THE AIK OR SWIMMINO BLADDER OF FISHES. 
379 
maigre {Scicena aquila) presents an air-bladder with a fringe 
all around its edge, which consists of numerous ramified pneu- 
matic cseca. In Gadus Navavaga, Professor Owen tells us, 
^^the lateral productions expand and line corresponding ex- 
pansions or excavations of the abdominal parapophyses, thus 
foreshadowing the pneumatic bones of birds. In Kurtus the 
air-bladder is encircled by expanded ribs, curving and meeting 
below it.” In Amia the air-bladder exhibits such a cellular 
structure that Cuvier could compare it to nothing else than 
the lung of a reptile; this cellular subdivision is still more 
strikingly shown in the Lepidosiren or mud-fish. Thus we have, 
as it were, a complete series of modifications, from the simple 
elongated cylinder of the perch or herring, through the various 
forms exhibiting csecal processes, up to the completed lung-like 
structure of the air-bladder of the Lepidosiren. The air- 
bladder consists of two membranes, an external tendinous mem- 
brane of a silvery colour, and an internal mucous membrane 
lined with flat or plaster epithelium,” and supplied with 
blood-vessels which either divide into fine branches, in a fan- 
like form, pr else form what have been termed o^etia mirabilia at 
particular points. These retia mirabilia, or vaso-ganglions, 
may readily be seen in the air-bladder of an eel, in the form of 
red glandular-like bodies of an oval shape. In some fishes a 
duct leads from the air-bladder to the stomach or the oesophagus : 
this is known by the name of the dmctus pneumaticus. It is a 
simple membranous tube, presenting, however, much diversity 
in length and diameter, and also in its point of communication 
with the digestive apparatus. ‘^In the herring the ductus 
pneumaticus is produced from the posterior attenuated end of 
the cardiac division of the stomach, and opens into the fusiform 
air-bladder at the junction of the middle and posterior thirds 
of that organ. The long narrow and flexuous ductus pneu- 
maticus is continued from the fore part of the posterior division 
of the air-bladder in the Cyprinoids, and opens into the dorsal 
part of the oesophagus; the short, straight, and wide ductus 
pneumaticus in the Lepidosteus opens also into the dorsal part 
of the oesophagus, the orifice being served by a sphincter. In 
the Erythrinus the air-duct communicates with the side of the 
oesophagus ; in Polypterus, as in Lepidosiren, with the under or 
ventral part of the beginning of the oesophagus.” * The contents 
of the air-bladder of fishes have been analysed by Humboldt, 
Biot, and others, and, strange to say, remarkable differences are 
stated to exist, according as the fish are fresh-water or marine. 
The air consists of a mixture of nitrogen and oxygen, with 
traces of carbonic acid gas : in fresh-water fishes the largest 
* Comparative Anatomy and Physiology of Yertehrates/’ vol. i. 
D D 2 . 
