ON THE ANATOMY OF FISHES. 
271 
air to and from the air-bladder through the ductus pneurnaticus in any Physostomi. 
Many Fishes frequently resort to the surface and swallow air, wiiicli may either act 
directly on the gills, or on accessory respiratory organs, but the somewhat prevalent 
impression that some Physostomi are capable of inhaling air through the pneumatic 
duct rests upon no satisfactory foundation and may be dismissed from consideration. 
On the other hand, there is a large body of experimental evidence for the belief 
that the air-bladder has an important, though secondary, relation to respiration by 
acting as a reservoir for the superabundance of oxygen which is taken into the blood 
through the gills, and subsequently re-absorbed from the air-bladder into the blood 
when the Fish is in water containing but little of that gas in solution.t It seems 
clear, nevertheless, that, however important this secondary I’elation of the air-bladder 
to respiration may be in Fishes in general, there can be no physiological connection 
between it and the Weberian mechanism. The function of the air-bladder as an 
oxygen reservoir is, to say the least, as characteristic of those Fishes in which a 
Weberian mechanism is absent, as it is of those in which it is present. In fact, 
Moreau’s researches go much further than this, and prove to the point of actual 
demonstration that of the Fishes with which he experimented those provided with a 
Weberian mechanism had relatively but little capacity for absorbing oxygen from the 
air-bladder, when placed under conditions likely to cause asphyxia, i.e., in water 
deprived of its oxygen, whereas many other Fishes belonging both to the Physoclist 
and Physostome divisions of the Teleostei, in which the Weberian apparatus is 
wanting, had a far greater capacity for absorbing oxygen under similar conditions 
than the former. Moreau has also shown that a marked capacity for oxygen absorp- 
tion is always associated with the presence of retia mirabilia in the walls of the air- 
bladder, and these structures, as we have already pointed out, are invariably al)sent 
in all Siluridce, and, so far as we are acquainted, in all the remaining Ostariophyseae. 
Dismissing respiration, we may next consider the much more difficult and debatable 
question whether these structures can be regarded as in any way subsidiary to the 
function of audition. 
Our knowledge of the physiology of hearing is mainly confined to the higher air- 
breathing Vertebrata, and but comparatively little is experimentally or certainly 
known at present about this function in Fishes, The conditions of subaqueous 
audition are in several respects very different from those in aii’. Not only do sound 
vibrations in water travel witli more than quadruple the ra})idity with which they are 
transmitted in air, but they can be appreciated at far greater distances. Tliere is, in 
fact, a closer analogy between water and solid bodies than between water and air, so 
far as the transmission of sound vibrations is concej’ued. Tliese facts may, to some 
extent perhaps, be physiologically illustrated by the statement that the relations of 
the auditory organ of a Fish to a sounding body in water are not very dissimilar to 
* Jobert’s experiments (20) to the contrary are, in our own opinion, not conclusive. 
t Sec Milxe-Edwards (25), and Gouriei (13a). 
