HEARING IN FISH 47 



consists of a very friable membrane composed of fibres running in a, 

 criss-cross fashion, \vliile the inner wall, which is only loosely 

 adherent to the outer wall, can be rejuoved with its contained 

 gases without interfering with the attachments of the ossicles. 

 The membrana tympani of man has a very similar fibrous structure 

 to that of the external coat. Other minute details might be des- 

 cribed, and these can be studied in the more teclinical papers that 

 have appeared in the Proceedings of the Royal Society and the 

 Transactions of the Royal Society of Medicine (Evans). 



Before we leave the subject of the accessory organ of hearing 

 in carps, it wiU be of great interest to study the modifications of 

 the swim-bladder and Weberian ossicles in a tropical air-breathing 

 fish. We were very fortunate to be given by Mr. Burne, the physio- 

 logical ciu-ator of the Royal College of Surgeons, a specimen of the 

 electric eel, which belongs to a family closely related to the carps. 

 The diagram (Plate 9) of its swim-bladder and the auditory con- 

 nections will enable the reader to follow these modifications, and to 

 compare them with the corresponding organs in a carp. The most 

 striking fact is the very large size of the posterior sac, or buoyancy 

 tank, which has a volume ten times larger than an average sized 

 carp. If the pneumatic duct is traced back to the gullet it will be 

 found to enter an ovoid chamber before it actually enters the 

 oesophagus. This chamber is the air-pump, which pumps the 

 swallowed air into the sac. From this chamber there also passes 

 a fine duct that runs forwards and enters a small air-vesicle which 

 lies in the anterior part of the abdomen. From the front of the air- 

 vesicle the small bones, or Weberian ossicles, pass forwards to end 

 in the duct that communicates with the internal ear or saccule. 



The most striking point in this arrangement is the complete 

 separation of the hydrostatic sac from the auditory vesicle which 

 makes the similarity of the duct leading to the air vesicle, to the 

 Eustachian tube of man, very convincing. 



In the course of our dissection of this fish other points were 

 observed of interest which hitherto had not been completely investi- 

 gated. The famous surgeon and scientist, John Hunter, noticed 

 and described a number of fohate projections in the floor and roof 

 of the mouth ; he did not, however, recognise the function of these 

 outgrowths. W^hen these foliate papillse were examined micros- 

 copically, it was found that they had a superficial layer of air spaces 

 like the alveoli of a lung, and there can be no doubt that, when the 

 mud flats are parched by the sun, the electric eel is able to obtain 

 air by this accessory mouth-breathing organ. 



It is not necessary to go into the details of the connections of 



