96 The Organs of Respiration 



found i to 5 per cent of CO 2 , i to 87 per cent of O 2 , and the 

 remainder nitrogen. The most remarkable fact discovered 

 about this mixture was that it frequently consisted almost 

 entirely of oxygen, the per cent of oxygen increasing with the 

 depth of the water inhabited by the fish. The reasons for this 

 phenomenon have never been satisfactorily explained. 



"In 1820 Weber described a series of paired ossicles which 

 he erroneously called stapes, malleus, and incus, and which con- 

 nected the air-bladder in certain fishes with a part of the ear 

 the atrium sinus imparis. Weber considered the swim-bladder 

 to be an organ by which sounds striking the body from the 

 outside are intensified, and these sounds are then transmitted 

 to the ear by means of the ossicles. The entire apparatus 

 would thus function as an organ of hearing. Weber's views 

 remained practically uncontested for half a century, but re- 

 cently much has been written both for and against this theory. 

 Whatever the virtues of the case may be, there is certainly an 

 inviting field for further physiological investigations regarding 

 this subject, and more especially on the phenomena of hearing 

 in fishes. 



"Twenty years later Johannes Miiller described, in certain 

 Siluroid fishes, a mechanism, the so-called ' elastic-spring ' ap- 

 paratus, attached to the anterior portion of the air-bladder, 

 which served to aid the fish in rising and sinking in the water 

 according as the muscles of this apparatus were relaxed or con- 

 tracted to a greater or lesser degree. This interpretation of the 

 function of the ' elastic-spring ' mechanism was shown by 

 Sorensen to be untenable. Muller also stated that in some fish, 

 at least, there was an exchange of gas between blood and air- 

 bladder the latter having a respiratory function and regarded 

 the gas in the air-bladder as the result of active secretion. In 

 Malapterurus (Torpedo electricus) he stated that it is a sound- 

 producing organ. 



"Hasse, in 1873, published the results of his investigations 

 on the functions of the ossicles of Weber, stating that their 

 action was that of a manometer, acquainting the animal with 

 the degree of pressure that is exerted by the gases in the air- 

 bladder against its w r alls. This pressure necessarily varies with 

 the different depths of water which the fish occupies. Hasse 



