THE CO-ORDINATION OF MOVEMENTS. 107 
no apparent change is effected in it by the irritation at 
one end; but the rate at which the molecular change 
produced travels can be measured ; and, when it reaches 
the muscle, its effect becomes visible in the change of 
form of the muscle. The molecular change would take 
place just as much if there were no muscle connected 
with the nerve, but it would be no more apparent to 
ordinary observation than the sound of the tuning-fork 
is audible in the absence of the sounding-board. 
If the nervous system were a mere bundle of nerve 
fibres extending between sensory organs and muscles, 
every muscular contraction would require the stimulation 
of that special point of the surface on which the appro- 
priate sensory nerve ended. The contraction of several 
muscles at the same time, that is, the combination of 
movements towards one end, would be possible only if the 
appropriate nerves were severally stimulated in the proper 
order, and every movement would be the direct result of ex- 
ternal changes. The organism would be like a piano, which 
may be made to give out the most complicated harmonies, 
but is dependent for their production on the depression 
of a separate key for every note that is sounded. But it 
is obvious that the crayfish needs no such separate 
impulses for the performance of highly complicated 
actions. The simple impression made on the organs of 
sensation in the two examples with which we started, 
gives rise to a train of complicated and accurately co- 
ordinated muscular contractions. To carry the analogy 
