288 
rROFESSORS T. W. BRIDGE AND A. C. HADDON 
The physiological relations of the Weberian mechanism to the hydrostatic function 
of the air-bladder constitute an extremely difficult problem, and one that can only be 
satisfactorily settled by experimental inquiry. If retia mirabilia were present, it 
might be conjectured that the Weberian ossicles formed part of a reflex mechanism 
by which the varying tensions of the gases in the air-bladder were rendered capable of 
so affecting the central nervous system as to reflexly lead to modifications in the way 
either of acceleration or retardation in the rate at which such gases were secreted or 
absorbed, and possibly in this way facilitate the more rapid adjustment of the volume 
of internal gas to the varying pressures of different depths. That the secretion of 
gas, and possibly its absorption also, are under the control of the nervous system has 
been experimentally proved by Moreau in certain Physoclisti, but the absence of 
retia mirabilia in the Ostarioph}'-se0e seems to us conclusive against assigning more 
than a very subordinate part to the mechanism, if any, in the way of controlling the 
secretion or absorption of gas in the latter group. On the other hand, it seems to us 
more reasonable to suppose that the Weberian mechanism controls the escape of gas 
through the ductus pneumaticus in the process of pressure adjustment during slow or 
rapid ascent. A gradual distension of the air-bladder would be accurately measured 
by its recording lever, the Weberian ossicles, and the increasing intensity of the 
stimulus imparted to the sensory epithelium of the internal ear and 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 ductus pneumaticus, so that only so much gas will be 
eliminated as will suffice to maintain the Fish in a plane of least effort at all levels 
during ascent, notwithstanding the reduction of external hydrostatic px'essure. In a 
brief reference to this point Ramsay Wright (43) says that in Physostomous Fishes 
“accommodation to a new higher level is more quickly effected by the ejection of 
bubbles of gas through the air-duct,” and again, in referring to the Ostariophyseae, 
remarks that the Weberian mechanism “probably enables them to measure the precise 
amount of air which must be disengaged in order to restore equilibrium at a new 
higher level” (pp. 385-6). For reasons which we shall refer to subsequently, we 
cannot at present regard the first suggestion as applicable to the Physostomi in 
general, but, in so far as the Ostariophyseae are concerned both suggestions agree 
with the tentative conclusions at which we also have arrived. 
Unfortunately there is but little anatomical, and absolutely no experimental, 
evidence available as to how, or in what way, the escape of gas through the ductus 
pneumaticus is regulated or controlled in accordance with these conclusions. That 
the escape is in some way regulated is suggested by the fact that in the case of a 
TenclD* under artificially diminished pressure the air issued from the ductus in the 
form of bubbles, not continuously, but at intervals, as the pressure was gradually 
reduced. The intermittent escape of the gas certainly seems to indicate the existence 
* Moreau, loc. cit. 
