CH. XVII.] 



CELL-STATIONS. 



199 



U. 



s.c/ 



(S. C.) from which the motor nerve-fibre arises, and which carries 



on the impulse. The spinal cord cells 



are thus surrounded by arborisations 



(synapses) derived not only from the 



si-iisory nerves (S), 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 



impulses from the brain. 



The sheaths of the nerve-fibres are not 

 shown in the diagram. 



The system of relays is still more com- 

 plicated in the case of sensory impulses, 

 as we shall see later on ; the same is 

 true for the motor path to involuntary 

 muscle, accessory cell-stations being situ- 

 ated 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 pyramidal fibre were 

 cut across, the piece still attached to the 

 brain-cell would remain in a comparatively 

 normal condition, but the peripheral end 

 would degenerate as far as the synapse 

 round the spinal cell (S. C.), but not 

 beyond. We can thus use the degenera- 

 tion 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 as that 

 already described in the nerves, except 

 that, as there is no primitive sheath, 

 there can be no multiplication of its 

 nuclei ; there is instead an over-growth 

 of neuroglia. Degenerated tracts conse- 

 quently stain differently from healthy 

 white matter, and can be by this means 

 easily traced. 



Another method of research which 

 leads to the same results as the degenera- 

 tion method is called the embryological method. The nerve-fibres 

 which grow from different groups of nerve-cells become fully 



Fig. 205. Diagram of an 

 element of the motor 

 path. U.S., upper seg- 

 ment; L.8., lower seg- 

 ment ; C.C., cell of cere- 

 bral cortex ; 8.C., cell of 

 spinal cord, in anterior 

 cornu ; M . the muscle ; 8, 

 path from sensory nerve 

 roots. (After Oowen.) 



