266 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
exactly opposite and internal to the anterior extremity of the tripus, and hence the 
transversely-disposed interossicular ligament passes straight from one ossicle to the 
other, and no sesamoid or pulley -like action of the intercalarium is necessary. We 
are, therefore, inclined to regard the intei'calarium in the Cyprinidm as fulfilling its 
primary function of giving increased leverage to the action of the interossicular 
ligament, such a function being necessitated by the relative positions of the two 
ossicles between which this ligament extends ; while in the Siluridse, on the other hand, 
from the altered relation and more direct juxtaposition of the scaphium and tripus, 
the need of an intercalarium is no longer felt ; and consequently this ossicle in 
the vast majority of the normal Siluridas becomes the comparatively functionless 
vestige of a physiologically more important structure in the Cyprinidse. At all 
events this conclusion seems to receive considerable support from the fact that the 
difference in the relative positions of the scaphium and tripus seems to be a constant 
character in all hitherto investigated species of the two families, and correlated with 
such differences there are also certain almost equally constant modifications in the 
condition of the intercalarium, to the extent that in nearly all Siluridm the ossicle has 
lost all connection with its vertebral centrum, and becomes but a comparatively 
insignificant nodule in the interossicular ligament, whereas in all the Cyprinoid Fishes, 
including even those with abnormal air-bladders, the intercalarium I'etains both its 
insertion into the ligament as well as its fulcral connection with the centrum of the 
second vertebra. 
The function of the radial fibres of the tripus seems to be that of antagonizing the 
outward pull which those fibres that pass from the convex side of the same process 
into the dorsal and lateral walls of the anterior chamber exert upon the tripus during 
any lateral distension of that part of the air-bladder. When distension subsides the 
pull of the previously tensely stretched radial fibres causes the crescentic process to 
move inwards to its normal position of close relation to the lateral surface of the 
complex centrum. Normally the two sets of fibres exactly counterbalance each other, 
and according as the volume of the anterior chamber is increased or diminished by 
lateral expansion or contraction, so will the pull of one set of fibres predominate over 
that of the other. In one or two instances (e.ry., Aucheniptems) where the elasticity 
of the articular process of the tripus functionally replaces the I’adial fibres, the latter 
are but feebly developed. 
Any push or pull given to the spatulate process of the scaphium by the inward or 
outward movement of the anterior process of the tripus must also cause the former 
to move inwards or outwards, and by so doing slightly decrease or increase the size 
of the atrial cavities. The immediate effect of this will be to cause either a forward 
or a backward flow of the fluids of the two cavities. The extent to which the atrial 
cavities will be increased or diminished in size by these means will, of course, primarily 
depend upon the amount of lateral conti’action or distension to which the anterior 
chamber of the air-bladder is liable, but the result will be that any motion, whether 
