THE GAS-BUBBLE DISEASE OF FISH AND ITS CAUSE. 
37 
Experiments have shown that under compressed air the amount of oxygen absorbed 
by the blood increases with the pressure, according to the law of the absorption of 
gases. It may be the liberation of this oxygen and other gases under reduced pressure 
that causes death in the above cases. 
In recording experiments upon the influence of high pressures upon various 
animals, Monsieur P. Eegnard says: * 
A (ish without an air bladder, or one in which the air bladder has been emptied of gas, can be 
submitted to a pressure of 100 atmospheres, 1,470 pounds per square inch, without injury. When 
the air bladder is not emptied a very curious phenomenon is observed. Under pressure of several 
atmospheres the gas of the air bladder dissolves in the blood, and at, the moment of decompression 
it suddenly becomes disengaged in the blood vessels, forming a foam which stops all circulation, and 
the animal dies; for the same reason fish drawn from the depths of the sea are dead when they reach 
the surface. 
The bearing of this matter of pressure on geographical distribution should be 
considered. It might seem that there are no barriers to universal distribution in the 
sea save those indicated by the isotherms, but the isobars must indicate the location 
of effectual barriers for certain species. 
Since even slight changes of pressure are detrimental, and each fish swimming in 
the sea tends to remain at about one level, it is reasonable to suppose that, when a fish 
leaves this level the difference of pressure becomes uncomfortable and there is a 
tendency for it to return to its normal habitat. The air bladder is at present considered 
not an organ under the influence of the muscles of the fish, by means of which it alters 
its specific gravity, but rather a delicate gauge, which notifies the animal of changes 
of pressure above or below the normal. 
Were a fish to pass too far beyond its normal depth, either up or down, the expansion 
or contraction of the gas of the air bladder would so change the specific gravity of the 
animal that its muscular activity would no longer be able to cope with the upward or 
downward tendency, and the fish would be carried to the surface or the bottom. Tims 
the very presence of an air bladder works the destruction of the fish. Cases are 
reported where deep-sea fish have come to the surface in this way, evidently having- 
been carried too far from their normal level in struggles with their prey. 
It is true that slight changes of level may be brought about by the secretion or 
absorption of gas by the walls of the air bladder. That the walls of the air bladder 
secrete and absorb gas has been shown by experiment. 1 have repeatedly emptied the 
air bladders of fish and found a small amount of gas secreted in 24 hours. But this 
process of secretion and absorption is relatively slow, and permanent changes of 
habitat could be brought about only very gradually; moreover, only very slight 
changes are possible for each species. 
Thus we see that fish are surrounded by barriers of pressure, and that each species 
and probably each individual is constrained to remain at a certain level, and the 
restricted distribution of certain fish is thus explained. 
* Rechercliea experi men tales aur l’in 11 uence des tres li antes pressions sur les organismea vivants. 
Conipt. lien., xcviii, March 21, 1884, p. 745. 
