416 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
THE AIR-BLADDER. 
The air-bladder of the cat-fish takes up in length about one half 
that of the belly cavity, and measures across at its broadest end 
from one-half to two-thirds its length. It narrows posteriorly and 
has a rounded termination, while the anterior face is broad and is 
covered by the head portion of the renal organs. It is covered up 
on its lower surface by the peritoneal folds. 
The dorsal surface has a groove into which the vertebral column 
fits, elsewhere the surface is even. The duct arises from it at the 
commencement of the middle third, and passes forward and down- 
ward to the cesophagus. 
There are three cavities in the air-bladder, two of which each 
communicate with a third, the anterior one. The long axis of the 
last named is directed transversely and occupies the broadest portion 
of the bladder. The long axis of the two others are parallel and are 
directed backward. The connection of each of these with the 
anterior one is by an aperture narrower than its own transverse 
diameter. It is with the anterior chamber that the duct communi- 
cates, opening at its posterior lower edge. 
There are two coats in the wall of the air-bladder. The outer 
white, and of some thickness, exists as such at all points, except a 
part of the dorsal surface. On the sickle-like auditory ossicle and along 
several vertebral segments it is but a thin transparent membrane, 
closely connected with and united to the ossicles and vertebre. 
Opposite the opening of the duct into the bladder the membrane 
again becomes thick and opaque white. This coat alone is connected 
with the auditory ossicles, and to its thickness, as well as to its 
constituents, it owes some of its stiffness. 
The inner coat is very thin and membrane like, and is conformed 
to the walls of the various chambers. Between the median walls of 
the posterior chambers is a single wall due to the fusion of the two 
outer coats. The outer coat also surrounds and enters closely into 
the constrictions of the openings of the posterior chambers into the 
anterior one. 
The outer coat is formed of connective tissue fibres and elastic 
fibres. The former are long, needle-like, and whitish as if calcified. 
The stiffness of the outer coat is due wholly to these fibres. When 
put into dilute acetic acid for several hours they swell up into a 
