RESPIRATION IN FISHES. 309 



being about one per cent, of the whole volume. 

 A tench is able to breathe when the quantity of 

 oxygen is reduced to the 5000th part of the bulk 

 of the water, but soon becomes exceedingly 

 feeble by the privation of this necessary ele- 

 ment. The fact, however, shows the admirable 

 perfection of the organs of this fish, which can 

 extract so minute a quantity of air from water 

 to which that air adheres with great tenacity*. 



* The swimming bladder of fishes is regarded by many of the 

 German naturalists as having some relations with the respiratory 

 function, and as being the rudiment of the pulmonary cavity 

 of land animals ; the passage of communication with the oeso- 

 phagus being conceived to represent the trachea. The air con- 

 tained in the swimming bladder of fishes has been examined by 

 many chemists, but although it is generally found to be a mixture 

 of oxygen and nitrogen, the proportion in which these gases exist 

 is observed to vary considerably. Biot concluded from his expe- 

 riments, that in the air-bladder of fishes inhabiting the greatest 

 depths of the ocean, the quantity of oxygen is greater, while in 

 those of fishes which come often to the surface, the nitrogen is 

 more abundant ; and De la Roche came to the same conclusion 

 from his researches on the fishes of the Mediterranean. From the 

 experiments of Humboldt and Provencal, on the other hand, we 

 may conclude, that the quality of the air contained in the air- 

 bladder is but remotely connected with respiration. (Memoires 

 de la Societe d'Arcueil, ii, 359.) 



According to Ehrmann, the Cobitis, or Loche, occasionally 

 swallows air, which is decomposed in the alimentary canal, and 

 effects a change in the blood-vessels, with which it is brought 

 into contact, exactly similar to that which occurs in ordinary 

 respiration. It is also believed that in all fishes a partial aeration 

 of the blood is the result of a similar action, taking place at the 

 surface of the body under the scales of the integuments. Cuvier, 

 sur les Poissons, I, 383. 



