September 8, 1904] 



NATURE 



465 



motor neurones, evidence of two rhythms should appear, 

 for the muscle-fibres can respond to a much quicker rhythm 

 than the four per second. But in result the rhythm remains 

 unquickened and unaltered. Either so prevents the access 

 of sS to the motor nerones of Fc, or so's reflex having 

 impressed its own tempo on the neurones of fc, the stimuli 

 from s/3 fall within a refractory period of the neuronic 

 iipparatus. On either supposition, so and s/3 must play 

 upon the same individual neurones of the final path. A 

 like result is given by all other points I have tried in the 

 receptive field of the scratch refle.\. Again, in the inhibitions 

 previously mentioned, when there occurs the tonic contrac- 

 tion or the rela.xation of the flexor we find no intermittent 

 contraction of the scratch refle.x grafted on them, as would 

 be the case were that intermittent contraction still involving 

 some part of the whole muscle. These various reflexes seem 

 to treat the final common path as a unit. The diagram 

 therefore seems justified in representing the common path, 

 FC, as a unit. 



We have no time to multiply further now the categories 

 of relfexes playing upon the final common path FC. I might 

 cite the deep reflex arc which arises in the muscles them- 

 selves and is answerable for the mild reflex tonus that even 

 in the spinal animal maintains the tonic posture of the limb. 

 Or, instead of having taken arcs that arise in the skin of 

 the foot, we might have taken others arising above the 

 knee, and traced a refle.x influence different from the arcs 

 arising in the foot, but yet playing upon the same final 

 common path : or we might have taken arcs from the skin 

 of the tail, that inhibit the reflex ; or from the fore feet, or 

 the ears. 



There is, however, one instance of action upon this final 

 common path fc which I would quote. Suppose, while the 

 scratch reflex is being elicited from a point at the shoulder, 

 a second point, say lo centimetres distant, but also in the 

 dorsal field of skin, is stimulated. The stimulation at this 

 second point favours the reaction from the first point. This 

 is well seen when the stimulus at each point is of sub- 

 minimal intensity. The two stimuli, though each unable 

 separately to invoke the reflex, do so when applied both 

 together (Fig. 4). This is not due to overlapping spread 

 of the feeble currents about the stigmatic poles of the two 

 circuits used. -Mere cocainisation of either of the two skin- 

 points annuls it. Moreover, it occurs when purely 

 mechanical stimuli are used. It is evident that the arcs 

 from the two points, e.g. so and S|3 (Fig. i B), have such 

 a mutual relation that reaction of one reinforces reaction 

 of the other, as judged by the effect upon the pnal co^nmon 

 path FC. Such mutual reinforcement is usual between 

 reflexes of identical species evoked from one and the same 

 receptive field, e.g. the nociceptive of the foot. 



Not for all the arcs arising in the receptive field of the 

 scratch refle.x can, in my experience, this mutual reinforce- 

 ment be demonstrated. There seems a gradual fall in re- 

 inforcing power as the distance between the receptors of 

 the arcs increases. In this connection the following point 

 is noteworthy. The scratch reflex carries the foot broadly 

 toward the place of stimulation. In the spinal dog the 

 reflex does not succeed in bringing the foot actually to the 

 irritated point, yet when the irritation is far forward the 

 foot is carried further forward, and when the irritation is 

 far back the foot is carried further back. A scratch reflex 

 evoked by a stimulus applied far back and high up in the 

 dorsal skin is therefore not wholly like a scratch reflex 

 evoked from far forward and low down. Now, the mutual 

 reinforcement between the scratch reflex arcs in their action 

 on the final common path fc seems greater the greater the 

 likeness between the reflex actions they initiate. The 

 coalition between the reflexes gradually decreases as the 

 interval between their receptive points at the skin surface 

 becomes wider. Whether coalition fades into mere indiffer- 

 ence, or passes over into antagonism, my observations as 

 yet do not say. But there are various receptive regions of 

 the body surface that do, in the spinal dog, appear indifferent 

 for the scratch reflex. Were it not that the nervous system 

 is perforce mutilated in the " spinal " animal, the number 

 of these indifferent arcs might be fewer. In presence of 

 the arcs of the great projicient receptors and the brain there 

 can be few receptive points in the body the activities of 

 which are totally indifferent one to another. Correlation 

 of the activities of arcs from receptive points widely apart 



NO. 1819. VOL. 70] 



is the crowning contribution of the brain toward the nervous 

 integration of the individual. 



In the case before us, then, the final common path — the 

 motor neurone — to the hip fle.xor muscle is played upon by 

 various categories of refle.x spinal arcs. Of those mentioned, 

 one category (i.), the nociceptive from the leg itself, induces 

 strong, steady contraction in the muscle. A second (ii.), 

 the scalptor or scratching from the dorsal skin, induces 

 rhythmic contraction in the muscle. A third (iii.), from the 

 deep structures of the limb itself, induces the mild enduring 

 contraction known as spinal tonus. A fourth (iv.), e.g. the 

 nociceptive from the opposite foot, depresses the activity of 

 the muscle probably by excluding from it the activity of 

 the other arcs which would excite the final path, the motor 

 neurone. .\nd there are many more we could trace from 

 various regions of the body ; also, pyramidal and other 

 influences from brain for which our final path is likewise 

 common. The arcs witltin one category may reinforce each 

 other's action on the common path, but those in separate 

 categories are generally correlated in their action on their 

 final common path in such a way as to antagonise one 

 another. They are rivals for possession of their final 

 common path, rivals as retinal points may be rivals for 

 possession of the visual sensorium. 



The extent to which in the nervous system this competition 

 for possession of the common path obtains is very great. 

 The multiplicity of the conflict seems extreme. The afferent 

 fibres — that is, private paths — entering the central organ 

 are much more numerous than are the final common paths. 

 We owe to Donaldson and his pupils enumerations which 

 show that the afferent fibres entering the human spinal cord 

 three times outnumber the efferent which leave it. Add 

 the cranial nerves and the so-called optic nerves, and we 

 may take the afferent fibres to be five times the greater. 

 The receptor system bears therefore to the efferent paths a 

 relation like the wide ingress of a funnel to its narrow 

 egress. The simile is bettered by supposing that within 

 the general systemic funnel the conducting paths of each 

 receptor may be represented as a funnel inverted, so that 

 its wider end is more or less co-extensive with the whole 

 plane of emergence of the final common paths. All these 

 private paths converge in the nervous system to the great 

 central organ, the spinal cord and brain, whence on the 

 other hand all the final common paths irradiate. This 

 central organ is, to return to our earlier metaphor, a vast 

 network the lines of which follow a certain pattern. But, 

 as we see from the instances cited — more could be given 

 abundantly, had we time — the pattern is unstable, the details 

 of connection shift from moment to moment. We might 

 compare the central organ with a telephone exchange, where 

 from moment to moment the connections between starting 

 and end points are changed to suit passing requirements. 

 In order to realise the exchange at work, one has to add 

 to its purely spatial plan the temporal datum that within 

 certain limits the connections of the lines shift to and fro. 

 The connections of any entrant path not only offer different 

 degrees of resistance, but their resistances, both absolutely 

 and relatively, vary from occasion to occasion. It is not 

 merely that general conditions of nutrition, of blood-supply, 

 Src, affect these resistances. The functional conductive 

 activity of the nervous organ itself produces from moment 

 to moment the temporary opening of some connections and 

 the temporary closing of others. A good example is the 

 " reciprocal innervation " of antagonistic muscles — when 

 one muscle of the antagonistic couple is thrown into action 

 the other is thrown out of action. This is only a widely 

 spread special case of a general principle. The general 

 principle is the mutual interaction of arcs which embouch 

 upon one and the same common path. Unlike arcs have 

 successive use, but not simultaneous use of the common 

 path. Like arcs mutually reinforce each other in their 

 action on the common path. Expressed teleologically, the 

 common path, although economically subservient for various 

 purposes, is yet used only for one purpose at a time. 



Thus the reaction initiated by one receptor while in 

 progress excludes in various directions the reactions of other 

 receptors. Tn this way the motor paths at any moment 

 accord in a united pattern for harmonious synergy, co- 

 operating for one effect. In the case of simple antagonistic 

 muscles, and in the instances of simple spinal reflex arcs, 

 the shifts of pattern of the conductive network from occasion 



