ON THE ANATOMY OF FISHES. 
239 
departed from, that the fibres of both strata vary their course and direction in different 
parts of the bladder, although at any one point the fibres of one layer rarely agree 
in direction with those of the other, but intersect at a greater or less angle. It is 
only as the two strata approach their common attachments to the skeleton that then* 
component fibres coincide in direction. The variable course of the fibres is largely 
due to the differentiation of special tracts of fibres in certain regions of the air- 
bladder for skeletal attachments. 
Not only is the anterior compartment of the air-bladder more or less completely 
invested by bone on its dorsal and anterior surfaces, but its walls are attached to 
rigid portions of the axial skeleton and to movable ossicles at certain special points. 
As to the nature and extent of the fixed skeletal attachments, there is substantial 
uniformity in the different members of the group, and the physiological effect of such 
skeletal connections is, in the great majority of cases, the same, viz , to render the 
anterior, dorsal, ventral, and posterior walls incapable of participating in any disten- 
sion of the chamber, which, consequently, must solely depend upon the movement of 
the lateral walls. The posterior wall, i.e., the primary transverse septum, is always 
attached at its dorsal margin to the ventral and lateral surfaces of either the complex 
or the fifth centrum — rarely to the sixth centrum ; laterally to this the dorsal edge of 
the septum is invariably attached to the ventral surfaces of the transverse processes 
of the fifth vertebra, or to those of the fourth vertebra, or exceptionally to the corre- 
sponding processes of the sixth vertebra ; and, in addition, a sheet of fibres is often 
prolonged forwards, on either side of tlie complex centrum, into the dorsal wall, where 
it eventually becomes attached to the dorsal lamina of its side. These attachments 
we have elsewhere referred to as the “ posterior pillars ” of the compartment. As 
the anterior wall is usually more or less efficiently buttressed by the transverse 
processes of the fourth vertebra, or by post-temporal plates, or median subvertebral 
processes, the extent of its attachment to the skeleton varies inversely with the 
extent to which it is invested or supported by bone. The median portion of the 
wall is always attached dorsally to the ventral surface and sides of the anterior 
portion of the complex centrum, often by means of laterally situated, oblique, bony 
ridges, and also to the radial nodules. Laterally to this, on each side, the anterior 
wall may be so completely invested by bone as to be free from any special connexion 
or attachment to rigid portions of the axial skeleton {e.g., Macrones) ; or in correla- 
tion with a less complete bony support, the outer stratum of the tunica externa of 
the anterior wall may separate dorsally from the inner stratum, and become firmly 
inserted into the decurved anterior margin of the transverse process of the fourth 
vertebra {e.g., Arius, Aiichenoglcinis, Pimelodn.s). The dorsal attachment of the 
median portion of the anterior wall to the radial nodules and to the complex centrum 
occurs in all the normal Siluroids, and may be regarded as constituting the “anterior 
pillars” of the compartment. The ventral wall may also be considered as rigidly 
attached to the skeleton, both in front and behind, inasmuch as its inner stratum 
