38 WITH HARD CHEEKS. 



present, are nourished by blood-vessels, Avliicli are very 

 apparent. 



The air-bladder does not occur iu all fishes : some fishes, 

 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, Avith which the bladder is 

 more or less distended, can escape. Muscles for compress- 

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

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

 service in all. 



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

 subject of repeated investigations. Priestley 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 percent., depending 

 on the depth at which the diflPerent species usually remained. 

 The Gurnards were frequently selected for these experiments, 

 their air-bladders having no canal of communication admit- 

 ted of being removed without losing their contents. 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 

 membrane, 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 trouble of examination. 



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



