238 
and the Dipnoids and Ganoids on the other. This pneumatic 
tube possesses the same coats as the air-bladder, is of various 
lengths, and said to be occasionally tortuous. As a rule it opens 
upon the dorsal surface of the alimentary canal, but in some of 
the herring family directly into the stomach. The glandular 
body observed upon as existing inside the air-bladders of the 
Physoclisti, is not so well developed, as a rule* in the 
Physostomi. 
As this pneumatic tube has no muscular coat it can hardly 
alter in its diameter except when acted upon by other forces, 
it would therefore be useless for inspiration. 
If we examine a carp we find its air-bladder is generally a 
simple sac, with a constriction between its anterior third and 
posterior two-thirds, but not sufficient to close the communi- 
cation. From the posterior portion of the air-bladder+ springs 
the pneumatic tube, as already described; or else this organ 
may be in the form of two rounded portions placed side 
by side beneath the bodies of some of the anterior vertebra, 
and not communicating with each other: but the two pneu- 
matic tubes coalesce previous to entering the dorsal wall of the 
alimentary canal. Not only in the Cyprinide but also in the 
Characinide and Siluride a chain of auditory ossicles connects 
the air-bladder with the internal ear, instead of a tube filled 
with gas, as remarked upon in the Physoclisti. Three ossicles 
on either side pass forwards along the under surface of the body 
of the first vertebra, connecting the outer wall of the air- 
bladder with the atria of the vestibule. These ossicles were 
first pointed out by Wexner; and since his time it has been 
shown that they belong, like the capsules of the special organs: 
of sense, to the Splanchnoskeleton. 
* Tn the eel there are two, placed one on each side of the duct commu- 
nicating between its two portions: the distribution of its vessels in the upper 
compartment of its air-bladder QurEKeErt likened to the cellular lung of reptiles. 
+ In carps the anterior portion of the air-bladder is very elastic, the 
posterior but slightly so. Muir observed that “in proportion as the fish 
rises in the water the anterior bladder, which is the most elastic, must con- 
siderably increase in volume, and thus keep the head of the animal up, while 
the contrary must be the case when the fish descends.” 
