328 THE BIOLOGY OF BIRDS 



people cannot shoot or look down a microscope without 

 forcibly suppressing the vision of one eye by closing it, 

 but it is possible by effort to learn to suppress the vision 

 of one eye while both are kept open. 



True binocular vision is familiarly illustrated by our- 

 selves, where two images of an object are formed, each on 

 the same half of each eye. In such a case there is what is 

 called partial decussation of the optic nerves, which requires 

 a little explanation. In all animals from fishes to man the 

 optic nerves cross one another (in a chiasma) before entering 

 the brain. In bony fishes all the fibres passing from the 

 right eye pass to the left side of the brain, and all the fibres 

 from the left eye pass to the right side of the brain. This 

 is called the decussate condition and it is unique. In other 

 cases the crossing of the optic nerves is more intricate 

 — for some of the fibres from the right eye go to the right 

 side of the brain, though most go to the left ; and some of 

 the fibres from the left eye go to the left side of the brain, 

 though most go to the right. 



§ 2. The Ear 



In contrast to most mammals, birds have no external 

 trumpet or pinna, which aids in the collection of the waves 

 of sound, and by movement in the location of their origin. 

 The absence of this structure in birds may be interpreted 

 in reference to the mobility of the head and neck and to the 

 advantage of reducing friction. The pinna is absent in 

 aquatic mammals such as whales and seals, thus reducing 

 friction ; its muscles are vestigial in man, who has a notable 

 power of moving his head about in trying to locate sounds. 

 The large ear-opening lies behind and below the eye, gene- 

 rally well hidden by featliers, and immediately surrounded 

 by bristles. It is very naked in ostrich and vultures. Just 

 within the margin of the ear-opening there is often a slightly 

 muscular fold of skin. This is represented in owls by a 

 muscular lid in front of the opening, and doubtless corre- 

 sponds rather to the ear-valve in the crocodiles than to an 



