ON THE ANATOMY OF FISHES. 
291 
higher centre whereby the processes involved in re-adjustment to constantly varying 
external pressures are brought under the voluntary control of the Fish, 
A little additional light may, perhaps, be thrown on the nature of the regulatory 
mechanism by some remarks of DuFOSsi: (Hb). This writer describes the ductus 
pneumaticus in certain Ostariophysese, such as the Carp {Cyprinus carpio), the 
Barbel {C. harhatus), and the Miller’s Thumb [Cobitis harhatula, Linn.) among the 
Cyprinidse, as widening out into a trumpet-shaped orifice as it opens into the 
oesophagus, and that in this dilated part of the duct the mucous membrane presents 
several small duplicatures, which form valves so disposed as to prevent the exit of gas, 
the will of the animal and not the mere accumulation of gas being indispensable to 
overcome the valvular obstruction. Dufosse, however, regards these valves as 
forming a regulatory mechanism controlling the escape of air from the air-bladder by 
which these Fishes are rendered capable of producing certain voluntary sounds 
classified by him under the name of breathing noises, or “ bruits de soufflement,” In 
the absence of experimental evidence that such valves are related solely to the 
production of voluntary sounds, as Dufosse believes, it may not be altogether 
unreasonable to suggest that these structures may have something to do with 
regulating the expulsion of gas from the air-bladder under the influence of reduced 
hydrostatic pressure, and further that the breathing noises may only be the 
accidental concomitants of the latter function. At any rate, the existence of 
valvular structures in the ductus pneumaticus of, at all events, some few Ostario- 
physeae afibrds an additional clue to the nature of the mechanism by which such 
Fishes may voluntarily or reflexly control the escape of air from the air-bladder. 
Whatever may be the precise nature of the regulatory mechanism, the advantage 
to the Fish of some such method of carefully graduated adjustment to pressure 
variations as that we have suggested is sufficiently obvious. Without any kind of 
regulatory control, and with an open ductus pneumaticus in free communication with 
the exterior, it may be surmised that the escape of gas would be continuous and 
unchecked, and might even involve a more or less complete exhaustion of the gas in 
the air-bladder as the pressure diminished, with the contingent disadvantage that the 
normal equilibrium of the Fish in the water would be greatly disturbed, and a 
considerable demand be made on the secretive activity of the bladder for the 
subsequent restoration of the gas. On the other hand, the existence of a controlling 
mechanism would ensure that only so much gas would be evolved under such 
circumstances as might suffice to maintain the Fish in a plane of least effort, and at 
the same time secure the needful economy in the liberation of the gas. 
There is another point of view from which, if the foregoing suggestions have any 
weight, the Weberian mechanism must be a source of considerable gain to all Fislies 
in which it is present. There can scarcely be any doubt that all Fishes, whether 
with or without this mechanism, must in ordinary locomotor movements encounter 
more or less extensive changes of level and pressure, although no doubt within limits 
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