THE GAS IN THE SWIM-BLADDER OF FISHES. 
127 
adjusted to the increase or decrease in external pressure that has taken place. The 
fish, therefore, makes no use of any muscles in regulating the volume of its air- 
bladder. The animal can accommodate itself only gradually to considerable changes 
in depth of water, but can live equally comfortably at varying depths, provided that 
the change has been gradual enough. Moreau’s experiments also convinced him that 
the gas is actually secreted into the air-bladder, and that there is a constant exchange 
of gas between it and the blood. In these investigations he has also noticed that 
section of the sympathetic nerve fibers supplying the walls of the air-bladder hastens 
the secreting of the gas into the empty bladder. Since then Bohr * * * § has shown that 
section of the vagus nerve causes the secretion to cease. Moreau noticed in one fish 
( Trigla ) having an air-bladder supplied with muscles that the latter served to make 
the air-bladder produce sound. 
Again, in 1885, the Weberian mechanism was brought to our attention with a 
new function attributed to it by Sagemehl,f who stated that this mechanism exists 
not for any auditory purposes nor to tell the animal at what level of the water it is 
swimming, but to indicate to the fish the variations in the atmospheric pressure. 
Sorensen tersely contrasts the views of Hasse and Sagemehl Ivy saying that “ Hasse 
considers the air-bladder with the Weberian mechanism as a manometer; Sagemehl 
regards it as a barometer.” The theory of Sagemehl has, naturally enough, met 
with little favor. Sorensen J (1895) held that there is but little evidence for attribut- 
ing to the air-bladder the function of a lung. It is to be remembered, however, 
that, according to Sorensen’s criterion, no matter what exchange of gases takes place 
between blood and air-bladder, it can not be considered an organ of respiration 
“unless its air is renewed by mechanical respiration.” 
Sorensen also refutes, from anatomical and experimental grounds, the many 
objections to Weber’s theory of the function of the ossicles. He would thus attribute 
to the air-bladder the function of hearing; indeed, in certain species, the only reason 
for the survival of the air-bladder is that “the organ is still of acoustic importance; 
that it acts as a resonator.” This idea, Sorensen states, is borne out by the anatomical 
structure found in Misgurnus and Clarices, which resembles the celebrated “Colladon 
resonator.” This author attributes to the air-bladder, with its “elastic spring” and 
various muscular mechanisms, the production of sound as its chief function. 
From the foregoing brief historical summary of the function or the swim-blad- 
der it is readily seen that these investigators have not thrown an} 7 direct light on the 
function of the gas itself which is contained in the bladder. It would, indeed, seem 
strange and contrary to general biological laws for certain gases to exist in the 
bladder and not be of any use. Why, indeed, should we find a high per cent of 
oxygen in the bladders of fish taken from 55 to 70 fathoms of water, especially as 
the gas is actively secreted § by the, fish? Such a specialized expenditure of energy 
would certainly not be accounted for on the basis of acoustic and phonation functions, 
as the normal gas composition of the surrounding medium would answer all these 
requirements. In the voyage of II. M. S. Challenger || it was found that “in the 
* Journal of Physiology, vol. 15, p. 494 et seq. 
t Journal of Anat. and Physiology, vol. 29, pp. 544 et seq.: “ Theory of Sagemehl,” by Sorensen. 
I Journal of Anat. and Physiology, vol. 29, pp. 109, 205, 390, and 518. 
§ J. S. Haldane, Science Progress, vol. 7, 1898. 
|| Challenger Reports, vol. I, pt. 1, p. 226. 
