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PROFESSORS T. W. BRIDGE AND A. C. HADDON 
tonic contraction of this sphincter is maintained, and secondly, the contraction of the 
walls of the duct and gall-bladder. The cystic duct in Man is always somewhat 
tortuous at its commencement, and this may furnish an explanation of the fact that 
very considerable manual pressure is required to expel the contents of the gall- 
bladder, at all events, after death, and at the same time suggests that the muscular 
contraction by which the bile is normally expelled is of a gradual and peristaltic 
character. Moreover, the tortuosity of the cystic duct may have some physiological 
significance in the sense that it acts as a valve and prevents mere abdominal pressure 
from producing a flow of bile into the intestine. 
The air-bladder and pneumatic duct certainly exhibit some structural analogies to 
the gall-bladder and cystic duct, and it is by no means improbable that a close 
physiological parallelism may also exist with regard to the expulsion of their respec- 
tive contents. It is true that the walls of the air-bladder are devoid of both intrinsic 
and extrinsic muscles, and they can have no power of expelling the gases they enclose 
by direct muscular compression, but muscular action is here unnecessary, inasmuch as 
the augmented tension and volume of the contained gases under diminished pressure 
will furnish the needful expulsive force. Mere manual pressure on the air-bladder has 
but little, if any, effect iji expelling its gaseous contents, and that this is so is probably 
due to the tortuosity of the pneumatic duct, aided possibly by the action of a terminal 
sphincter, at or near its gastric or oesophageal extremity. On the other hand, the 
ductus pneumaticus in some Slluridse, and possibly in all the normal species, contains 
unstriped muscular fibres. The reflex mechanism in the two cases may also be 
similar. Diminished hydrostatic pressure conditions an expansion of the volume of 
gas in the air-bladder, which will ultimately result in the transmission of a stimulus 
to the sensory epithelium of the internal ear, and the initiation of afferent impulses in 
the auditory nerve. The afferent impulses may find their final expression in the 
peristaltic contraction of the walls of the pneumatic duct and the expulsion in suc- 
cessive bubbles of an amount of air that will suffice to restore equilibrium at a new 
level. It is probable that the pneumatic duct is not to be regarded as a mere channel 
for the escape of gas from the air-bladder, but rather as a structure which, under 
reflex control, actively participates in the process, and, at the same time, regulates 
the quantity of the expelled air. Slight reductions of pressure, the result of equally 
slight variations of level, may not necessarily lead to an escape of air, for probably no 
inconvenience to the locomotion of the Fish would result from them, in fact, the 
tortuosity of the pneumatic duct, aided, it may be, by a special sphincter, can be 
regarded as of the nature of a safeguard to prevent the unnecessary elimination of gas 
under such conditions. But more extensive reductions of pressure may at once call 
into play the reflex mechanism to secure the necessary readjustment to the more 
superficial level. That the reflex mechanism is under the direct control of some 
special nerve centre is highly probable, and it is at least within the range of 
possibility that the activity of this centre may in turn be dominated by some still 
