Chap, ii.] THE BRAIN. 777 



bear in mind that in a voluntary movement fatigue is much 

 more of nervous than of muscular origin. 



Besides the active muscles, if we may so call them, which 

 directly carry out the movement, the metabolism of which sup- 

 plies the energy given out as work done, other muscles, some of 

 which are antagonistic to the active muscles and some of which 

 may be spoken of as adjuvant, enter into the whole act. In 

 flexion for instance of the forearm on the arm it is not the 

 flexor muscles only but the extensors also which are engaged. 

 According to the immediately preceding position and use of the 

 arm, and according to the kind and amount of flexion which is 

 to be carried out, the extensors will be either relaxed, that is to 

 say inhibited, or thrown into a certain amount of contraction. 

 And in some of the more complicated voluntary movements the 

 part played by adjuvant muscles is considerable. Hence in a 

 voluntary movement the will has to gain access not only to the 

 active muscles, but also to the antagonistic and adjuvant mus- 

 cles ; and every voluntary movement, even one of the simplest 

 kind, is a more or less complex act. 



The impulses which lead to the contraction of the active 

 muscles reach the muscles along the fibres of the anterior roots, 

 (we may for the sake of simplicity take spinal nerves alone, 

 neglecting the peculiar cranial nerves,) and such evidence as 

 we possess goes to shew that the impulses governing the antag- 

 onistic and adjuvant muscles travel by the anterior roots also; 

 the question whether the inhibition of the antagonistic muscles 

 when it takes place, is carried out by inhibitory impulses passing 

 as such along the fibres, or simply by central inhibition of pre- 

 viously existing motor impulses need not be considered now. 

 These anterior roots are connected as we have seen with the 

 grey matter of the cord, and in each hypothetical segment of 

 the cord we may recognize the existence of an area of grey 

 matter which, though we cannot define its limits, we may, led 

 by the analogy of the cranial nerves, call the nucleus of the 

 nerve belonging to the segment; and we may further recognize 

 in such a nucleus what we may call its efferent and its afferent 

 side. 



Every voluntary movement, even the simplest, is as we have 

 repeatedly insisted a coordinated movement, and in its coordi- 

 nation afferent impulses play an important part. The study of 

 reflex actions, § 462, has led us to suppose that each spinal seg- 

 ment presents a nervous mechanism in which a certain amount 

 of coordination is already present, in which efferent impulses 

 are adjusted to afferent impulses. But the results obtained by 

 stimulating separate anterior nerve roots shew that, in the case 

 of most muscles at all events, the especially active muscles of 

 the limbs for instance, each muscle is supplied by fibres coming 

 from more than one nerve root, that is to say the spinal nucleus, 



