464 



NATURE 



[September 8, 1904 



leg's flexors, is cut short concomitantly with or preparatory 

 to the entrance into contraction of their antagonists, the 

 left extensors. Fig. 3 shows a record of this. This in- 

 hibition of the flexor scratching movement occurs some- 

 times when the contraction of the extensors is minimal or 

 hardly perceptible (Fig. 3). As before, the inhibition may 

 temporarily interrupt a reflex or may delay its. onset, or 

 simply cut it short, the result depending on the time re- 

 lations of the applications of the stimuli to the conflicting 

 arcs. 



It is obvious from this that the final common path, FC, 

 to the flexor muscle can be controlled by, in addition to 

 the before-mentioned arcs, others that actuate the extensor 

 muscles, for it can be thrown out of action by them. The 

 final path, FC, is therefore common to the reflex arcs, not 



effect 



the signal line marking the period of stii 

 shoulder skin. '1 be strength of stimulu! 

 FC is not obtained, s^. the signal line r 

 of shoulder skin 3 centimetres from sa. 

 to evoke thi 



sa and s^ therefore reinforce one another 

 of seconds. Read from left to right. 



; and s^ of Fig. i B. 



: the fl. 



uscle of the hip 

 lUlation ot tne skin belonging to arc sa (Fig. i B) c 

 is arranged to be subminimal, so that a reflex respon 

 arking the period of stimulation, alsosubminimal, of a 

 Though the two stimuli applied separately are each u 



applied contemporaneously they quickly evoke th' 

 ' \ their action on the final 



only from the same-side foot (Fig. i B, l) and shoulder skin 

 (Fig. I B, so, s/3), but also to arcs "from the opposite foot 

 (Fig. I B, r), in the sense that it is in the grasp of all of 

 them. In this last case w^e have a conflict for the mastery 

 of a common path, not, as in the previous instance, between 

 two arcs both of which use the path in a pressor manner 

 although differently, but between two arcs that, though 

 both of them control the path, control it differently, one in 

 a pressor manner heightening its activity, the other in a 

 depressor manner lowering or suppressing its activity. 



I said that the scratch reflex is unilateral. If the right 

 shoulder be stimulated, the right hind-leg scratches; if the 

 left shoulder be stiinulated, the left hind-leg scratches. If 

 both shoulders be stimulated at the same time, one or the 

 other leg scratches, but not the two together. The one 

 reflex that takes place prevents the occurrence of the other. 

 The reason is, that although the scratch reflex appears 

 unilateral it is not strictly so. Suppose the left shoulder 

 stimulated. The left leg then scratches. If the right leg 



NO. 1819, VOL. 70] 



is then examined it is found to present slight, steady 

 extension with some abduction. This extension of the leg 

 which accoinpanies the scratching movement of the opposite 

 leg contributes to support the animal on three legs while 

 it scratches with the fourth. 



Suppose stimulation at the left shoulder evoking the 

 scratching movement of the left leg, and the right shoulder 

 then appropriately and strongly stimulated. This latter 

 stimulus often inhibits the scratching movement in the 

 opposite leg and starts it in its own. In other words, 

 the stimulus at the right shoulder not only sets 

 the flexor muscles of the leg of its own side into scratch- 

 ing action, but it inhibits the fle.\or muscles of 

 the opposite leg. It throws into contraction the ex- 

 tensor muscles of that leg. In the previous example there 

 was a similar co-ordination. The 

 motor nerve to the flexor muscle 

 Is therefore under the control not 

 only of the arcs of the scratch 

 reflex from the homonymous 

 shoulder, but of those from the 

 crossed shoulder as well. But in 

 regard to their influence upon 

 this final common path, the arcs 

 froin the homonymous shoulder 

 and the opposite shoulder are 

 opposed. The influence of the 

 latter depresses or suppresses 

 activity in the common path. 



Experiments by Verworn dis- 

 iillow any view that this kind of 

 depression has its field in the 

 motor nerve itself. Many circum- 

 stances connect it with the place 

 where the converging neurones 

 come together in the grey matter 

 at commencement of the common 

 path. The field of competition 

 between the rival arcs seems to 

 lie in the grey matter, where they 

 impinge together upon the final 

 or motor neurone. That is 

 equivalent to saying that the 

 essential seat of the phenomenon 

 is the synapse between the motor 

 neurone and the axone-terminals 

 of the penultimate neurones that 

 converge upon it. There some of 

 these arcs drive the final path 

 into one kind of action, others 

 drive it into a different kind of 

 action, and others again preclude 

 it from being activated by the 

 rest. 



My diagram (Fig. i B) treats 

 the final common path as if it 

 consisted of a single individual 

 neurone. It is, of course, not so. 

 The single neurone of the 

 diagram stands for several 

 thousands. It may be objected 

 that in the various given actions 

 these motor neurones are implicated in particular sets 

 — one set in one action, one set in another. That view 

 seems unlikely. In the scratch reflex, I think we can 

 exclude it. The rhythm of that reflex has the same 

 frequence whether it be excited strongly or feebly : thus, 

 whether the extent of the contractions be great or small 

 they recur with practically the same frequence. That a 

 muscle contracts feebly under feeble stimulation of its nerve 

 may be due in some cases to a fraction only of the nerve- 

 fibres and muscle-fibres of the preparation being then active. 

 But in the scratch reflex the whole group of motor neurones 

 seem to act, even when the grade of contraction exhibited 

 is quite weak. Let the reflex be excited by stimulation of 

 the skin-point sa (Fig. i B), and let the stimulus be weak, 

 producing only a feeble reflex. Then let another skin-point, 

 sB (Fig. I B), be stimulated while so is being stimulated, 

 and let the stimuli at s/3 be timed so as to fall alternately 

 with those applied at Sa. Then if the two paths impinge 

 on two different sets of units in the compound group of 





1 fifths 



