906 REFLEX ACTIONS. [BOOK m. 



complex reflex action by a slight pressure on or other stimulation 

 of the skin than by even strong induction-shocks applied directly 

 to a nerve trunk. If, in a brainless frog, the area of skin supplied 

 by one of the dorsal cutaneous nerves be separated by section 

 from the rest of the skin of the back, the nerve being left attached 

 to the piece of skin and carefully protected from injury, it will be 

 found that slight stimuli applied to the surface of the piece of 

 skin easily evoke reflex actions, whereas the trunk of the nerve 

 may be stimulated with even strong currents without producing 

 anything more than irregular movements. In ordinary mechanical 

 and chemical stimulation of the skin it is not a single impulse but 

 a series of impulses which passes upwards along the sensory nerve, 

 the changes in which may be compared to the changes in a motor 

 nerve during tetanus. In every reflex action, in fact, the central 

 mechanism may be looked upon as being thrown into activity 

 through a summation of the afferent impulses reaching it. Hence 

 while a reflex action is readily called forth by even feeble induction- 

 shocks applied to the skin if they be repeated sufficiently rapidly, 

 a solitary induction- shock is ineffectual unless it be strong enough 

 to cause in the skin or nerves changes of an electrolytic nature 

 sufficient to give rise of themselves to a series of impulses. 



585. When a muscle is thrown into contraction in a reflex 

 action, the pitch of the sound which it gives forth does not vary 

 with the stimulus, but is constant, being the same as that given 

 forth by a muscle thrown into contraction by the will. From 

 which we infer, even bearing in mind the discussion in 80 

 concerning the nature of the muscular sound, that in a reflex 

 action the afferent impulses do not simply pass through the centre 

 in the same way that they pass along afferent nerves, but are 

 profoundly modified. And in accordance with this we find, as we 

 shall see, that a reflex action takes up an amount of time, the 

 greater part of which is spent in the carrying out of the central 

 changes, and which though variable is always much longer, and 

 may be very much longer, than that taken up by the mere passage 

 of a nervous impulse along a corresponding length of nerve fibre. 

 The term reflex action is therefore an unsuitable one. The 

 afferent impulse is not simply reflected or turned aside into an 

 efferent channel ; on its arrival at the centre it starts changes of 

 a different nature from and more complex than its own ; and the 

 issue of efferent impulse is the result of those more complex 

 changes not the mere continuation of the simpler afferent impulse. 

 In other words, the interval between the advent at the central 

 organ of afferent, and the exit from it of efferent impulses, is a 

 busy time for the nervous substance of that organ ; during it 

 many processes, of which we have at present very little exact 

 knowledge, are being carried on. 



586. The character of the movement forming part of a 

 reflex action is also influenced by the intensity of the stimulus. A 



