S§9 French NdthM^MitUte. 



Employed to change the specific gravity of fishes, and that 

 for this purpose the animal, by means of its muscles, 

 compressed this organ, and thus varied its dimensions ac- 

 cording as it was desirous to remain in equilibrium, to as- 

 cend or descend in the medium in uliich it exists. As to 

 the way in which the air gets admission, it has been thought 

 that it was by means of the canal in such bladders as are 

 furnished with it, and by n)eans of the glands by secretion 

 in those which have no communication externally. Besides, 

 we know from the experiments of M. Biot, that this air 

 H a mixture of oxygen and azote, and that its nature varies 

 according to the depth at which the fish lives : so that the 

 species which arc procured from the bottom of the sea 

 contain a greater proportion of oxygen, while those which 

 are found near the surface yield more azote. M. Delaroche 

 having collected a great number of fishes in the Mediterra- 

 nean, has described their swimming-bladders, and made u& 

 acquainted w ith some which were unknown before : he has 

 confirmed the experiments of M. Biot ; and as to the uses of 

 this organ, he adopts nearly the same results with preceding 

 naturalists. 



This air-bladder has also been the subject of some re- 

 searches by Messrs. Humboldt and ProvenQal. They 

 vvishcd-to ascertain what were the relations between this 

 organ and respiration. The chief results of their experi- 

 ments arc as follow': — The air contained in the swimming- 

 bladder does not depend upon the air brought into contact 

 with the bronchiae ; the absence of this organ does not affect 

 the process of respiration, but it seems to prevent the pro- 

 duction of the carbonic acid gas. Lastly, they have seen 

 tench, from which the swimming-bladder had been taken, 

 swim about, rise and sink in water, with as much facility 

 as those which are furnished with them. 



These labours have given rise to a very detailed report of 

 M. Ctivier, in which he brings to view all the' inquiries 

 which have been undertaken as to the swimming-bladder of 

 fishes, and in which he again takes notice of the various 

 questions which have arisen on the subject. After a pro- 

 f(umd discuss'on, he arrives at some general results which 

 we have mentioned above, and details every thing which 

 fttili remains doubtful on the subject*. 



There are some other experiments from which physiolo- 

 gists may derive great advantage. These relate to the action 

 exercised by substances of various kinds on the bodies of 



* See tii6 prescj« volume, p. 291—302. 



