300 
PROFESSORS T. W. BRIDGE AXD A. C. HADDON 
the retention of perfect equilibrium at all levels within the range of their normal 
movements. 
(c.) The Extrinsic Muscles of the Air-hladder in the PimelodincB . — A function sub- 
stantially similar to that of the “ elastic spring” apparatus may, in all probability, be 
assigned to the powerful compressor muscles of the Pimelodinm. These muscles 
cannot possibly have any share in dilating the air-bladder and rarefying the contained 
gases in order to facilitate ascent, but it would certainly seem that they enable these 
j)articular Siluridae to exercise a still more effective control over its distension, 
inasmuch as the muscles are apparently able to compress the air-bladder at all times, 
although more effectively, no doubt, Avhen the latter is more or less distended. By 
the contraction of these muscles during rapid movements of ascent the tendency to 
over-distension on the part of the air-bladder will be promptly counteracted, while a 
forcible expulsion of gas through the pneumatic duct would enable the Fish to 
speedily adjust its volume and specific gravity to a new plane of least effort at the 
more superficial level. As in the case of the “ elastic spring ” apparatus of other 
Siluridre, it is of course possible, theoretically, that the special advantage of the 
compressor muscles may be realised without necessarily involving the mechanical 
liberation of gas from the air-bladder. Excessive distension may be guarded against, 
and pressure adjustment effected by compression alone, the contained gases being so 
far condensed that no harmful reduction of the specific gravity of the Fish can take 
place, and, as before, accuracy and rapidity of pressure adjustment would, by this 
method, be correlated with the further advantage of economy in gaseous secretion. 
In both series of Fishes it is extremely interesting to recall the existence of a 
special arrangement by means of which the compression of the air-bladder, either by 
the action of the “ elastic springs,” or by the contraction of special compressor 
muscles, is prevented from imparting a too violent shock to the Weberian mechanism, 
and more especially to the fluids and sensory epithelia of the internal ear. In the 
Pimelodinm the contraction of the muscle which we have named the tensor trlpodis, 
taking place coincidently with that of the compressor muscles, will effectually limit 
the violent excursions of the crescentic process of each tripus, which, otherwise, 
would certainly take place when the anterior chamber of the air-bladder is forcibly 
compressed in the way described. In those Siluridoe provided with the “ elastic 
spring ” mechanism, a precisely similar result is brought about by the existence of a 
groove at the junction of each terminal bony plate with its flexible and elastic root, 
into which the outer margin of the crescentic process of the tripus is received. 
During the compression of the anterior chamber by the “ elastic springs,” the 
outward movement of the crescentic process is controlled by the inward movement of 
the elastic sju’ing itself, which has the result of effectually checking any sudden or 
extensive excursion on the part of the tripus, and, at the same time, prevents the 
transmission of a too violent impulse to the Weberian ossicles and internal ear. 
The extreme difficulty of attempting to arrive at a satisfactory solution of the 
