ON THE ANATOMY OF FISHES. 
265 
ear, vibrations or other movements of the lateral walls of the anterior chamber 
that are of slight amplitude and rapid recurrence from whatever source derived. In 
fact, it would almost seem as if the interposition of a lax, or j-ather compressible 
ligament between any two of a series of three movable ossicles, were a special 
contrivance for the express purpose of preventing the internal ear from becoming 
susceptible to any but the comparatively grosser changes that take place in the 
internal condition of the air-bladder. It is also difficult to avoid the conclusion 
that the Weberian ossicles constitute a mechanism which, while not sufficiently 
delicate to be capable of transmitting to the internal ear slight variations in 
the condition of the gaseous contents of the air-bladder, is, nevertheless, veiy 
susceptible to all such changes that exceed a certain minimum. 
The function of the intercalariurn is not very clear in those Siluridfle in which it is 
represented by a mere nodule of bone imbedded in the interossicular ligament, unless 
it is to impart some slight rigidity to that structure, but even if this be the case the 
ligament still remains sufficiently lax or compressible to warrant the physiological 
conclusion to which reference has just been made. In the very limited number 
of instances {e.g., Macrones, Pseudohag^'iis, Liocassis, etc.) where the intercalariurn 
not only consists of a horizontal process with its distal end imbedded in the 
interossicular ligament, but in addition, of an ascending process lying in the fibrous 
wall of the neural canal between the exoccipital and the arch of the third vertebra, 
it is also difficult to see that the ossicle can have any other function assigned to it 
than that of slightly stiffening the ligament in question. Some light, however, may 
be thrown on this point by a comparison of the position and relations of the 
intercalariurn in the two families of the Siluridse and Cyprinidie. In the latter 
family, where the ossicle is represented by a Y-shaped bone having its two arms 
articulated with the lateral sin-face of the conjoined second and third vertebrae, and 
the distal extremity of its outwardly- directed stem inserted into the centre of the 
interossicular ligament, the scaphium is somewhat more anterior in position than the 
anterior extremity of the tripus, and consequently the ligament in passing between 
the two ossicles, assumes a direction which is somewhat oblique from behind forwards 
and inwards. Not only is the ligament more oblique in direction, but it is of also 
greater relative length and thinness than in the Silurida^. In this case there can be 
but little doubt that the intercalariurn functions as a pulley, and by moving through 
the arc of a small circle coincidently with the motion of the tripus, so alters 
the direction of the pull of the interossicular ligament as to enable the latter more 
directly to exert its full force on the scaphium. In the absence of an intercalarium, 
or if that ossicle were in the same condition as it is in tlie great majority of the 
Siluridae, the length, thinness, and compressibility of the ligament would be such 
that it is difficult to see how any but the most extensive volumetric variations of the 
air-bladder could possibly have any effect either on the scaphium or the internal ear. 
In the Siluridae, on the contrary, the spatulate process of the scaphium is situated 
MDCCCXCIII. — B. 2 M 
