ON THE ANATOMY OF FISHES. 
97 
that side (fig. 19, o.st.). The most posterior of the fibres, that is, those that are 
attached to the posterior margin of the crescentic process, describe a very similar 
curvature, but in this instance, in the dorsal wall itself, the convexity of the curve 
being directed outwards and the concavity inwards, and subsequently pass into the 
dorsal wall of the neighbouring lateral compartment, there becoming obliquely 
arranged with regard to the long axis of the chamber (fig. 19). In nearly all parts 
of their course the curvilinear fibres of the outer stratum cross those of the inner 
stratum at a greater or less angle, and only coincide with the latter in direction as 
they both approach their attachments to the tripodes. It is obvious that all fibres 
that ultimately become inserted into the moveable tripodes at one extremity of their 
course are at the opposite extremity absolutely or relatively fixed, either as is the 
case with those forming the inner stratum of the tunica externa, through their direct 
continuity with the posterior pillars, or, as with those forming the outer stratum, 
through their extension into the walls of the inexpansible posterior or lateral 
chambers of the air-bladder. The specialisation of the dorsal wall of the anterior 
chamber to form special tracts of fibres in the form of two broad and stout triangular 
sheets, separated from each other by a thin skeletally attached medio-dorsal area, 
each of which is composed of two intersecting strata of fibres, and extends outwards 
from its attachment to the tripus to form the lateral and antero-lateral walls, where 
it becomes intimately related to the inner surface of the adjacent lateral cutaneous 
area, is extremely characteristic not of Macrones only but of nearly all other normal 
Siluridm. 
(6.) That portion of the tunica externa which occupies the concavity of the 
crescentic process of each tripus is specialized to form a series of tightly stretched 
fibres connecting that process with the radial nodule. The fibres are firmly inserted 
into the nodular prominence at the dorsal extremity of the radial nodule, and from 
that point radiate outwards and are eventually attached to the inner margin of the 
ventral ridge and the whole extent of the inner concave edge of the crescentic process 
(figs. 19 to 22, r.f.). The prommence on the radial nodule occupies, as it were, the 
centre of a circle of which the crescentic process itself forms a large arc, and the 
fibres in question, like the radii of a circle, converge from the process to the nodule. 
For these fibres we would suggest the term “ radial fibres.” A thin layer of tunica 
interna invests the ventral surface of the crescentic process as well as the inner 
surface of the nodular prominence, but without forming any special attachment to 
either ; the removal of the layer, which can be readily effected, at once exposes tlie 
surfaces of both ossicles. 
Although, as we have previously pointed out, the typical disposition of the fibres 
of the two strata of the tunica externa is transverse or circular for the inner stratum 
and longitudinal for the outer, it is obvious that this arrangement has been widely 
departed from in particular portions of the bladder. Thus, while the disposition of 
the fibres of the inner stratum in the lateral and anterior walls of the anterior chamber 
MDCCCXCIII. — r.. 
o 
