494 ELEiMENTARY PHYSIOLOGY less. 



(iv) The Pre-pyramidal Tract. —Like the descending 

 tracts which we have already mentioned tlie pre-pyraniidal 

 tract ends in connection witli the anterior horn cells. 

 The impulses which its fibres carry come, however, not 

 from the cereljrum but from the cerebellum We have 

 seen (p. 471) that a sensory impulse reaching the central 

 nervous system by a fibre in the posterior root may 

 expend itself either in the cerebrum, the cerebellum or 

 the anterior horn ; we now see that an anterior horn 

 cell (motor neuron) may be played upon by impulses 

 coming either from the cerebrum, the cerebellum or the 

 sensory neurons of the posterior roots. 



Such are the functions of the spinal cord, taken as a 

 whole. The spinal nerves arc, as we have said, chiefly 

 distributed co the muscles and to the skin. But other 

 nerves, such as those for instance belonging to the blond- 

 vessels, the so-called vatto-motor nerves (Lesson II. p. 6J)), 

 though many of them run for long distances in the sym- 

 pathetic system, may ultimately be traced to the spinal 

 cord. Along the spinal column the spinal nerves give off 

 branches which run into and join the sympathetic system. 

 And the vaso-motor fibres which run along in the sym- 

 pathetic nerves do really spring from the spinal cord, 

 finding their way into the symi)athetic system through 

 these communicating or commissural branches. Besides 

 which, some vaso-motor fibres run in spinal nerves along 

 their whole course. 



Experiments, moreover, go to show that the nervous in- 

 fluences which, through these vaso-motor nerves, regulate 

 the blood-vessels, now forcibly constricting them, now 

 allowing them to dilate, and now keeping them in a state 

 of moderate or tonic constriction, proceed from the spinal 

 cord. 



The cord is, therefore, spoken of as containing centres 

 for the vaso-motor nerves or, more shortly, vaso-motor 

 centres. 



For example, the muscular walls of the blood-vessels 



