ON THE ANATOMY OF FISHES. 
241 
Radial fibres arising from the radial nodules and inserted into the concave inner 
margins of the crescentic processes of the tripodes or into the corresponding margins 
of their ventral ridges are invariably differentiated from the dorsal wall of the anterior 
chamber. In two instances {Auchenipterus, Oxydoras), where the function of the 
radial fibres is taken by the flexible and highly elastic ai’ticular process of the tripus, 
the former are but scantily and feebly developed. 
As we have previously pointed out, the lateral compartments of the air-bladder are 
neither invested by bone nor are they directly attached to the skeleton, but project 
freely into the anterior portion of the abdominal cavity. The most important feature 
in connection with their structure, apart from their relatively greater capacity when 
compared with the anterior chamber, is their separation hy a common longitudinal 
septum and the frequently septate condition of their cavities. Physiologically, the 
longitudinal septum and the secondary transverse septa subserve the double function 
of rendering the lateral chambers almost incapable of distension, and at the same time 
diminishing the susceptibility to the effects of external pressure. Hence it follows 
that all variations that may take place in the volume of the air-bladder are dependent 
on the lateral expansion or contraction of the anterior chamber. 
Not the least remarkable feature in the air-bladder of the Siluroid Fishes is the 
liability of its walls to become invaded by ossified deposit, either by extension from 
certain of the adjacent skeletal elements which are always more or less intimately 
connected with the organ, or by apparently independent ossifications that subsequently 
coalesce with the contiguous skeletal structures. The oblique lateral ridges of the 
complex centrum are clearly ossifications of the transverse membrane along the line of 
its mesial and dorsal insertion into the lateral surfaces of the centrum. Similarly, the 
dorsal laminae are to be regarded as ossifications of the dorsal portion of the super- 
ficial coat of the air-bladder. In our previous references to the skeletal attachments of 
the dorsal margin of the transverse membrane we described the latter as not only 
inserted into the lateral ridges of the complex centrum, but also as being prolonged 
backwards on each side of the centrum and dorsad to the anterior chamber of the 
bladder in the form of a slip of fibres which is ultimately inserted into the anterior 
margin of the dorsal lamina of its side. The direction of the dorsal lamina, as it 
passes from the ventral surface of the transverse process of the fourth vertebra to its 
junction with the lateral ridge of the complex centrum and the radial nodule, exactly 
coincides with the course of this strip of fibres, so that the lamina appears as an 
ossified backward prolongation of the latter to an original insertion into the modified 
transverse process. In fact, there can scarcely be any doubt that both the lateral 
ridges and the dorsal laminm result from the ossification of the superficial coat of the 
bladder along the lines of its primitively wholly fibrous attachments to the sides of 
the complex centrum and to the ventral surfaces of the transverse processes of the 
fourth vertebra. The slightly free posterior margin of the dorsal lamina, as we have 
already mentioned, receives the insertion of the more laterally situated fibres of the 
MDCCCXCIII. — B. 2 I 
