42 M'lTH HARD CHEEKS. 



and those principally that live near the bottom of the water, 

 are without any. Among those species that have an air- 

 bladder, many appear on the closest examination to have no 

 canal or tube by which the air, with which the bladder is 

 more or less distended, can escape. Muscles for compressing 

 the air-bladder are obvious in some species, and wanting in 

 others, yet the air-bladder apparently performs the same ser- 

 vice in all. 



The gas contained in these air-bladders has also been the 

 subject of repeated investigations. Priestly and Fourcroy 

 determined the gas in the Carp to be nearly pure nitrogen ; 

 other chemists found the air in different fishes to consist of 

 nitrogen, oxygen, and carbonic acid ; the nitrogen in greater 

 proportion, and the oxygen in smaller, than in atmospheric 

 air. In the air-bladder of marine fishes the oxygen is in 

 excess, varying from forty to eighty-seven per cent., depend- 

 ing on the depth at which the different species usually re- 

 main. The Gurnards were frequently selected for these 

 experiments, their air-bladders having no canal of communi- 

 cation admitted of being removed without losing their con- 

 tents. It should be borne in mind that fresh water contains 

 more oxygen than that of the sea. 



The air thus found in these bladders, however variable in 

 its nature, is believed to be secreted by the inner lining mem- 

 brane, and in some instances by a red body, which appears 

 to form part of the walls of the air-bladder itself, and is made 

 up of minute blood-vessels arranged between the membranes. 

 This structure in the Conger Eel will amply repay the trou- 

 ble of examination. 



That the air found in this bladder is not taken in at the 

 mouth, is proved not only by the perfection of the valves of 

 the canal, which only open outwards, but also by the want of 

 uniformity in the quality of the air itself, and its existence in 

 those swimming-bladders that have no canal of communi- 



