ON THE ANATOMY OF FISHES. 
285 
Some light may possibly be thrown on this difficult question by a closer comparison 
of what is experimentally known of the methods of pressure adjustment in Fishes in 
general, and in the Ostariophysese, than has hitherto been made ; and if no satis- 
factory conclusion can be arrived at in this way, some clue at least may be gained a® 
to the direction in which the solution of the problem must be sought for in the 
future. 
Gaseous secretion and absorption are, as we have seen, highly important factors in . 
the majority of Fishes in the adjustment of the volume of gas in the air-bladder 
to varying hydrostatic pressures, and the conditions under which these pressures take 
place, have been experimentally investigated by Moreau. Of the conditions which 
determine an increase in the rate of secretion of gas into the air-bladder, the most 
important are (1) repeated abstraction of gas; (2) increase of external pressure 
(3) section of sympathetic nerve ; and (4) the presence in the wails of the bladder 
of special retia mirabilia, or “ vaso-ganglia.” The process of absorption, on the 
other hand, is not so well understood, but certain conditions undoubtedly favour 
its action ; these are (l) diminution of pressure, and (2) the existence of retia 
mirabilia. It will be obvious that the rapidity with which the amount of gas in an 
air-bladder can be augmented or diminished by secretion or absorption, is an 
important feature in this discussion, for on this will depend the capacity of the Fish 
to adjust itself to the varying pressures of different levels in ordinary locomotion ; 
but unfortunately Moreau’s experiments do not throw much light on the precise 
rate at which these processes take place. In many of his experiments, as, for 
example, in those dealing with secretion, the amount of gas previously abstracted 
was often so great that the time occupied by the process of secretion to restore 
equilibrium varied in different Fi.shes from several hours to several days, and similarly 
with absorption. No attempt has yet been made to obtain accurate measurements 
of the precise rate of secretion and absorption, under conditions involving relatively 
small and gradual variations of level and pressure. Tliree important facts appear 
however to be well established : (1) that gaseous secretion and absorption are 
relatively slow processes in all Fishes ; (2) that although retia mirabilia are not 
indispensable to these processes, there can be no doubt that both take place much 
more rapidly in Fishes that possess such structures than in those in which they are 
wanting ; and (3) that increased hydrostatic pressure accelerates the rate of secretion, 
while diminished pressure exerts a similar influence on absorption. With such data 
as these and other facts supply, we may consider more closely the methods by wliich 
in different Fislies the process of pressure adjustment is effected. 
In the case of the Physoclisti, which very generally pos.sess retia mirabilia, but no 
ductus pneumaticus, gaseous secretion and absorption must be the only means of 
* The effect of pressure on the secietion of oxygen is well illustrated by the observations of Bior (2), 
who found that the amount of this gas in the air-bladder increases in direct proportion to the depth at 
which the Fish is found, and may even amount to 87 per cent. 
