ox THE ANATOMY OF FISHES. 
283 
antecedent probability that the Weberian ossicles are directly related to this function. 
The general structure of the air-bladder, the mode of interconnection of the different 
Weberian ossicles, and their relations both to the air-bladder and to the internal ear, 
as well as the relations inter se of the different parts of the two last-mentioned 
structures, are perfectly consistent with this theory, against which no anatomical 
objections can be urged, and are equally inconsistent with any other at present 
suggested. 
The division of the air-bladder into a small, elastic, and expansible anterior portion, 
and a relatively large, inelastic, and inexpansible posterior division, renders that organ 
admirably adapted for the measurement of the varying volumes of the contained gases, 
inasmuch as the volumetric variations in the relatively large volume of gas contained in 
the whole bladder will find their expression almost exclusively in corresponding altera- 
tions in the size of the anterior chamber through the expansion or contraction of its 
lateral walls, which alone are directly connected with the recording Weberian ossicles. 
Regarded, in fact, as an article of physical apparatus, the air-bladder may be compared 
with such instruments as, for example, the thermometer, by which the expansion or 
contraction of mercury or alcohol under the influence of changes of temperature is 
accurately measured. In both instruments there is a relatively large reservoir in free 
communication with a much smaller one, and the expansion or contraction of the 
relatively large mass of gas, or fluid, as the case may be, in the larger chamber is 
measured by the more readily observed expansion or contraction of the contents of 
the smaller chamber ; and hence, there is a close functional parallelism between the 
anterior and posterior divisions of the air-bladder and the capillary tube and bulb of 
a thermometer respectively. Similarly it may also be said that the anterior chamber 
of the air-bladder is, to some extent, constructed on the principle of an aneroid baro- 
meter, inasmuch as the susceptibility of the recording lever (Weberian ossicles), 
connected with the former to the varying volumes of the contained gases, is consider- 
ably increased by the comparative rigidity of its anterior, posterior, dorsal, and ventral 
walls and the consequent restriction of any expansion or contraction to movements 
of its lateral walls alone, in much the same way that the expansion or contraction of 
the air contained within the metallic box of the aneroid will find its sole expression in 
corresponding movements of the flexible iq3per wall of the box, which alone is capable 
of transmitting such internal volumetric changes to the external recording lever ; in 
fact, in a certain sense it may be said that the air-bladder of the Ostariophysem 
combines the structural principles of both the thermometer and the aneroid barometer. 
The varying relative dimensions of the anterior and posterior chambers of the air- 
bladder in different Siluridm very probably bear some relation to corresponding varia- 
tions in the delicacy of the Weberian mechanism as a pressure register in various 
species, for, as is the case with the analogous parts of the tliermometer, the suscepti- 
bility of the mechanism should, within certain limits, increase as the |)osterior chambers 
become larger and the anterior chamber smaller. 
