rd's discovery. But a clear
light was not thrown on the subject until Bernard's experiments were
made in 1851. The experiments were soon after confirmed and extended
by Brown-Sequard, Waller, Budge, and numerous others, and henceforth
physiologists felt that they understood how the blood-supply of any
given part is regulated by the nervous system.

In reality, however, they had learned only half the story, as Bernard
himself proved only a few years later by opening up a new and quite
unsuspected chapter. While experimenting in 1858 he discovered that
there are certain nerves supplying the heart which, if stimulated,
cause that organ to relax and cease beating. As the heart is essentially
nothing more than an aggregation of muscles, this phenomenon was utterly
puzzling and without precedent in the experience of physiologists. An
impulse travelling along a motor nerve had been supposed to be able to
cause a muscular contraction and to do nothing else; yet here such an
impulse had exactly the opposite effect. The only tenable explanation
seemed to be that this particular impulse must arrest or inhibit the
action of the impulses that ordinarily cause the heart muscles to
contract. But the idea of such inhibition of one impulse by another was
utterly novel and at first difficult to comprehend. Gradually, however,
the idea took its place in the current knowledge of nerve physiology,
and in time it came to be understood that what happens in the case of
the heart nerve-supply is only a particular case under a very general,
indeed universal, form of nervous action. Growing out of Bernard's
initial discovery came the final understanding that the entire nervous
system is a mechanism of centres subordinate and centres superior, the
action of the one of which may be counteracted and annulled in effect
by the action of the other. This applies not merely to such physical
processes as heart-beats and arterial contraction and relaxing, but
to the most intricate functionings which have their counterpart in
psychical processes as well. Thus the observation of the inhibition of
the heart's action by a nervous impulse furnished the point of departure
for studies that led to a better understanding of the modus operandi of
the mind's activities than had ever previously been attained by the most
subtle of psychologists.

PSYCHO-PHYSICS

The work of the nerve physiologists had thus an important bearing on
questions of the mind. But there w