212 STUDIES IN ELECTRO-PHYSIOLOGY: 



The following illustration is given to explain reflex 

 action : 



t^ 



ACC 



Fig. 116. REFLEX ACTION. (After Hallibwton.) 



Excitation occurs, we will say, at a sensory surface S, 

 and the impulse is transmitted by the sensory nerve-fibre 

 to the central nervous system. " This fibre does not 

 become anatomically connected to any of the cells of the 

 central nervous system. The only cell -body in actual 

 continuity with the sensory nerve-fibre is the one in the 

 spinal ganglion (G) " (a storage cell). " On entering the 

 spinal cord the main fibre conveys impulses upwards which 

 ultimately reach the brain, but in the spinal cord it gives 

 off fine side branches or collaterals which terminate by 

 arborising around one or more cell -bodies and their den- 

 drons ; these cells are small ones situated in the posterior 

 cornu of the spinal grey matter ; one only (PCC) is shown 

 in the diagram. The short axon of this cell similarly ter- 

 minates by a synaptic junction with one or more of the 

 large multipolar cells of the anterior cornu of the spinal 

 grey matter ; one of these shown in the figure is labelled 

 ACC. This motor cell is thus stirred up to action and 

 sends an impulse by its axon to the muscular fibres it 

 supplies." (Halliburton.) 



I may remark, in parenthesis, that we have here 

 evidence of condenser-action, of cells changing the sign of 



