at great distances, 



Fig. 299. Tracing of the shadow of the pecten on the retina 

 in various birds. (After Pumphrey and Menner.) 



and is small and smooth in nocturnal birds (Fig. 299). However, it is 

 almost certain that the original function of the pecten was to bring 

 nourishment to the vitreous and retina. It has often been suggested 

 that the pecten is in some way connected with accommodation; it is 

 not likely that it actually assists in focusing, for instance, by pressing 

 forward the lens, and no changes have been seen in it during accom- 

 modation. However, it might possibly assist by adjusting the intra- 

 ocular pressure, which must be increased by the extensive changes in 

 the lens during accommodation. 



30. The ear of birds 



Both vestibular and auditory parts of the ear are well developed in 

 birds. The former are not known to possess special peculiarities, but 

 the large connexions with the cerebellum suggest great importance in 

 the operations of flight, presumably especially by the semicircular 

 canals. There is a distinct cochlea, slightly curved and especially well 



