778 VOLUNTARY MOVEMENTS. [BOOK in. 



or at least the spinal motor mechanism for any one muscle, 

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

 untary 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 adjoin- 

 ing 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 com- 

 plex, in the fingers for instance, not only is each muscle sup- 

 plied 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 

 mechanisms. 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 pyramidal 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 cerebrospinal axis. And in the 

 monkey and man, subject to the doubts already expressed as 

 to the potentialities of the human spinal cord, we may prob- 

 ably 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 may probably further infer that a care- 

 ful adjustment obtains between the beginnings of the pyra- 

 midal tract in the cortex and its endings in the cord, so that 

 the topography of ' areas ' or ' foci ' in the cortex above is an 

 image or projection of the spinal mechanisms below. 



The complex character, on which we insisted just now, of 

 almost every voluntary movement necessitates that in every 

 such movement a large area of spinal mechanism is involved. 

 But this is not all. The movements of any part, of the legs 

 for instance, are not determined, nor is the coordination of the 

 movements effected, simply by what is going on in the legs and 

 the part of the spinal cord belonging to them. The discussion 

 in a previous section has shewn that much of the coordination 



