300 Report on a Memoir of M, l)claroeJie, 



from the blood, wherefore does it vary so much when \\vc 

 greater part of the other secretions are so constant in their 

 nature? Above all, how can the animal body, so greedy 

 of oxygen in general, exhale it so precisely at depths where 

 it has the fewest methods of getting it from the external 

 medium ? M. Delaroclie, who juits these questions, admits 

 that it is difficult to answer them saiisfactorilv. 



He afterwards proceeds to tlie uses of the air-bladder. 



From its absence in many iishcs taken indiscriminately 

 from all classes, he concludes, with the authors of the com- 

 parative anatomy, that it cannot hold an important place 

 in the vital functions; and this makes him reject all neces- 

 sary connexion between the air-bladder and respiration. 



He would have even been inclined to conclude, from its 

 solute stoppage in the greater number of fishes that are fur- 

 nished with it, that it could not in general be eluployed in 

 the absorption of any useful matter, in the excretion of 

 any injurious substance, nor even in the production of 

 a substance to be employed in some other part of the body; 

 but that it is solely by itself as the air-vessel, and in its qua- 

 lity of considerable caj^acity, filled with a light elastic sul)- 

 gtance that it may be useful to the fish. 



Now in this respect it can only have a mechanical use, 

 either vvith respect to its station or movement. 



M. Delaroclie in the first place ascertains its use in the 

 station, and admits that it serves to render the whole fish 

 specifically lighter, and to place it in equilibrium with the 

 water in which it is suspended. 



This is one part of the most generally received opinion ; 

 but it is clear that the necessity of the bladder for this sole 

 purpose is any thing but demonstrated. . Nature would 

 rather have made all fishes of the same gravity as the water, 

 as she has done with those fishes that have no bladders; 

 thus, the common opinion is also composed of two other 

 integrant parts equally necessary with the former. The 

 one is, that the fish can compress as it pleases, to a certain 

 extent, its bladder, or dilate it; which we prove by the pe- 

 culiar muscles with which the bladder is furnished in cer- 

 tain fishes, and by the mediate action which the sides and 

 the muscles of the abdomen exercise on it in all those 

 which have it. 



M. Delaroche also adopts this secortd part of the common 

 opinion. 



Ee thinks even that it is in this way the fish supplies, 

 when it rises, the pressure exercised on its bladder in deep 

 water by the column of v/atcr above it. Were it otherwise, 



the 



