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Profs. T. W. Bridge and A. C. Haddon. [June 20, 



edge to the transverse process of the fourth vertebra, and only the 

 comparatively thin inner stratum, in addition to the fibres of both 

 strata from the lateral walls, extend into the dorsal walls and consti- 

 tute the triangular sheets. In the latter case but few, if any, of the 

 fibres of the inner stratum reach the tripodes, which, in consequence, 

 only receive the direct insertion of the outer stratum of the tunica 

 externa of the lateral walls. 



Radial fibres arising from the radial nodules and inserted into the 

 concave inner margins of the crescentic processes of the tripodes are 

 invariably differentiated from the dorsal wall of the anterior chamber. 

 In one instance (Auchenipterus) , where the function of the radial 

 fibres is taken by the flexible and highly elastic articular 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 connexion with 

 their structure, apart from their relatively greater capacity when 

 compared with the anterior chamber, is their separation by 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 their susceptibility to the effects of external pressure. 



Although we have never been able to detect the presence of 

 intrinsic muscular fibres in the walls of the air-bladder, powerful 

 extrinsic muscles are present in several Siluroids. In Platystoma 

 tigrinum, Pimelodus maculatus, P. ornatus, and Piramutana piramuta, 

 a powerful muscle takes origin from the posterior face of the skull, 

 on each side of the foramen magnum, and is inserted into nearly the 

 whole extent of the corresponding half of the ventral surface of the 

 anterior chamber. As the contraction of these muscles must forcibly 

 compress the anterior chamber we shall call them the " compressor 

 muscles " of the air-bladder. They probably occur in many other 

 Pimelodinae, but, so far as our investigations are concerned, are 

 probably confined to that group. 



The presence of compressor muscles is invariably associated with 

 the existence of a pair of much smaller muscles which arise from the 

 exoccipitals, and are inserted into the anterior wall of the anterior 

 chamber of the bladder. The tendon of each muscle has its insertion 

 into the anterior wall immediately external to the complex centrum, 

 and the insertion coincides with the extension of a slip of fibres from 

 the inner surface of the anterior wall to the ventral ridge and concave 

 inner margin of the crescentic process of the tripus. As the contrac- 

 tion of these muscles must evidently have the effect of limiting the 



