32 THE RIDDLE OF MIGRATION 



then the fiUngs are displaced. The crustacean 

 immediately turns itself sideways into the position 

 dictated by the magnet, i.e., the position in which 

 the iron filings press on the hairs that are normally 

 stimulated by the downward pull of gravity. With 

 an "ear" deprived of any particles whatever, the 

 animal loses all sense of stability. 



This same apparatus, but greatly elaborated, 

 exists in the vertebrate ear. It is no longer merely 

 a spherical sac but consists of a sub-divided major 

 compartment {vestibule) from which run three semi- 

 circular canals in three planes (Figs. 5B and 6) 

 Two are vertical and at right angles to each other. 

 The third is horizontal and at right angles to the 

 other two. If one of these is injured the bird w^ill 

 periodically fall over and show obvious inability to 

 walk normally. If all three are injured the bird 

 completely loses control over its movements. These 

 balancing canals, with their common chamber, are 

 the oldest part of the ear, still subserving the 

 original function of equilibrium. The canals are 

 very delicate structures both filled with liquid 

 (endolymph) and surrounded by it (perilymph) , the 

 latter in its turn contained in bony canals duplicat- 

 ing the true (membranous) canals. 



Sound waves are received from the outside by the 

 ear-drum (tympanum) and are transferred from it 



