810 THE CENTRAL NERVOUS SYSTEM 



is much more nearly proportional to the strength of the stimulus. Thus, 

 a feeble stimulus applied to the flank calls forth only a slight flexion 

 of the hind limb of the same side, whereas a stronger stimulus sets 

 going a typical scratching movement. 



3. After-effect. — When a stimulus is removed from a nerve, the effect 

 which it produces, as judged, for example, by the action current, im- 

 mediately disappears. There is no after-response. In reflex arcs, how- 

 ever, such a phenomenon is usually well marked. Particularly is this 

 the case in the flexion and scratch reflexes of the spinal dog. A mo- 

 mentary stimulus of optimal strength applied to the scratch skin-area 

 may produce no immediate response, but after its removal a violent 

 scratching movement may set in. This after-discharge, in cases in which 

 the. stimulus is strong, may indeed, as in the flexion reflex, be more 

 marked than the response during the time of application of the stimulus. 

 In this particular reflex, the after-discharge often takes the form of a 

 clonus, with a rate of contraction of from seven and a half to twelve 

 per second. The crossed extension reflex also has a very pronounced 

 after-discharge, which may outlast the stimulus for from ten to fifteen 

 seconds. Regarding the phenomenon of after-discharge, Sherrington 

 has stated that there is "no feature of the conduction of a reflex arc 

 which distinguishes its mechanism more universally from that of a 

 nerve fiber, tract or trunk than lengthy after-discharge." 



4. Summation. — When a subliminal stimulus — that is, one that has 

 in itself no visible effect — is frequently repeated in the case of a nerve, 

 no response occurs. In the case of a reflex arc, however, such repeti- 

 tion of subliminal stimuli ultimately calls forth response. This sum- 

 mation is wry evident in the case of the scratch reflex; e. g., one or 

 two elect l'ieal stimuli applied to the scratch field-area call forth, as a 

 rule, no movement of the coi'responding hind leg, but if these same 

 stimuli are frequently repeated, the typical reflex scratching movement 

 will occur. Evidently, then, in a reflex arc there is a considerable 

 amount of resistance towards a single stimulus, which resistance is 

 overcome by a succession of stimuli. In other words, the threshold of 

 the excitability of the reflex mechanism becomes lowered as a result 

 of its previous stimulation. Each stimulus excites the sensory surface 

 so that it responds more easily to the succeeding stimulus. 



5. Irreversibility of the Direction of Conduction. — This is well illus- 

 trated in the so-called Bell-Magendie law of conduction in the spinal 

 nerve roots. A motor impulse travels oul of the cord by the anterior 

 roots, while a sensory impulse travels in by the posterior. This direc- 

 tive influence can not depend on the nerve trunks or the nerve cells, for 

 nerve trunks conduct equally in both directions, and so also must the 

 nerve cell. The irreversibility must therefore depend on the synaptic 



