ORGANS OF THE SENSORI-MOTOR SYSTEM 113 



are necessary : (i) that which has a receptor as its peripheral 

 termination and (2) that which has a " motor-plate " (in a muscle) 

 as its peripheral termination. Some physical agency reacts with 

 the receptor and initiates an impulse w^hich flows along the 

 dendron, or aff"erent path, into a synapse in the ganglion. There 

 the impulse passes through the dendrites into the motor-cell 

 and then along the axon of the latter, or efferent path into the 

 motor plate which is its termination in the muscle-fibre. The 

 change thus set up in the motor-plate stimulates the muscle- 

 fibre to contract, that is, somehow it releases potential energy 

 in the latter effector organ. 



The description is a fiction (i) because more than one receptor 

 is involved in the original stimulation, (2) instead of one dendron 

 there is really a number of such making up a nervelet, (3) there 

 are many synapses, many efferent axons and many muscle-fibres. 

 But even then the scheme is much too simple. 



The actual reflex arcs. We may approximate further towards 

 actuality as follows : 



Spinxxl 

 cord. 



Receptor 

 trvskin 8cc. 



Fig. 21. — Diagram of the Minimal Combinations of Neurones in an 



ACTUAL Reflex Arc. 



The reflex may be the kicking movements of the hind leg in a 

 sleeping dog w^hen the skin of the flank is tickled. Two sets 

 of muscles are the effectors, (i) those that flex the leg and (2) 



