252 THK GROWTH OF THK BRAIN. 



nerves in the spinal cord and from their relations to 

 the muscles of the limbs. The same muscles may be 

 thrown into contraction by stimulating any one of 

 several of the motor roots. If, then, the group of 

 cells giving rise to these fibres is designated as the 

 nucleus for the nerve fibres controlling that muscle, it 

 is found that such a nucleus may extend through several 

 spinal segments, and that the muscles used in the fine 

 adjustments at the extremity of the limb, those of the 

 hand, for instance, have in the cord extensive nuclei. 

 It appears that by thus drawing into a long column a 

 group of cells which gives rise to the controlling fibres, 

 the muscle is not only put under the direction of a large 

 number of efferent cells, but that since these cells are 

 located at various levels, special groups of them are 

 directly accessible to different incoming impulses ; and 

 thus the co-ordinated responses of the muscles are 

 refined. What has been said strictly applies only to 

 the cells giving rise to the efferent fibres. Concerning 

 the distribution of the afferent fibres, there is much less 

 exact information. On this side, however, two relations 

 are well established : first, that by way of the afferent 

 fibres, impulses may directly reach the nuclei of the 

 efferent nerves ; and second, that also by way of them, 

 impulses may reach these cells via the cerebral hemi- 

 spheres, usually the hemisphere of the opposite side. 

 The accompanying figure illustrates this relation. 



The principal path followed by these impulses on 

 the way to the higher centres is either along the 

 dorsal columns of the cord, on the same side as 

 far as the lower end of the bulb, where the im- 

 pulses, having passed to a new set of cells, cross to 

 the opposite side of the axis and terminate in the 

 corresponding hemisphere ; or across the median 

 plane to the lateral column, and so cephalad to the 



