8oo A MANUAL OF PHYSIOLOGY 



reversed. (2) Its velocity over the whole reflex arc is much 

 smaller than over a nerve-trunk of equal length. Both of these 

 differences depend mainly on the fact that the impulses must be 

 transmitted from one neuron to another, and very likely on a 

 fundamental property of the synapse. (3) The reflex arc is easily 

 fatigued, easily affected by deprivation of oxygen and by drugs, 

 in comparison with the nerve-trunk. This difference is due to 

 the portion of the arc in the grey matter, including the synapse 

 or synapses. (4) The reflex end-effect may much outlast the 

 stimulus in other words, a marked ' after-discharge ' is charac- 

 teristic of reflexes. The more intense the stimulus which 

 liberates the end-effect, the greater is the duration of the after- 

 discharge. For example, the ' crossed extension reflex ' (ex- 

 tension at the knee, ankle, and hip, produced in the spinal dog 

 by stimulation of the skin of the opposite or contralateral hind- 

 limb), when provoked by a stimulus of more than a certain 

 intensity, may outlast the stimulation by ten or fifteen seconds, 

 and the after-discharge may be stronger than any other part of 

 the reflex (Sherrington) . (5) A succession of impulses may 

 easily pass along a reflex arc when one of the series would fail 

 to pass (temporal summation). This does not occur in a nerve- 

 trunk. The first stimulus, though itself unable to produce the 

 reflex effect, facilitates the action of succeeding stimuli, so that 

 summation of the impulses occurs in the cord (Stirling). A 

 stimulus e.g., a make-induction shock, far too weak to produce 

 the scratch-reflex when applied once only to a point of that area 

 of skin from which the reflex is normally elicited has been 

 seen to cause the reflex after more than forty shocks had been 

 delivered at the rate of eighteen per second. (6) The rhythm 

 and intensity of the reflex end-effect correspond much less 

 closely with the rhythm and intensity of the stimulus than in 

 nerve-trunks. (7) The phenomena of refractory period (p. 141), 

 inhibition and ' shock,' are much more conspicuous in the reflex 

 arc than in nerve-trunks. 



Inhibition in Reflex Action. Special emphasis must be laid 

 upon the part played by inhibition in reflex actions. For the 

 proper carrying out of many reflex movements it is necessary 

 not only that the appropriate effector organ, the appropriate 

 muscle, or group of muscles, should be caused to contract at 

 the proper time, but that their contraction, or that of other 

 muscles, should be diminished or abolished by inhibition, or even 

 rendered for a certain period impracticable by the establishment 

 somewhere in the reflex arc of a refractory state, which is itself 

 a phenomenon of inhibition. There is good evidence that this 

 is a central inhibition i.e., it depends on some process occurring 

 in the spinal portion of the reflex arc. 



