496 



ANATOMY or VEUTEBRATES. 



329 



oesophagus in the Eel ; but De la Roche had well describccl the 

 ol>li(|ne aperture,' and accurately cites the whole family of the 

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

 pneumatic duct. It had been sup])0sed that the vascular ' air- 

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

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

 unquestionably in those fishes wliich have the shortest and 

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

 siren, Polypterus) 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 differci^t jiropor- 

 tions in the venous and the 

 arterial blood, it may be 

 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.^ 



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 : ^ but in the higher organised species above- 

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

 air to the bladder. 



Tlie 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 gas : but in the air-bladder of sea-fishes, 

 and especially of those which frequent great depths, oxygen 

 predominates.'' 



In tlie genera Avchenipterus , Si/nodon, Alalapterurus, and some 

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



' cxvii. p. 201. = .\xi. 1841, p. 98. See also Pr. J. IlaTy, in Phil. Trans, 1S38. 



" Neither the air-duct nor the elasticity of the air-blacklor are equal to jnevcnt the 

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

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

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

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



' Humboldt found the gas in tl)B air-bladder of the electric Gynmotus to consist of 

 90° of nitrogen and 4^ of oxygen. Biot found 8?° of oxygen in some of tlie deep- 

 sea Mediterranean lislies, the rest nitrogen, witli a trace of carbonic acid. No Iwdro- • 

 gen has over been delected in the air-l)laddcrs of lishcs. 



