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SCIENCE PROGRESS 



the axial core of a nerve fibre : this is termed the axon, the 

 others are termed dendrons. All nervous action is reflex, and 

 the simplest reflex act is the first term of a series, of which 

 the most complex volition is the last. Therefore before pro- 

 ceeding to discuss the brain, the most complex organ in nature 

 both as regards structure and function, let me call your atten- 

 tion to the simplest form of nervous system illustrated in this 

 diagram. You observe S (fig. 1) is a sensory nerve-cell with 





Fig. 1. 



branching processes; one branch ends in the skin, the other 

 branch proceeds centrally, and this you see breaks up into a 

 number of fine terminals which are brought into relation with 

 the branching processes of M 1 , a motor cell ; proceeding away 

 from this cell is a process, the motor nerve, which terminates in 

 a muscle connected with the sensitive skin. Stimulation of the 

 sensory nerve in the skin, it matters not whether it is chemical 

 or physical, produces what is known as an afferent nervous 

 stimulus, which travels in the direction shown by the arrow to 



