NERVE 107 



The fibres enter the spinal cord by the posterior roots of 

 the spinal nerves (fig. 46). 



(i.) Section of a series of posterior roots leads (a) to loss of 

 sensation in the structures from which the fibres come, and 

 (b) to a loss of muscular co-ordination, as a result of cutting 

 off the afferent impulses connected with the muscle-joint 

 mechanism (p. 105), and (c) to loss of tone in these muscles. 



As a result of this section, the parts of the fibres cut off 

 from the cells of the ganglia on the posterior root die and 

 degenerate (p. 76). Therefore, if the root is cut between 

 the ganglion and the cord, the degeneration extends inwards 

 and up the posterior columns of the cord, and, if it is cut 

 outside the ganglion, the degeneration passes outwards to the 

 perij^hery. 



Sthnulation (a) of the central end may cause reflex 

 movements and pain ; (6) of the peripheral end may, in 

 certain situations, cause dilatation of blood-vessels (p. 98). 



(ii.) Section of the anterior root causes paralysis of the 

 muscles and other structures supplied by the outgoing 

 fibres, and the fibres die and degenerate (p. 76). 



Stimulation of the peripheral part causes contraction of 

 the muscles supplied. 



2. Course of the upgoing Fibres in the Spinal Cord. 



In the spinal cord the course of the upgoing fibres has 

 been traced by experiment, by clinical observation, and by 

 pathological investigation, chiefly in man and in the ape. 



(1) Section of Posterior Roots. — As already pointed out, 

 when the posterior roots are cut reflex action is abolished and 

 all sensation in the part of the body supplied by the nerve 

 likewise disappears. 



(2) Section of Spinal Cord. — If the spinal cord is cut across, 

 after a certain period of spinal shock, reflex action 

 occurs and may be increased, but all sensation is absent 

 below the level of the section. 



It must therefore be concluded (1) that these ingoing 

 nerves act upon the effector mechanism in the cord by 



