ON VOLUNTARY MOVEMENTS. 687 



to the antagonistic and adjuvant muscles; 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 show that the impulses governing 

 the antagonistic 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 previously existing motor impulses, need 

 not be considered now. These anterior roots are connected, as we have 

 seen, with the gray matter of the cord, and in each hypothetical segment 

 of the cord we may recognize the existence of an area of gray 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 coordination afferent impulses 

 play an important part. The study of reflex actions ( 502) has led us to 

 suppose that each spinal segment 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 show 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, or at least the spinal motor mechanism for any one muscle, 

 extends over two or three segments. Hence a fortiori in a voluntary move- 

 ment, involving as this does in most cases more than one muscle, the spinal 

 mechanism engaged in the act spreads over at least two or three segments, 

 thus allowing of increased coordination. In that coordination the impulses 

 serving as the foundation of muscular sense play an important part, but 

 other afferent impulses, such as those from the adjoining skin, also have their 

 share in the matter ; and it is worthy of notice that not only is the skin over- 

 lying a muscle served, broadly speaking, by nerve-roots of the same segment 

 as th*e muscle itself, afferent in one case, efferent in the other, but in the 

 parts of the body where coordination is especially complex, in the fingers 

 for instance, not only is each muscle supplied from more than one segment, 

 but also each piece of skin is supplied in the same way by the posterior roots 

 of more than one nerve. 



In the case of the frog it is clear that in reflex movements a large amount 

 of coordination is carried out by these various spinal mechanisms ; and as we 

 have urged, we may safely infer that in the voluntary movements of the frog, 

 the will makes use of this already existing coordination, whatever be the 

 exact path by which in this animal the will gains access to the spinal mechan- 

 isms. In the dog we may conclude that in voluntary movements the spinal 

 mechanisms, with coordinating functions, are also set in action, in this case 

 by impulses passing straight from the cortex to the mechanisms by the pyram- 

 idal tract, though apparently, in the absence of the pyramidal tract, the 

 will can work upon the mechanisms by changes travelling through other 

 parts of the cerebro-spinal axis. And in the monkey and man, subject to 

 the doubts already expressed as to the potentialities of the human spinal 

 cord, we may probably also infer that in each voluntary movement some, 

 perhaps we may say much, of the coordination is carried out by the spinal 

 mechanism set into action through impulses along the pyramidal tract. We 



