EELATION BETWEEN STRUCTURE AND FUNCTION. 11 



the whole root is excited ; but the action of each muscle 

 is feebler. The collection of nerve-fibres contained in the 

 rootlet is therefore, in miniature, a collection like that 

 contained in the whole root. This fact must be taken 

 together with the fact that, as can be shown by degeneration 

 experiments, the fibres contained in each rootlet repre- 

 sent the nerve-fibre processes, or axis cylinder processes, 

 of the motor nerve-cells lying in that particular transverse 

 plane or level of the spinal cord, from which the rootlet 

 makes its emergence. The two facts together show that 

 the nerve-cells which innervate the different muscles 

 belonging to any one myotom in the limb, lie commingled 

 together, so that any one section through the spinal 

 cord, taken at right angles to its length, will meet nerve- 

 cells belonging to all the muscles represented in that 

 segment. This is the same as saying that the nerve-cells 

 belonging to any one limb -muscle, lie not gathered 

 together at any one particular level of the spinal segment, 

 but scattered throughout its length. And, inasmuch as 

 each muscle is innervated from several segments, the 

 nerve-cells for each muscle are scattered in a continuous 

 series through the length of a series of spinal segments ; 

 and throughout this extent are commingled with the cells 

 of a great number — in some cases as many as forty and 

 more — of other muscles. 



It is, therefore, evident how it comes about that no 

 traumatic injury of the spinal cord can ever paralyse a 

 single muscle alone and apart from its fellows. Even 

 the severance of a whole single motor nerve-root cannot 

 paralyse a single limb-muscle ; the effect of such an injury 

 is to partially impair a large number of the muscles. 



It has been urged by many observers that the motor 

 nerve-fibres gathered together in each motor spinal-root 

 are a collection connected for functional co-operation. 



