ON THE ANATOMY OF FISHES. 
91 
lateral or posterior in position, and longitudinally arranged (fig. 18). The anterior 
chamber [a.c.) forms about one-third, and the two lateral chambers (/.c.) the 
remaining two-thirds of the antero-posterior extent of the bladder. The transverse 
septum {t.s.) is much the thicker of the two principal septa and divides the anterior 
chamber from the two lateral compartments, which, in turn, are separated from each 
other by the median longitudinal septum [l.s.). The longitudinal septum is continuous 
in front with the posterior face of the transverse septum, and both are continuous by 
their dorsal and ventral edges with the corresponding walls of the air-bladder. The 
transverse septum is not quite vertical, but inclines forwards from its dorsal to its 
ventral margin, the latter being more anteriorly situated than the former, but this 
statement more especially applies to the mesial portion of the septum. Hence, as in 
fig, 18, the transverse septum has its ventral margin somewhat V-shaped, with the 
apex pointing forwards, while the obliquity of the septum causes the two lateral 
compartments to slightly overlap the anterior chamber when viewed from the ventral 
surface. Along its ventral edge the transverse septum is almost co-extensive with 
width of the air-bladder, but dorsally the septum contracts from the lateral walls, so 
as to leave on each side an arch-like opening through which the anterior chamber 
communicates with each of the two lateral compartments. There is no communication 
between the lateral chambers except indirectly through the anterior chamber. In 
addition to the principal transverse and longitudinal septa there are three or four 
secondary transverse septa in each of the lateral compartments, which gradually 
diminish in width from before backwards (fig. 18, t.s.'). Each secondary septum, like 
the principal transverse septum, is of greater width dorsally than it is ventrally, and 
stretches from the longitudinal septum outwards towards the lateral or outer wall of 
the compartment, but before arriving at that point terminates in a deeply concave 
arch-like free margin. Dorsally as well as ventrally, the secondary septa are con- 
tinuous with the corresponding walls of their respective compartments, and round 
their free margins the smaller cavities into which they subdivide each lateral chamber 
communicate witE one another and with the anterior chamber. The principal trans- 
verse septum is really but the most anterior of these septa, although it differs from 
them in several important particulars, and notably in the firm attachment of its dorsal 
edge to the skeleton. In addition to the septa already mentioned, numerous buttress- 
like ridges of fibres arise from the dorsal and ventral margins of the longitudinal and 
the primary and secondary transverse septa and extend, root-like, into the adjacent 
roof and floor of the lateral compartments, thus still further adding to the sacculated 
or camerated structure of these chambers. With the exception of a few fibrous ridges 
that pass from the ventral margin of the anterior face of the primary transverse septum 
and blend with the adjacent ventral wall of the anterior chamber, the latter has per- 
fectly smooth walls and is entirely devoid of ridges, sacculations, or septa. 
The ductus pneumaticus (fig. 17, cZ.^9.) communicates with the cavity of the anterior 
chamber by an opening situated in the inedian line of the ventral wall and close to 
N 2 
