ON THE ANATOMY OF FISHES. 
297 
influence of diminished hydrostatic pressure, althougli it is possible that the rate of 
overflow may in some way be regulated. In some few Siluridm, however, there does 
seem to exist a special mechanism by which, under certain conditions, the air-bladder 
may be subjected to considerable compression, and the air which it contains either 
forcibly expelled, or greatly reduced in volume by condensation. This mechanism 
presents two important modifications, viz., the “elastic spring apparatus” and the 
powerful extrinsic muscles of the Pimelodinae. In the case of the “ elastic spring 
apparatus ” it would seem that by the contraction of its powerful protractor muscle each 
spring can be drawn forwards towards the head, while the subsequent relaxation of the 
muscle will at once enable the spring to exert its full pressure on the air-bladder. 
Johannes Mullee regarded this apparatus as a mechanism through which the 
condensation or rarefaction of the gases in the air-bladder became a motive force in 
causing the Fish to rise or fall in the water. On this view it is presumed that the 
contraction of the protractor muscles will pull forwards the anterior wall of the air- 
bladder to which the elastic spi'ings are attached, and by that means lead to the 
expansion of the bladder and the rarefaction of its contained gases ; while, on the 
contrary, the relaxation of the muscles will at once allow the springs, by their own 
elasticity, to compress the air-bladder, condense the gases within it, and consequently 
Increase the specific gravity of the Fish. As Mullee remarks, condensation and rare- 
faction are placed under the action of two powerful and opposite forces, in such a 
manner that condensation results from the elasticity of a recoil, while rarefaction 
depends upon the action and persistence of an essentially muscular force which 
annihilates the recoil. 
We cannot quite agree with Mullee that the elastic springs can have any share 
in dilating the air-bladder, or rarefying the gases which it contains. The bony plates 
in which the two springs terminate distally are only applied to the anterior wall of 
the air-bladder, and the fibres of the latter are not directly attached to, or inserted 
into, the plates at their points of contact, and it is, therefore, diflScult to see how the 
forward movement of each spring can possibly draw the antero-lateral wall with it, 
or rarefy the contained gases by increasing the internal capacity of the air-bladder. 
It is quite true that the transverse membrane which is so closely applied to the 
anterior wall of the bladder is also continuous with the inner (or anterior) and ventral 
margins of the terminal long plates, but we have never been able to detect any 
connection between the latter and the component fibres of the tunica externa of the 
antero-lateral reofions of the bladder itself. But even if it be true that the antero- 
lateral walls can be drawn forwards in the way which Mullee’s theory requires, it 
still seems to us extremely doubtful that any increase of internal capacity would 
necessarily result, inasmuch as any outward bulging in the antero-lateral regions 
would almost certainly be attended by an inward bulging elsewhere. The mobility 
and elasticity of the transverse process which forms each spring will certainly give to 
the lateral portions of the anterior wall that capacity for sharing in the distension of 
MDCCCXCIir. — B. 2 Q 
