380 
POPULAR SCIENCE EE TOW. 
percentage of nitrogen occurs ; in salt-water fishes oxygen is 
said to occur in the largest proportion. Some, I believe, have 
maintained that the air of the swim-bladder of carps consists of 
pure nitrogen. Humboldt, who experimented on the electric 
eel {Gymnotus electricus), found the gas of its air-bladder to- 
consist of 96° of nitrogen and 4° of oxygen. Biot, on the other 
hand, experimenting on some deep-sea Mediterranean fishes, 
discovered 87° of oxygen, the rest nitrogen, with a trace of 
carbonic acid. This, if an undoubted fact, is certainly a singular 
anomaly, and I confess I share, with the late Dr. Davy, some 
doubt as to the accuracy of the experiments. “ That the same 
organ should secrete two gases so very different in their nature, 
appears anomalous, and deserving of further enquiry. Indeed, 
does not the entire subject need more minute enquiry? At 
present the facts relating to it are few, and seem far from 
adequate to allow of very satisfactory conclusions being drawn 
as to the use of the bladder and its secretion in the animal 
economy ; except of a mechanical kind, as effecting the specific 
gravity of the fish. Were the gas uniformly of one kind, were 
it constantly azote, it might be easy to assign it a plausible end, 
the function of the air-bladder might be inferred to be auxiliary 
to that of the kidneys. The secretion of oxygen is the anoma- 
lous fact, so contrary is it to the ordinary course of changes in 
living animals, in which the general tendency is to the con- 
sumption of oxygen — a priori^ one might almost as much 
expect oxygen to be exhaled from the lungs, in respiration, as 
to be separated from the blood by secretion by the air-bladder; 
and had we not the authority of so accurate an observer as M. 
Biot, we might be led to suspect that the statement of its being 
so was founded on error.” f 
How does the air gain admittance to the air-bladder ? In 
the case of those fish whose air-bladders possess no ductus 
pneumaticus, it is clear that the gas must be secreted by the 
inner membrane of the bladder from the blood; but in fishes 
which are provided with a ductus- pneumaticus, so as to lead to 
a communication with the oesophagus, the air may in a great 
measure be derived from the atmosphere, as in the case of trout 
when they rise at a fly. This opinion was held by Kathke and 
Dr. Davy. Further experiments on this interesting subject are 
very desirable, for we seem to know at present very little 
positively about it. We have seen how varied is the form of 
the air-bladder in those fishes which possess one ; let us now 
notice its presence or absence in the different orders. In the 
Givrostomi and Cydostomi, represented by the British lancelot 
[Branchiostoma) and lamprey (Petromyzon) respectively, the 
t ^^Physiological Eesearches,” p. 271. 
