.386 



PHYSIOLOGY 



The anatomical basis of the various events involved in the carrying 

 out of such a reflex as that just studied is shown in the diagram (Fig. 

 172). In this diagram the nerve-fibre a represents the pain-receiving 

 or nociceptive nerve from the skin of the foot. This passes by a 

 posterior root into the spinal cord, where it divides and gives off a 

 number of collaterals. These collaterals, as we have already seen, 

 pass in various directions ; some to the neighbouring grey matter, some 

 to the centres in the higher parts of the nervous system. Neglecting 



oc 



FIG. 172. 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 + indicates an excitatory 

 effect, the sign an inhibitory effect. (SHERKINGTON.) 



the latter and any intermediate neuron which may be intercalated 

 between the afferent fibre and the motor cell, we see that those col- 

 laterals which affect the motor cells of the muscles of the two hind 

 limbs can be divided into two sets, one of which always produces 

 during activity excitation in certain efferent neurons, whilst the 

 other produces inhibition of the efferent neurons of the antagonistic 

 muscles. The single afferent nerve fibre is therefore, with regard to 

 one set of its central terminal branches, specifically exciter, and, in 

 regard to another set of its central endings, specifically inhibitor. In 

 the case in point the central terminal branches of the nerve a are 



