The Air-Bladder 131 



bladder to the earj and since the ears were known to have 

 the equilibrating sense in the semicircular canals and their 

 auxiliaries, it was thought not unreasonable to assume that 

 they have this depth sense also. 



The proponents of the hearing-mechanism theory offered 

 two points in rebuttal. First, even if the air-bladder in these 

 fish does change size with change in depth and pressure, 

 which has never been proved, does it ever become slack (like 

 the left-hand diagram)? For as long as the fish remains 

 under water there will be pressure on the bladder, and as 

 long as there is pressure on it, its walls will remain tense 

 and will be capable of receiving and transmitting sound- 

 waves. Second, of what use would such an elaborate depth- 

 measuring or pressure-registering device be to these fishes? 

 The great majority of them live in comparatively shallow 

 waters, and their opportunity to change levels is thereby 

 very much limited. For that same reason — ^because for a 

 great many of them the bottom is so near the top — they 

 might find a device which strengthened their hearing very 

 useful, especially if it enabled them to detect enemies above 

 the surface. 



It remained for the German physiologist whose experi- 

 ments were described in the section on "Hearing" (page 

 105) to settle the matter in a scientific manner. It will 

 be recalled that he worked with a minnow, a member of the 

 carp family, which is equipped with the Weberian ossicle 

 connection between the ear and the air-bladder. He had 

 found that his minnow had such an acute sense of hearing 

 that it would show response to a tuning fork at a distance 

 of 200 feet sounded so softly that a man under water (in 

 the form of one of his students who volunteered to occupy 

 the same tank with the fish) could hardly hear it. However, 

 when he operated on the minnow and removed its air- 

 bladder, its hearing became much poorer. It could still be 



