CH. XVII.] 



NERVE-CELLS. 



197 



nerve-cell in actual continuity with the sensory nerve-fibre is the 

 one in the spinal ganglion (G) from which it grew. 



The terminal arborisation of the sensory nerve-fibre merely 

 interlaces with the dendrons of the motor nerve-cell ; yet simply 

 by this synapse, the motor nerve-cell (M C) is affected and sends 

 an impulse by its axis cylinder process to the muscle (M). 



A very rough illustration which may help one in realising this 

 may be taken as follows : Suppose two trees standing side by 

 side ; their stems will represent the axis cylinders ; their branches 

 the dendrons. If the trees are close together the branches of one 



Axon 



Vijf. 203. Cell of PurkinJK from the human cervbvlhiui. 

 Oolgi's methoil. (After Szyinonowicz.) 



will intermingle with those of the other : there is no actual 

 branch from the one which becomes continuous with any branch 

 of the other ; but yet if the stem of one is vigorously shaken, 

 the close intermixture of the branches will affect the other so 

 that it also moves. 



Another very important general idea which we must next get 

 hold of, is that a nervous impulse does not necessarily travel 

 along the same nerve-fibre all the way, but there is what we 

 may term a system of relays. The nervous system is very often 

 compared to a telegraphic system throughout a country. The 

 telegraph offices represent the nerve-centres, the afferent nerve- 



