304 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
characteristic crescentic curvature — a modification which is often associated with the 
more or less complete atrophy of the fibres that normally converge to this ossicle from 
the lateral walls of the air-bladder, and even when these fibres are present the loss of 
curvature may be due to the fact that reduction in the size of the bladder is not 
accompanied by a corresponding diminution in the size of the tripus itself, which, 
therefore, receives the insertion of fewer fibres than in most Siluridse normales. The 
scaphium as frequently retains but a vestige or loses all trace of its ascending 
process, and, as the condylar process may also disappear, the ossicle often becomes 
reduced to the condition of a simple concavo-convex structure in relation with the 
external atrial aperture. 
As regards the effects of degeneration of the air-bladder on the structure of those 
portions of the internal ear which are specially related to the Weberian ossicles, we 
are satisfied that in, at least, some Siluridse abnormales the sinus endolymphaticus has 
completely disappeared, although the ductus endolymphaticus, the cavum sinus imparls, 
and the atrial cavities remain, and retain also their normal relations to one another 
and to the scaphia. The absence of a sinus endolymphaticus in several genera with 
rudimentary air-bladders is at least suggestive of the probability that it is also wanting 
in other Siluroids in which the air-bladder has undergone a like degeneration ; on the 
other hand, we entertain no doubt as to the presence of an apparently functional sinus 
in a few Siluridse abnormales. Physiologically, the suppression of the sinus endolym- 
phaticus must prove a serious obstacle to the transmission of afferent impulses 
generated by varying external pressures to the sensory epithelium of the sacculi, even 
supposing that the air-bladder and Weberian ossicles still retained their normal 
structure and mutual relations ; but that it is a fatal obstacle cannot, in our opinion, 
be so readily affirmed, for movements in the fluids of the atrial cavities and cavum 
sinus imparls might still impinge on that portion of the ductus endolymphaticus which 
is in relation with the anterior aperture of the cavum. 
Even if the diminutive air-bladder of the Siluridae abnormales retained its structural 
integrity in other respects, the almost universal atrophy of the lateral chambers would 
certainly have the effect of rendering the bladder far less delicate as a register of 
varying hydrostatic pressures than it is in the normal section of the family, but taken 
in conjunction with such other retrogressive changes both in the air-bladder and 
Weberian mechanism as those above mentioned, it becomes almost impossible to 
believe that any function of the kind suggested can be assigned to these structures, 
or that they do otherwise than present various stages of retrogressive modification 
towards the condition of vestigial and functionless organs ; and this conclusion 
seems to us equally inevitable whatever may have been their original function — 
whether acoustic, barometrical, or hydrostatic. Considering the high degree of 
specialisation which characterises the air-bladder and Weberian mechanism in the 
Slluridre, it is by no means surprising that these structures should be liable to rapid 
degeneration when from any cause the necessity for the exercise of their special 
