THE ANATOMY OF FISHES. 
263 
are directly connected with the tripodes, the Weberian ossicles are rendered more 
susceptible, and, therefore, capable of responding to smaller variations of internal 
gaseous tension, by whatever cause produced, than if the anterior chamber were equally 
elastic and expansible in all directions. The increased delicacy of the Weberian 
mechanism in the normal Siluridse, as compared with other Ostariophysem, is probably 
the cause of the more extensive anchylosis of the anterior vertebrae, and their 
rigid articulation with the skull. If the anterior vertebrae were flexibly articulated 
with one another and to the skull, so as to be able to participate in the lateral flexion 
of the vertebral column in ordinary locomotion, while at the same time these vertebrae 
and their processes retained their close relation to the dorsal surface of the air- 
bladder, the anterior chamber and Weberian ossicles could hardly fail to be affected 
by muscular compression. Consequently, the anchylosis of those vertebrae which are 
specially related to the anterior chamber becomes almost a necessity in the Siluridae 
if the Weberian mechanism is to remain unaffected by the more or less violent shocks 
produced by variations of muscular contraction and relaxation. The posterior portion 
of the air-bladder, represented by the two lateral compartments, lies free in the 
abdominal cavity and is not specially related to any skeletal elements, and anchylosis 
in this region does not, therefore, take place. From the position and shape of the 
tripus and its mode of articulation with the centrum of the complex vertebra, as well 
as from the relations and attachments of its crescentic process to the fibres converging 
from the corresponding lateral wall of the anterior chamber, it will follow that any 
inward or outward movement of the lateral wall will necessarily cause the crescentic 
process to move either inwards or outwards, while the anterior process of the same 
ossicle will execute similar movements, but in inverse order. In all such movements 
the tripus constitutes a lever of the first order, the fulcrum of which is represented 
by the articulation of the articular process of the ossicle with the complex centrum- 
The relative leno-ths of the two arms of the lever, that is to say, the anterior and 
crescentic processes, vary somewhat in different Siluroids, although within limits so 
narrow that in the great majority of the normal forms that came under our notice 
the two were of approximately equal length. Very exceptionally the anterior process 
may be slightly the longer of the two, and, consequently, like the long arm of a 
lever, gains in amplitude of movement what it thereby loses in force. Any inward 
or outward movement of the anterior process of the tripus will impart either a push or a 
pull to the interossicular ligament, and finally to the spatulate process of the scaphium, 
which, therefore, moves either inwards or outwards by a corresponding rotation of 
that ossicle on its condylar and ascending processes, and with every such movement 
of the scaphium a forward or backward impulse will be given to the perilymph of the 
atrial cavities. The characteristic curvature of the crescentic process of the tripus 
apparently serves no other purpose than that of increasing the surface required for 
its connection with the fibres of the air-bladder. 
The precise mode in which movements of the anterior process of the tripus are 
