THE AIU-BLADDEH IN TISHES. 173 



throiigli tlie skin may perfectly supply the place of respii-ation 

 through any special organ — is offered by frogs, which usually 

 breathe through lungs. Milne-Edwards the elder showed long 

 since that frogs, when prevented coming to the surface, were 

 able to live under water so long as they were not cut oflf from 

 the possibility of obtaining food and were freely supplied with 

 fresh water. In such a case general skin respiration must 

 necessarily take the place of lung respiration. Since then, 

 Paul Bert '^ has shown that skin respiration can only take the 

 place of lung respiration when, in the cold season, the tempe- 

 rature of the water varies between zero and 13° centigrade. 



The instances here adduced prove at once that the absolute 

 amount of oxygen needed for respiration and absorbed from the 

 water varies according to the peculiarities of the different species, 

 and perhaps even in individuals ; and moreover that its ab- 

 sorption depends on certain external conditions, above all on 

 the temperature. From this it further follows that there must 

 be for every individual animal an optimum quantity of oxygen 

 needed in a given time ; if this optimum is not attainable by 

 the ordinary organs of respiration, either the animal dies of 

 suffocation, or else the deficiency must be supplied by some 

 other means, as, for instance, in Milne-Edwards' experiment 

 on the frog, by general skin respiration. To this category 

 belongs too, in a certain sense, the air-bladder of fishes, which, 

 according to the most recent investigations, may under some 

 circumstances be regarded as an organ auxiliary to respiration. 

 A body of gas is deposited in it from the blood which also 

 contains oxygen, and this is rapidly used up if the fish is in 

 water which holds but little oxygen. Now, although generally 

 no air can be transmitted to the air-bladder from outside, still, 

 as it would seem, it serves as a reservoir for the superabundance 

 of oxygen which is introduced into the blood by the absorbent 

 action of the gills and the skin. "Very numerous experiments 

 have been made on this matter, but they have yielded so many 

 contradictory results that it is superfluous to go here into any 

 closer discussion of them ; and I refer those who are interested 

 in the matter to Milne Edwards' well-known work, 'Legons 

 d'Anatomie et de Physiologie compar^e.' In a note '* I have 



