580 THE HUMAN BODY. 



parts of the brain. Thence they travel down the white 

 columns of the cord to its gray matter, which they enter at 

 different levels, each in the neighborhood of a centre for 

 producing a given movement. If they there radiated far 

 and wide no definite movement could result, for all the 

 muscles supplied from the cord would be made to contract,, 

 and not merely those necessary to bend the index finger, 

 for example. We must here again, therefore, assume a path 

 of least resistance for the propagation of nerve impulses from 

 a given fibre coming down from the brain, to the efferent 

 fibres going to a certain muscle or group of muscles. The- 

 path between the two is almost certainly not direct; a 

 co-ordinating spinal centre intervenes, and all that the 

 brain has to do is to excite this centre, which then secures 

 the proper muscular co-ordination. If the hand be laid flat 

 on the table and its palm be rolled over, many muscles, in- 

 cluding thousands of muscular fibres, have to contract in 

 definite order and sequence. Persons who have not studied 

 anatomy and who are quite ignorant of the muscles to be 

 used can perform the movement perfectly; and even a 

 skilled anatomist and physiologist, if he knew them all 

 and their actions, could not by conscious effort combine 

 them so well as the cord does without such direct interfer- 

 ence. "Wejijiy^jthen^to look onjthe_cord^as_containing^a. 

 host of co-ordinating centres for different muscles. These 

 centres are put in nervous connection, on the one liancL. 

 with certain regions of the skin, and, on the other, with 

 regions of^jthe brain, and may be excited from eillimJjE 

 the former case the movement is called jreflex: in the latter 

 it jmayjbe reflex, or may bj^ccjmipariiprl by a feeling of 

 "jalL^jmdjs then called voluntary^- The more accurately 

 the required centre, and no other, is excited, the more 

 definite and precise the movement. 



The Education of the Cord- Much of what is called 

 edu^ting our tonch_or our muscles j^reaIIy~^diLi'caiio5IIb 

 the^spinal^cord. A person who begins to play the piano 

 finds at first much difficulty in moving his fingers inde- 

 pendently; the nervous impulses from the brain to the cord 

 radiate from the spinal centres of the muscle which it is 



