124 SCIENCE PROGRESS. 



secretion and absorption of gas is in some sense under 

 voluntary control. On increasing or diminishing the pres- 

 sure within a large vessel of water in which a fish is 

 swimming, it is easy to see that the animal by swimming 

 downwards or upwards endeavours to counteract the ten- 

 dency of the expansion or compression of the air in the 

 swimming-bladder to carry it in the opposite direction. 

 The fish is evidently made very uncomfortable by altera- 

 tions in the volume of its air-bladder, and the discomfort 

 causes it to assume the depth at which the volume of the 

 air-bladder is normal. An animal swimming in the sea 

 will thus tend to be kept at about one depth. Permanent 

 alteration of depth will at any rate only be brought about 

 gradually. The air-bladder thus seems to be a contrivance 

 of such a nature as to prevent a fish from suddenly altering, 

 except to a very limited extent, the depth at which it lives. 

 Presumably it is of advantage to the animal to remain at 

 about one level ; on the other hand, gradual changes of 

 depth will be compensated for by secretion or absorption 

 of gas, so that if the change is only made gradually 

 enough the animal will be able to live equally comfort- 

 ably at the most varying depths. Moreau remarks that 

 flat fish have no air-bladders, and this is what might be 

 expected from the fact that they rest on the bottom on 

 their sides. 



I now come to the facts with regard to the air-bladder 

 which are of the chief interest to physiologists. Early in 

 the present century it was found by Biot x and other 

 observers that the gas in the swimming-bladder is not air, 

 but a variable mixture of oxygen and nitrogen, together 

 with about i or 2 per cent, of carbonic acid. The most 

 remarkable fact discovered about this mixture, however, 

 was that it frequently consisted almost entirely of oxygen, 

 especially in deep-sea fishes. 



The following table gives the results of a number of 

 analyses by Biot : — 



1 Memoires de la Societe cCArcueil, 1807. 



