1092 
ward pressure acting upon the fish, does not cause the swimming- 
reactions by the stimulation of some other organ such as eye, laby- 
rinth or lateral line. 
Some of my experiments lead me indeed to suspect that the 
pressure modification to which a fish is subjected, is not exclusively 
perceived by means of the bladder and the organ of Wesrr. 
Morrav pointed out that the fish by modifying the gas-density in 
the bladder can adapt itself slowly to a modification in the pressure. 
I therefore caused a perch to adapt itself to a decrease of 10 cen- 
timetres of mercury. If such a fish is exposed again to normal cir- 
cumstances, it responds to this pressure, which is now too high for 
it, by swimming upward. After some time it sinks to the bottom 
and remains there panting. To the back of the animal a cork was 
now fastened by means of thin copper-wire, which drew the animal 
to the surface. On the bladder now presses 1 atmosphere i.e. 10 
centimetres of mercury — 25 centimetres of water more than that 
to which the fish adapted itself. Yet the animal continually tries to 
swim downward. Being pulled up by the cork, it remains lying at 
the surface at last. 
This swimming-reaction takes place, therefore, in a direction 
exactly opposite to that which was caused by the pressure-modification. 
That in the ostariophysi the transmission of stimuli is not 
exclusively due to the organ of WrBerR becomes evident from the 
following fact. 
The sensitiveness of tenches to pressure-changes is not affected if, 
on one or on both sides, the nervus octavus between vestibular- 
apparatus and brain, is cut through. Since, after the nerve has been 
eut on both sides, there was no longer any connection between the 
brain and the statie organ, the stimulus must have awakened the 
swimming-reflex by another route. 
Summarized in a few words my conclusions, drawn from the 
above disquisition, are the following: 
1. Active volume-changes can under careful exclusion of disturbing 
influences, be demonstrated in fishes. Not, however, in the sense in 
which it is understood by Borerrrs theory. The facts observed by 
me have been overlooked by Morrav owing to the short duration 
of his experiments, by his adherents and opponents owing to tech- 
nical defects in their experiments. 
Cause and significance of the changes have not been discovered yet. 
2. In agreement with BaGriont we must look upon the air-bladder 
as a hydrostatic organ of sense. 
