96 Instinct and Intelligence 



effective struc- 

 tures. Soon 

 after leaving its 

 cell the axis 

 cylinder be- 

 comes encased 

 by an inner 

 sheath of fatty 

 matter, and an 

 outer sheath 

 known as its 

 neurilemma ; it 

 is along the 

 central axis or 

 conducting core 

 of the fibre 

 that impulses 

 travel to and 

 leave the body 

 of nerve cells. 

 Until the axis 

 cylinder of a 

 nerve cell has, 

 in the course of its development, become en- 

 closed in its protecting sheaths (or, technically, 



t 



FIG. 12. Diagram of a ganglionic 

 nerve-cell (neuron), d d d, dendrons ; 

 n, nucleus and nucleolus ; a.c, axis- 

 cylinder ; m, medullary sheath ; s, neuri- 

 iemma ; t, terminal branches. (Human 

 Speech, p. 147.) 



