496 ANATOMY OF VERTEBRATES. 



oesophagus in the Eel ; but De La Roche had well described the 

 oblique aperture,, 1 and accurately cites the whole family of the 

 Eels as fishes having both the so-called f air-gland ' and the 



o o 



pneumatic duct. It had been supposed that the vascular 4 air- 

 gland ' was present only in those fishes which could not derive 

 the gaseous contents of their swim-bladder from without; and 

 unquestionably in those fishes which have the shortest and 

 widest ducts (Sturgeon, Amia, Erythrinus, Lepidosteus, Lepido- 

 siren, Polyptcrus) the supposed air-secreting vaso-ganglions are 

 not developed. Since Professor Magnus has determined the 



existence of free carbonic 

 acid gas, of oxygen, and of 

 azote in the blood, and dis- 

 solved in different propor- 

 tions in the venous and the 



Parallel vessels of the vase-ganglion of the air-bladder, arterial blood, it may be 

 Eel. CCLXVIII. -,.-, n i .-, 



readily conceived that the 



venules of the vaso-ganglions may withdraw carbonic acid gas 

 from the arterioles, and that these may reach the inner surface of 

 the air-bladder richer in oxygen and poorer in carbonic acid than 

 when they penetrated the vaso-ganglions. 2 



The air-duct may allow the gas to escape under certain circum- 

 stances ; and the small size and obliquity of its orifice in many 

 Osseous Fishes (Carp, Eel) seem only to adapt it to act as a 

 safety-valve against sudden expansion of the gas when the fish 

 rises to the surface : 3 but in the higher organised species above- 

 cited, with short and wide air-ducts, these may, likewise, convey 

 air to the bladder. 



The contents of the air-bladder consist, in most freshwater- 

 fishes, of nitrogen, and a very small quantity of oxygen, with a 



trace of carbonic acid ;as : but in the air-bladder of sea-fishes, 



~ ' 



and especially of those which frequent great depths, oxygen 

 predominates. 4 



In the genera Auvhenipterus, Synodon, Malapterurm , and some 

 other Siluroids, the axis vertebra sends out on each side a slender 



1 cxvn. p. 201. 2 xxi. 1841, p. 98. See also Dr. J. Davy, in Phil. Trans. 1838. 



5 Neither the air-duct nor the elasticity of the air-bladder are equal to prevent the 

 consequences of a too rapid removal from the enormous pressure which fishes sustain 

 at great depths in the sea; those that are drawn up quickly by the hook are often 

 found to have the air-bladder ruptured, and sometimes the stomach is protruded from 

 the mouth by the pressure of the suddenly extricated and expanded gas. 



4 Humboldt found the gas in the air-bladder of the electric Gymnotus to consist of 

 96 of nitrogen and 4 of oxygen. Blot found 87 of oxygen in some of the deep- 

 sea Mediterranean fishes, the rest nitrogen, with a trace of carbonic acid. No hydro- 

 gen has ever been detected in the air-bladders of fishes. 



