THE SPINAL CORD AND REFLEX ACTIONS. 595 



collected on their way to the encephalon. It does, it is true, 

 contain the paths for the conduction of all those impulses 

 which, originating in the cerebrum, give rise to voluntary 

 movements of the trunk and limbs ; also for all the centrally 

 travelling impulses which give rise to sensations ascribed to 

 those parts ; and it is also the path for certain impulses^giving 

 rise to involuntary movements as, for example, those which, 

 originating in the respiratory centre, travel to the phrenic and 

 intercostal nerves. 



If, however, the cord were merely collected and continued 

 nerve-roots it ought to increase considerably in bulk as it ap- 

 proached the skull, and this it does not do in anything like 

 the required proportion; a histological enumeration also shows 

 that the total number of fibres cut across in a transverse sec- 

 tion of the cord in the upper cervical region is far less than 

 the total number of fibres in all the spinal nerve-roots. Most 

 of the root-fibres, in fact, pass at once into the central gray 

 mass and their axis cylinders end in its cells, or lose their in- 

 dividuality by joining its network of cell branches and fine 

 non-medullated fibres. Most of the fibres of the anterior root 

 end in nerve-cells near the level at which they join the cord, 

 especially in the cells of the anterior horns : many of the fibres 

 of the posterior roots also join the gray network, either at or 

 a little above or a little below the level at which they reach 

 the cord, but some appear to run on to the brain without en- 

 tering the gray core. Those which do pass into it probably 

 break up in its network and are not directly continued into 

 a cell, but this is still uncertain. In correspondence with the 

 fact that most of the spinal nerve-fibres have their primary ter- 

 mination in it near their point of entry, is the fact that the 

 amount of gray matter at any level is greater or less accord- 

 ing as the nerve-roots at that level are large or small: the 

 cervical and lumbar enlargements for example are almost 

 entirely due to increase of gray matter in those regions. 

 When we make a voluntary movement of a limb the impulse 

 orginating in the brain does not pass directly to the motor 

 nerves of the muscles concerned, but to a mechanism in the 

 gray matter of the cord, which is in connection with those 

 muscles ; and when we feel an object touching the finger, the 

 afferent impulses probably, though not so certainly, first enter 

 the gray core of the cord and thence make a fresh start to 

 the brain. When the blood-vessels constrict on painful stimu- 



