84 PHYSICAL EXPRESSION. 



being acted on by external stimuli (a), the other 

 is distributed to muscular fibres, which on contrac- 

 tion cause diminution of the body cavity (d). Im- 

 pressions made on the sensory surface are conveyed 

 by the afferent fibres to the central ganglion, 



Fig. 3. Nervous system of an Ascidian (Carpenter), a, the mouth ; I), the vent ; 

 c, the ganglion ; d, the muscular sac. 



whence an impulse is sent along the efferent fibres, 

 causing contraction of the muscles. Such an action 

 is termed a reflex action, a term derived from the 

 reflection, as it were, of the afferent impression 

 back to the periphery." * The ganglion produces 

 the motor-power ; the muscular fibres are the motor- 

 apparatus, acting when they receive an impulse 

 from the ganglion. 



Now, passing up to the vertebrates. Here we 

 find a well-defined portion of the body, the nerve- 

 mechanism which, when properly nourished, pro- 

 duces motor-force; there are also well-defined 

 muscles, connected by nerve-fibres with the nerve- 

 centres. When a stimulus passes from the nerve- 



* Ferrier/'qp. cit., p. 16. 



