THE REGULATING MECHANISM 211 



extent and order of a given group of muscular 

 contractions must be regulated to obtain a 

 required result. Thus in writing, many 

 muscles of the arm, fore-arm, and fingers act. 

 Again, in walking, complicated groups of 

 muscles must combine. It seems that in all 

 such mechanisms sensory impulses, or rather 

 impulses from the periphery, of which we may 

 or may not be conscious, start the mechanism. 

 Thus, in walking, impulses may come from the 

 skin of the feet and from the muscles and 

 tendons of the limb or from the eyes. If these 

 impulses reach the cerebrum, we may be 

 conscious of them. Without these impulses 

 even voluntary motion is irregular and ineffi- 

 cient. But many may find their way to 

 the back part of the cord and from it to the 

 cerebellum by what are called the inferior 

 peduncles of that body, which connect it 

 with the cord. The structure of the grey 

 matter of the cerebellum is extremely com- 

 plicated, and although many details are known 

 to histologists, we can form no conception of 

 its mechanism. But the grey matter shows 

 the usual plan of neurones of various forms, in 



