1892.] Contributions to the Anatomy of Fishes. 143 



are insuperable objections to this theory ; and, moreover, the Fish 

 could have no appreciation of the direction of the sounds conveyed 

 through this mechanism. Finally, it may be affirmed that, contrary 

 to what might be expected if so complicated a structure as the 

 Weberian mechanism is an accessory to audition, there is absolutely 

 no evidence of the existence of exceptional powers of hearing in the 

 Siluridse or any other Ostariophyseae. For these reasons we conclude 

 that the Weberian ossicles are in no way related to the function of 

 hearing, even to the subordinate and qualified extent tacitly suggested 

 by Hasse and Ramsay Wright. 



The only remaining view is that the ossicles under consideration 

 are accessory to the hydrostatic function of the air-bladder. Moreau, 

 and later, Charbonnel-Salle, have completely refuted the older theory 

 of this function, which has usually been associated with the name of 

 Borelli. Summarising the conclusions which the experimental re- 

 searches of these authors and certain other facts appear to warrant, 

 it may be affirmed for Fishes in general : 



(a.) The function of the air-bladder is to render the Fish, bulk for 

 bulk, of the same weight as the medium in which it lives. In this 

 mean condition, or plane of least effort, the Fish acquires a capacity 

 for the maximum amount of locomotion with a minimum of muscular 

 effort. 



(6.) la its movements of ascent or descent the Fish becomes exposed 

 to augmented or diminished pressure, which, in each case, varies in 

 amount according to the variable height of the superimposed column 

 of water, and this leads to an expansion or contraction of the air in 

 the air-bladder, and consequently to an increase or diminution in the 

 volume of the Fish itself, and thereby to a corresponding alteration 

 in its specific gravity, which may temporarily remove the animal from 

 its normal plane of least effort. 



(c.) The Fish has no power of varying the capacity of its air-bladder 

 by direct muscular contraction, and its readjustment to a new plane 

 of least effort results from a gradual increase or decrease in the 

 amount and volume of the air contained within the air-bladder to an 

 extent proportional to the new pressure and due to a corresponding 

 modification of the processes concerned in the secretion or absorption 

 of the contained gases. Hence, by this apparently automatic method 

 of adjustment, the Fish will find, sooner or later, and whatever may 

 be the depth of the water and the amount of external hydrostatic or 

 atmospheric pressure, a plane of least effort, where it will again possess 

 exactly the density of the water. 



(d.) That Johannes Miiller's theory of the displacement of the centre 

 of gravity upon a longitudinal axis in the case of Fishes with a two- 

 chambered air-bladder has no foundation in fact. 



(e.) That, despite the obvious advantages which an air-bladder con- 



