THE SPINAL CORD AS A REFLEX CENTRE 



369 



accompanying diagram (Fig. 163) from Sherrington shows the manner 

 in which the skin fields of these segments are combined to make up 

 the total skin area in the hind limb of the monkey. 



THE WAY OUT 



Primitively the motor nerves also represent fibres passing from 

 a collection of ganglion- cells to the muscles of the corresponding 

 body segment. In the dorsal region this segmental arrangement 

 of motor nerve fibres is still traceable in the adult animal. 

 In all other parts the morphological has become subservient to a 

 physiological arrangement. Every muscle of the limbs contains 

 elements from several segments, and is innervated therefore from 



Tad- 



r uenfsaiL 



titit. of ' triL 



several anterior spinal roots. Hence it follows that stimulation of 

 one anterior root produces no definite movement of a group of muscles- , 

 but partial contraction of a number of muscles which do not normally 

 contract simultaneously. Thus stimulation of a sensory nerve may 

 evoke either flexion or extension of a limb, but not both simultaneously. 

 Stimulation of the motor roots will cause simultaneous contraction 

 of both flexor and extensor muscles. It is this subordination of 

 morphological to physiological arrangement in the limbs which 

 has necessitated the formation of limb plexuses. The nerve-root 

 is a morphological collection of fibres ; the nerve issuing from a 

 limb plexus and passing to a group of muscles is a physiological 

 collection. When it is stimulated it evokes a contraction of a group 

 of muscles which are normally synergic, i.e. co-operate in various 

 movements. 



The fibres passing to the skeletal muscles are large, about 14 />. 

 to 19 M in diameter, and their axis cylinders represent the axons of 



24 



