The Senses and the Nervous System 107 



day fishes. This is a hole in the top of their heads which 

 leads by a passage to the mouth. It is an accessory of their 

 breathing-system, and has nothing to do with their hearing. 

 In the course of evolution, however, it has become part of 

 the auditory apparatus, for the higher animals have made 

 from it the cavity of the middle ear, and the passage called 

 the Eustachian tube which leads from the inside of the ear- 

 drum to an opening in the throat. The business of this tube 

 is to keep the pressure on the two sides of the ear-drum the 

 same. Its lower end, in the throat, is closed except during 

 the act of swallowing. When your ear-drums feel funny 

 going through the Holland Tunnel, it is because atmospheric 

 pressure under the Hudson River is greater than the atmos- 

 pheric pressure on New York streets. The former is on the 

 outside of your ear-drum 5 the latter, brought in by you, is 

 on the inside J and the ear-drum has to keep the two apart. 

 When you swallow, the Hudson River atmosphere enters 

 the Eustachian tube, the pressure on the two sides of the 

 drum is equalized, and you are a happier and healthier 

 man than if the shark had no spiracle. 



And while we are on this trend of thought, it is of note 

 that the little bones called the hammer, anvil, and stirrup 

 which transmit the vibrations from our ear-drums to our 

 inner ears are modifications of three bones at the rear end of 

 the fish's jaw. This is one of the more recent discoveries of 

 science, and it explains why the fish has no hammer, anvil, 

 and stirrup. He is still using them to eat with. 



EQUILIBRIUM 



In the equilibrium of the fish, the sacculus and lagena 

 play no part. The German investigators mentioned in the 

 preceding section found that removal of these destroyed true 

 hearing but left equilibrium unimpaired 3 whereas hearing 



