ON THE ANATOMY OF FISHES. 
193 
with the bony or fibrous wall of its investing capsule, at all events, when the sac has 
not shrunken from the action of preservative fluids, or collapsed from injury to its 
walls. The walls of the bladder are composed of an outer fibrous sheet, the tunica 
externa, lined throughout by a thin delicate inner tunic with an epithelium on its 
inner surface. A slender ductus pneumaticus passes from the intermediate tubular 
portion of the bladder to the oesophagus, precisely as Taylor described; 
Notwithstanding the abnormal condition of the bladder, the general structure of its 
walls and the extent and nature of its attachments to various skeletal elements are sub- 
stantially similar to those of an anterior chamber in a perfectly normal organ (fig. 81). 
The tunica externa of the ventral, anterior, and the greater part of the dorsal wall of 
each sac is extremely thin, but for a portion of its extent in the posterior wall, is 
somewhat thickened by the prolongation inwards between the ventral process and the 
transverse process of the fifth vertebra of a sheet of fibres derived from the fibrous 
investment of the ventral surface of the funnel which blends with the posterior wall 
and more or less firmly attaches the latter to the inner surface of the transverse 
process. In addition to this the dorsal edge of the posterior wall {p-2^.) is firmly 
inserted into nearly the whole length of the posterior margin of the dorsal lamina, 
from the point where the latter becomes apparent as a faint ridge on the inner surface 
of the transverse process of the fourth vertebra in the roof of the funnel to its free 
inner termination in the radial nodule (r.n.). The median tubular section of the 
bladder is also attached by its anterior and dorsal walls respectively to the roots of 
the ventral processes and to the ventral surface of the complex centrum. Such rigid 
skeletal attachments are strictly comparable to the dorsal insertion of the primary 
transverse septum into the skeleton, and to the medio-dorsal attachment to the 
complex centrum, respectively, in a normal anterior chamber. The anterior pillars of 
the normal bladder are apparently represented in Clarias by the dorsal attachment 
of the inner or mesial portion of the anterior wall of each air-sac to the complex 
centrum, to the radial nodule, and to the ventral surface of the root of the crescentic 
process of the tripus, but unlike Bagarius, Glyptosternum, and some other abnormal 
Siluridse, the remainder of the anterior wall is free from any special attachment or 
connection with the transverse process of the fourth vertebra. 
The tripus is an unusually large but extremely thin flattened ossicle (fig. 81, tr.). 
The anterior process {tr.a.) is provided with a flattened inner face at its anterior 
extremity for the attachment of the interossicular ligament. From the junction of 
the anterior and crescentic processes an articular process (tr.ar.) is given off, which, 
by means of its pointed distal extremity, articulates with the lateral surface of the 
complex centrum at the bottom of a deep pit-like depression. Both the anterior and 
articular processes occupy the longitudinal groove on the lateral surface of the complex 
vertebra and basioccipital, and lie also in a saccus paravertebralis, the latter coinciding 
in position and in length with the groove itself (fig. 78, tr.s.pv.). The root of the 
crescentic process (fig. 81, tr.c.) enters the cavity of the corresponding funnel and 
MDCCCXCIIl. — B. 2 C 
