242 THE IMMUNITY OF THE SWIM-BLADDER 



and a piece of lead is tied on to the ventral fin of another just 

 heavy enough to sink it to the bottom. The animals by next day 

 will have adjusted their specific gravity by means of their swim- 

 bladder, and both will be found freely swimming about. The cork 

 and lead are now removed the fish relieved of the cork irresistibly 

 sinks to the bottom, and the other rises to the surface, and there they 

 stay until a new adjustment of the bladder has been made (Moreau). 



Analyses of the gas in the bladder of deep-sea fish show a high 

 percentage of oxygen. In the case of a fish (Synapko-branchus 

 pinnatus) caught at a depth of 4500 ft., analysis of the gas yielded 

 85 per cent. O 2 and 12 per cent. N 2 . The pressure at this depth 

 is 150 atm., and thus the tension of the oxygen in the bladder 

 equalled 127 atm., and that of the nitrogen about 18 atm., 1 while 

 the tension of these gases dissolved in the surrounding sea- water was 

 only i atm. and 4 atm. respectively. 



Bohr brought up some cod in a bow net from a depth of 45 ft. ; 

 the gas in their swim-bladders expanded to such a degree that the 

 fish were forced to swim on their backs. Analysis of the gas showed 

 52 per cent. 2 . Next morning the fish were again the right way 

 up and of normal size. Analysis gave 10-16 per cent. O 2 . From 

 one fish 8 c.c. of gas were drawn off by a trocar, and found to contain 

 15 per cent. 2 ; forty-eight hours later the bladder yielded 7 \ c.c. 

 arid 79 per cent. O 2 ; twenty-four hours later 7 c.c. and 84 per cent. 

 2 . After cutting the intestinal branch of the vagus by making 

 a small opening just behind the gill-slit, Bohr found that the secretion 

 of oxygen ceased. In fish with a closed bladder, without a ductus 

 pneumaticus, there is a vascular area, " the oval," which is perme- 

 able to oxygen. The extent of the oval and the dilatation of its 

 blood-vessels are controlled by muscle and the escape of gas from 

 the bag regulated. The epithelium of the bladder is in another 

 place differentiated into a glandular-like structure, " the red body/' 

 which has the function of secreting the gas. In the red body are 

 to be seen the debris of red corpuscles. Jaeger suggests that the 

 gland secretes a lysin which, by producing haemolysis of the corpuscle, 

 sets oxygen free. But Bohr calculates that on this theory the fish 

 would soon have no red corpuscles. It is evident that the epithelium 

 of the swim-bladder is non-susceptible to the poisonous influence of 

 high-tension oxygen, for, while 5 atm. of O 2 rapidly poisons all 

 other forms of protoplasm, we find the Synaplio-branchus pinnatus 

 with an oxygen tension of 127 atm. in its bladder. Whales, too, 

 must be immune to oxygen-poisoning, if they sound to great depths, 



1 A. Jaeger is of opinion that nitrogen enters the bladder by diffusion, after 

 the fish have been brought to the surface. 



