1 8 Foster, Physical Basis of Psychical Events. 



impulse, or a group or series of impulses travelling along a 

 sensory fibre, along the axon of a sensory afferent unit, 

 impinges on the body or dendrites of one or of more than 

 one motor unit ; forthwith there issue along the axons of 

 these motor units, impulses which are wholly incommen- 

 surate with the exciting impulse. That exciting impulse 

 or that series of exciting impulses may be exceedingly 

 feeble and yet may, under suitable circumstances affecting 

 the motor unit, give rise not only to numerous but to most 

 powerful impulses in the motor unit. A reflex action is 

 not, as the term would seem to imply, the mere reflection 

 of a centripetal afferent sensory impulse into a centrifugal 

 efferent motor impulse. The issuing motor impulses of a 

 reflex action are the outcome of the special activity of the 

 motor unit. The characters of that activity are, in the 

 main, determined by the features, properties, circum- 

 stances of the motor unit itself; the afferent impulses 

 do little more than start the activity of the motor 

 unit, they are to be regarded as of the nature of a 

 stimulus which awakes the irritability of the motor unit, 

 somewhat as they themselves were started by an external 

 stimulus applied to a sensory unit. The manifestations of 

 the motor unit may, it is true, be not only started, but also 

 in a measure determined by the characters of the sensory 

 impulses, a strong, sensory impulse producing effects 

 different from those of a weak one, and so on ; but the real 

 features of the motor act are determined by the individu- 

 ality of the motor unit. 



Moreover, the effect of the advent of afferent impulses 

 at the doors of a motor unit may be, not as in an ordinary 

 reflex act, the development of motor impulses, and so of 

 movement, but the very reverse, the arrest of movement ; 

 the effect may be inhibitory. And this, we have reason 

 to think, may take place without our being able to detect 

 any differences in the nature of the afferent impulses 



