342 PHYSIOLOGY 



for the extensor muscles of the same side and for the flexor muscles of the 

 opposite side. 



The ascending branches of the nerve fibre in the same way will have 

 endings which, while inhibitor for the greater number of other possible 

 reflex changes, will be excitor in a slight degree for certain efferent 

 neurons whose action is allied to that of the primary reflex. The diagram 

 shows also that the contraction of the flexor muscle, set up as the result 



FIG 171 Diagram indicating connections and actions of two afferent spinal 

 root-cells a and a', in regard to their reflex influence on the extensor and flexor 

 muscles of the two knees. The sign -f indicates an excitatory effect, the 

 sign an inhibitory effect. (SHEBBINGTON.) 



of stimulating a, itself initiates a secondary reflex process from muscle up 

 the nerve fibre a and back again to the muscle by the efferent neuron. This 

 muscular afferent nerve also has central terminations of two signs excitor 

 to itself and inhibitor to the antagonistic muscles. For the sake of clearness 

 the diagram omits a number of other channels coming from other regions of 

 the cord, or from other efferent nerves, the sign of which would be negative, 

 i.e. which would tend to inhibit the activity of the whole reflex arc. 



We see therefore that from every sensitive point on the surface of the body 

 impulses can be initiated which will set into action whole chains of neurons, 

 and will have a widespread influence throughout the central nervous system. 

 It is important to note that the efferent path innervating, say, the flexor 

 muscles of one side is common to many reflexes. It is used, for instance, by 

 mutually antagonistic reflexes such as the scratch reflex and the flexor or 



