CHAP. IL] THE BRAIN. 1067 



(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 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 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 coordina- 

 tion afferent impulses play an important part. The study of reflex 

 actions, 589, has led us to suppose that each spinal segment 

 presents a nervous mechanism in which a certain amount of co- 

 ordination 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, or at least 

 the spinal motor mechanism for any one muscle, extends over two 

 or three segments. Hence a fortiori in a voluntary movement, 

 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 overlying a 

 muscle served, broadly speaking, by nerve roots of the same 

 segment as the 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 



