MENTAL DEVELOPMENT 297 



the terminals of the sensory neurone S, where it excites the 

 terminals of the motor neurone M, giving rise to an outgoing 

 efferent current which stimulates the muscles and causes its 

 contraction. 



Let us suppose the stimulus to be a painful and therefore 

 a harmful one, the effect of the neuro-muscular mechanism will 

 be a protective reflex action, the contracting muscle with- 

 drawing the skin surface from the cause of the pain. You 

 will observe that the diagram shows that the sensory neurone 

 consists of a cell with a process which divides into two branches ; 

 one proceeding to the skin — this is the sensory nerve — the other 

 branch dendron proceeding centrally to end in a terminal 

 arborisation. The current of nervous action resulting from the 

 stimulus always proceeds towards the centre ; it is afferent ; the 

 fine terminals of the central projection of the nerve cell are in 

 physiological (that is functional) but not anatomical connection 

 with the branching processes, dendrites of M, the motor cell- 

 This alterable functional connection is spoken of as the synapse ; 

 the motor cell, M 1 , gives off one process which becomes the 

 essential conducting axial core of a motor nerve fibre which ends 

 in the muscle ; and the current of nervous action along this is 

 always outgoing or efferent. We have thus two systems of 

 neurones : (a) afferent sensory, {b) motor efferent. There is yet 

 another neurone, A, which you observe associates the synapse 

 of S and M 1 with a second motor neurone element M 2 , which 

 innervates another muscle that is antagonistic in its action to 

 that supplied by M 1 . Stimulation of the sensory nerve in the 

 skin may give rise not only to reflex contraction of the muscle 

 supplied by M 1 , but also through the association neurone A, to 

 relaxation by inhibition of contraction of the muscle supplied 

 by M 2 . 



The special function of the brain is inhibition or control of 

 instinctive reflex action, and this is done by its associative 

 memory of past experiences. 



The neurones, I have said, are independent nervous units ; 

 they are in anatomical contiguity but not in continuity. The 

 cerebro-spinal and sympathetic nervous systems are made 

 up of neurones which we may regard as complex highly 

 differentiated cells obeying, however, the same laws of nutrition, 

 repair, and waste as other cells of the body. 



The neurones are the essential nervous elements, and they, 



