92 
PROFESSORS T. W. BRIDGE AND A. 0. HADDON 
the ventral edg-e of tlie primary transverse septum. The duct is not straight but 
somewhat sigmoid, with secondary flexions near its junction with the oesophagus. 
As we have previously pointed out, the dorsal wall of the anterior chamber is closely 
related to the under surfaces of the expanded transverse processes of the fourth and 
fiftli vertebrae, and also to the subvertebral keel which the complex and fifth vertebral 
centra, with their investing superficial ossifications, combine to form (fig. 23, a.h.). 
In fact the median portion of the dorsal wall, as far posteriorly as the primary trans- 
verse septum, is deeply grooved externally by the impression of the subvertebral keel, 
and this inpushing of the dorsal wall, which is visible internally as a prominent medio- 
dorsal ridge, to some extent constricts the anterior chamber into two laterally bulging 
halves, while the vertex of the A -shaped transverse septum has the effect of con- 
tinuing this constriction into the posterior wall of the chamber (figs. 18 and 23). On 
the other hand, the lateral compartments lie comparatively free in the abdominal 
cavity, or at all events are only related laterally and dorsally to the musculature of 
the body wall, the slender transverse processes and ribs, and to the mesonephros, but 
in no way are they attached to, or protected by, any special outgrowths or modifica- 
tions of the skeleton as is the case with the anterior chamber. 
The dorsal anterior and posterior walls of the anterior chamber are not only more 
or less intimately related to certain characteristic modifications of the axial skeleton 
but they are directly attached thereto at certain special points. But before referring 
to the skeletal attachments of the anterioi’ chamber we may, with advantage, briefly 
describe the general arrangement of the principal sheets of fibres that form the walls 
of the air-bladder. Omitting the peritoneal investment on its ventral surface, and 
its superficial coat, the walls of the air-bladder are composed, as in all other Teleostei, 
of two principal layers of fibres, viz. : — the tunica interna and the tunica externa. 
The former consists of an extremely thin fibrous layer, supporting on its inner surface 
an epithelium of flattened cells, and furnishes a complete and continuous inner lining 
to the various primary and secondary cavities of the air-bladder, of uniform structure 
and thickness throughout. In specimens that have been preserved for any length of 
time in sj)irit, or have undergone maceration for a shorter period, the tunica interna 
readily separates from the tunica externa. In all the figures of the interior of the 
air-bladder whicli illustrate this paper the tunica interna has been removed so as to 
show the disposition and skeletal attachments of the fibres composing the outer tunic. 
After the removal of the inner tunic, and the exposure of the tunica externa, the free 
edge of the oblique ventral ridge of tripus, and the ventral surfaces of the heel-like 
process and of the posterior portion of the crescentic process of that ossicle, are 
clearly distinguishable on each side of the air-bladder, imbedded in the dorsal wall of 
the anterior chamber near its anterior extremity and a little to the outer side of the 
complex centrum (figs. 18 to 20). The tunica externa is always much thicker and 
consequently more rigid than the tunica interna ; it is, in fact, entirely owing to the 
thickness and strength of the former coat, combined with its more or less extensive 
