part of it. Now
the great pain of the secondary part of the train, as soon as it commences,
employs or expends the sensorial power of sensation belonging to the whole
associated train of motions; and in consequence the motions of the primary
part, though increased by the stimulus of an extraneous body, cease to be
accompanied with pain or sensation.

If this mode of reasoning be just it explains a curious fact, why when two
parts of the body are strongly stimulated, the pain is felt only in one of
them, though it is possible by voluntary attention it may be alternately
perceived in them both. In the same manner, when two new ideas are
presented to us from the stimulus of external bodies, we attend to but one
of them at a time. In other words, when one set of fibres, whether of the
muscles or organs of sense, contract so strongly as to excite much
sensation; another set of fibres contracting more weakly do not excite
sensation at all, because the sensorial power of sensation is pre-occupied
by the first set of fibres. So we cannot will more than one effect at once,
though by associations previously formed we can move many fibres in
combination.

Thus in the instances above related, the termination of the bile duct in
the duodenum, and the exterior extremity of the urethra, are more sensible
than their other terminations. When these parts are deprived of their usual
motions by deficiency of sensorial power, as above explained, they become
painful according to law the fifth in Section IV. and the less pain
originally excited by the stimulus of concreted bile, or of a stone at
their other extremities ceases to be perceived. Afterwards, however, when
the concretions of bile, or the stone on the urinary bladder, become more
numerous or larger, the pain from their increased stimulus becomes greater
than the associated pain; and is then felt at the neck of the gall bladder
or urinary bladder; and the pain of the glans penis, or at the pit of the
stomach, ceases to be perceived.

2. Examples of the second mode, where the increased action of the primary
part of a train of sensitive association ceases, when that of the secondary
part commences, are also not unfrequent; as this is the usual manner of the
translation of inflammations from internal to external parts of the system,
such as when an inflammation of the liver or stomach is translated to the
membranes of the foot, and forms the gout; or to the skin of the face, and
form