AIR-BLADDER. 205 



considerably in excess of the oxygen, which amounts to from 

 9 to 20 per cent. In fishes taken from deeper water the per- 

 centage of oxygen increases, to as much as 87 to 90 per cent, in 

 fishes taken from a great depth. Further, if the air-bladder 

 be artificially emptied, the fish sinks to the bottom, but it 

 slowly recovers by gas-secretion ; the gas so secreted is richer in 

 oxygen than air. At the same time nitrogen must also be 

 secreted, and sometimes appears to be the only gas secreted.* 

 Recently Bohr f has shown that section of the vagus prevents 

 the secretion of gas into the air-bladder. 



The evidence that it acts as a respiratory organ is very slight. 

 Many fishes swallow air, but there is nothing to show that the 

 air is taken into the air-bladder. In some cases however it 

 has been shown (e.g. by Budge tt, op. cit., in young Gymnarchus) 

 that the fish dies if it is prevented from coming to the surface 

 to take in air. It has however been suggested with more 

 probability that the oxygen secreted into the bladder may 

 serve as a store when the fish enters water in which the supply 

 of oxygen is too small. 



In some fishes, e.g. the Ostariophysi, in which the posterior 

 chamber is non-distensible and often enclosed by bone, while the 

 anterior chamber is distensible and connected by ossicles to the 

 membranous labyrinth, and in other fishes (see above) in which 

 the air-bladder is in connection with the ear, it has been surmised, 

 though not in any way proved, that the air-bladder acts as a re- 

 sonator in intensifying sound vibrations and transmitting them to 

 the auditory apparatus. On this view the Weberian apparatus 

 may be of use in increasing the acuteness of hearing. It has 

 also been suggested that it acts as a sound-producing organ, as 

 a consequence of the incomplete septa and membranes which 

 project into it being set in vibration by a movement of the con- 

 tained air, caused by contraction of the extrinsic and intrinsic 

 muscles which are contained in its walls. This suggestion rests 

 on observation, for many fishes possess the power of producing 

 sound (grunting, drumming, hissing, etc.), as many Sciaenidae, 

 some Siluridae (Doras, Platystoma, etc.), Trigla gurnardus, 

 Dactylopterus volitans, Malapterurus electricus, and many others ; 

 and in some cases the sound has actually been detected 



* See Hiifner, in Arch. f. Anat. und Physiologic, 1892, p. 54. 

 t C. Bohr, Influence of section of vagus on gas secretion in air-bladder, 

 Journ. of Physiology, 15, 1894, p. 494-500. 



