ON THE ANATOMY OF FISHES. 
289 
of some obstacle to what would otherwise be a continuous and gradual evolution of 
gas, and also suggests the presence of some check or regulating mechanism which 
periodically limited the outflow in accordance with the requirements of a gradually 
diminishing pressure, and thereby enabled the animal to retain a constant plane of 
least effort, for even in a partially exhausted air-pump the Fish still swam about 
freely. On this point Ramsay Weight (43) remarks, “the mode in which the air is 
discharged in Amiurus is not known to me, but the duct, tortuous where it opens 
into the oesophagus, must be much straighter when the ventral wall of the anterior 
part of the air-bladder is distended than when such is not the case. Further 
investigation must show whether the duct participates actively in disengaging the 
air-bubbles, and if so, under the control of what nerve it does so” (p. 386). We 
cannot altogether agree with Ramsay Weight’s suggestion that the escape of air 
from a distended air-bladder may be facilitated by the straightening of a tortuous 
pneumatic duct, for, as we have elsewhere shown, the skeletal attachments of the 
anterior chamber are such that it has but little capacity for distension except through 
the outward bulging of its lateral walls, in which the ventral wall can scarcely, if at 
all, participate. Nevertheless, we are strongly inclined to agree with his suggestion 
that the ductus pneumaticus does actively participate in promoting the escape of gas 
and in controlling the rate of exit, and on this point some clue as to the nature of 
the process may be gained by a comparison of substantially similar mechanisms in 
other organs with certain facts of structure in connection with the air-bladder and 
pneumatic duct of the Siluridm, but their truth or falsity only experimental inquiry 
can determine. 
Comparative physiology furnishes many illustrations of the various methods by 
which the escape of partially solid or wholly fluid substances from the body, or from 
the organs, in which they are formed or temporarily stored, is regulated. A special 
spliincter muscle may surround the terminal outlet and be maintained in a normal 
condition of tonic contraction by impulses from a special nerve centre, the activity of 
which may be increased or inhibited by afferent impulses generated by appropriate 
external or internal stimuli. The sphincter ani may be taken as an example of this 
form of regulatory mechanism. Much more to the point, however, is the method by 
which the flow of bile from the gall-bladder to the small intestine in Mammalia is 
controlled. Both the cystic duct and the gall-bladder have muscular walls, while the 
terminal oriflce of the common bile duct is said to be suiTounded by a sphincter 
muscle. The relaxation of this sphincter and the contraction of the muscular walls 
of the gall-bladder and its duct may be brought about by a reflex action, in which the 
afferent impulses are due to the stimulus afibrded by the passage of the acid contents 
of the stomach over the intestinal orifice of the bile duct, but nothing appears to be 
satisfactorily known either as to the nerve centre or to the path of the afferent 
impulses. It is clear that two distinct results follow the application of the stimulus 
and the initiation of afferent impulses, first, the inhibition of the centre by which the 
MDCCCXCIII. — B. 2 P 
