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 



