ON THE ANATOMY OP FISHES. 
93 
attachments to the skeleton^ that the air-bladder never collapses, even after its walls have 
been perforated, or portions removed, but invariably retains its normal shape and outline. 
The tunica externa consists of two principal strata of fibres, an outer in which the 
general direction of the fibres is in the main longitudinal without being constant for 
all parts of their course ; and an inner stratum composed of fibres with a general 
tendency to a transverse arrangement. The fibres composing either stratum, 
although they may vary their course and direction in different portions of the air- 
bladder, from longitudinal becoming oblique or nearly transverse, and vice versa, and 
may describe various curves, very rarely coincide in direction at any one point, the 
fibres of one stratum crossing those of the other at more or less of a right angle. 
This characteristic arrangement is always fairly obvious in the anterior, lateral, and 
dorsal walls of the anterior chamber, and in the dorsal and lateral walls of the lateral 
compartments, where the two strata can be readily separated and the component 
fibres traced, but becomes more obscure in the ventral wall ; there the fibres of the 
outer stratum pursue a somewhat irregular course and betray a tendency to interlace 
with one another, so as to be only traceable with some difficulty. As a rule it is only 
when the two strata approach their common attachments to rigid or moveable parts 
of the skeleton that their respective fibres exhibit a tendency to coincide in direction. 
In the walls of the lateral compartments, at all events in so far as the dorsal and 
ventral walls are concerned, the fundamental disposition of their fibres is to some 
extent obscured by the extension into them of the fibres composing the longitudinal 
septum, the primary and secondary transverse septa and their buttress-like ridges. 
In the septa themselves the component fibres are always vertical. 
The series of semi-diagrammatic figures, 19-22 inclusive, are intended to illustrate 
the arrangement and skeletal attachments of the physiologically more important 
fibres of the outer and inner strata of the dorsal wall of the anterior chamber as 
seen from the interior, and also to some extent the corresponding features of the 
anterior, posterior, and lateral walls. In all cases it may be presumed that the 
ventral wall of the bladder and the whole of the tunica interna have been removed. 
In figs. 19 and 20, the inner stratum of the tunica externa is shown on the right side, 
and the fibres forming the outer stratum on the left side. In figs. 21 and 22 the 
insertions of the fibres of the outer and inner strata, respectively, into the crescentic 
processes of the tripodes are represented. 
The disposition of the principal sheets of fibres in the tunica externa of the anterior 
chamber and their respective attachments to fixed or moveable portions of the 
skeleton are as follows : — 
(1-) Attachments to rigid portions of the skeleton. 
(a.) The posterior wall of the chamber, that is, the primary transverse septum, is 
composed of two vertically arranged strata of fibres, a posterior thin stratum, which 
dorsally and ventrally is continued into the dorsal and ventral walls respectively of 
the two lateral compartments, and a much thicker anterior stratum. Traced dorsally 
