n 



nu 



Wr\-^ / 



HI 



su ^ 



h.c ,^" 



N.C 



SPG 



ii±f^v:^MI A ^\ 



i^F 



21. — Diagrams of reflex actions. — Modified from 



Bayliss's Principles of General Physiology. 



I. In a sea-anemone a stimulus from a sensory nerve-cell or neurone 

 (S.C.) at the surface (SU.) may pass directly bv a sensory fibre 

 (S.F.) to a muscle (MU.). 

 II. In some cases, e.g. in the earthworm, the stimulus passes into the 

 ventral nerve-cord (N.C.) and directly affects a motor nerve-cell or 

 neurone (M.C.). Thence an impulse travels by a motor fibre (M.F.) 

 to the muscle (MU.). 



III. More usually in the earthworm and similar types there are associative 



neurones (A.) interpolated in the nerve-cord (N.C.) between the 

 branches of the sensory nerve fibre (S.F.) and the dendrites of the 

 motor neurones (M.C.). 



IV. In Vertebrates from a nerve-ending (N.E.) on the surface (SU.) the 



stimulus passes by a sensory fibre to the sensory neurones in the 

 spinal ganglia (on the dorsal roots of the spinal nerves) and thence 

 into the spinal cord (SP.C). Associative or internuncial neurones 

 (A.) connect the branches of the sensory nerve-fibre with the den- 

 drites of the motor neurone (M.C.). The impulse travels along the 

 axis cylinder or motor fibre to the muscle (MU.). 



