1892.] Contributions to the Anatomy of Fishes. 147 



walls of the air-bladder. Conversely, the typical Physoclisti have the 

 advantage during descent. Moreau points out that a Fish incurs 

 more danger by rising above the plane of least effort than by sinking 

 below it. It is conceivable that a Physoclist, in the course of rapid 

 ascent, might so far depart from its normal plane of equilibrium as to 

 be forcibly carried to the surface of the water, and in that helpless 

 condition fall an easy and conspicuous prey to predaceous Birds or 

 Fishes. No special danger or inconvenience would result from a 

 sudden and rapid descent, and it is at least possible that the in- 

 creased secretion of gas which augmented pressure certainly condi- 

 tions in the Physoclisti may not be altogether without effect in 

 bringing about a more speedy re-adjustment to the greater pressure of 

 a deeper level, even in the Ostariophysese, in spite of the absence of 

 retia mirabilia. From these considerations it follows that, as com- 

 pared with the Physoclisti, the OstariophyseaB possess a far greater 

 capacity for adapting themselves to rapid and extensive changes 

 of level, more particularly in the direction of ascent, and many 

 well-known facts in connexion with their habits support this con- 

 clusion. 



The physiological relation of the Weberian ossicles to the hydro- 

 static function of the air-bladder is a problem which can only be 

 satisfactorily solved by experimental inquiry. The evidence seems 

 conclusive against assigning more than a very subordinate part to this 

 mechanism, if any, in the way of controlling the absorption or secre- 

 tion of gas ; but it may, nevertheless, control or regulate the escape 

 of gas through the ductus pneumaticus. A gradual distension of the 

 air-bladder would be accurately measured by the recording lever 

 (Weberian ossicles) and the increasing intensity of the stimulus im- 

 parted to the sensory epithelium of the auditory organ and to the 

 saccular branches of the auditory nerve. The consequent reflex or 

 voluntary efferent impulses may find expression in the exercise of 

 some form of regulatory control over the liberation of gas through 

 the pneumatic duct, so that only so much gas will be eliminated from 

 time to time as may suffice to enable the Fish to retain its plane of 

 equilibrium at all levels during ascent, notwithstanding the reduction 

 of external hydrostatic pressure. Unfortunately there is but little 

 anatomical and absolutely no experimental evidence as to how, or in 

 what way, the escape of gas is regulated in accordance with these 

 suggestions. Valvular structures have been found in the ductus 

 pneumaticus in some OstariophyseaB (e.g., Cyprinidse), and we have 

 found unstriped muscle cells in its walls. It is probable, as suggested 

 by Ramsay Wright, that the duct is not to be regarded as a mere 

 channel for the escape of gas from the air-bladder, but rather as a struc- 

 ture which, under reflex control, actively participates in the process, 

 possibly by peristaltic contractions. The air-bladder and pneumatic 



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