190 



NERVE-CENTRES 



[CH. XVI. 



part of the cord which controls the arm movements, and terminates 

 by arborising around small cells at the base of the posterior cornu 



(P.C.C.); thence the im- 

 pulse is transferred to the 

 large motor cells of the 

 anterior cornu (A.C.C.), 

 and the final link in the 

 chain is formed by the 

 motor nerve-fibres to the 

 muscular fibres (M). The 

 spinal cord cells are thus 

 surrounded by arborisa- 

 Itions (synapses), derived 

 not only from the sensory 

 [nerves but by fibres from 

 the upper part of the 

 nervous system. We now 

 see how it is possible that 

 reflex actions in the cord 

 may be controlled by im- 

 pulses from the brain. 

 [>;f| The system of relays 

 is still more complicated 

 in the case of sensory im- 

 pulses, as we shall see later 

 on; the same is true for 

 the motor path to involun 

 tary muscle, accessory cell- 

 stations being situated in 

 the sympathetic ganglia. 



We may now return for 

 a moment to the subject of 

 degeneration. If the nerve- 

 fibre is cut off from its 

 connection with the spinal 

 nerve-cell, the peripheral 

 end degenerates as far as 

 the muscle. 



Suppose, now, the pyra- 

 midal fibre were cut across, 

 the piece still attached to the brain-cell would remain in a compara- 

 tively normal condition, but the peripheral end would degenerate as 

 far as the next synapse. We can thus use the degeneration method 

 to trace out tracts of nerve-fibres in the white matter of the central 

 nervous system. The histological change in the fibres is here the same 



FIG. 185. Diagram of the neurons of the motor path 



