466 



NA TURE 



[September 8, 1904 



to occasion are but of small extent. The co-ordination 

 covers one limb or a pair of limbs. But the same principle 

 extended to the reactions of the great arcs arising in the 

 projicient receptor organs of the head, e.g. the eye, that 

 deal with wide tracts of musculature as a whole, involves 

 much further-reaching shift of the conductive pattern. The 

 singleness of action from moment to moment thus assured 

 is a keystone in the construction of the individual whose 

 unity it is the specific office of the nervous system to 

 perfect. Releasing forces acting on the brain from moment 

 to moment shut out from activity whole regions of the 

 nervous system, as they conversely call vast other regions 

 into play. The interference of unlike arcs and the reinforce- 

 ment of like arcs seem to lie at the very root of the great 

 psychical process of " attention." I will not trench on 

 psychological aspects of the problem. 



I have urged that the struggle between dissimilar arcs 

 for mastery over their final common path takes place in the 

 synaptic field at origin of the final neurones. Mutual re- 

 inforcement by similar arcs seems also referable to the same 

 synaptic field. As to the nature of the physiological pro- 

 cesses involved, little, it appears to me, can be said. The 

 final common path seems an instrument more or less passive 

 in the hands of the various arcs that use it. Thus in the 

 scratch reflex one arc can impress one rhythm on it, another 

 another. And in " fatigue " FC reveals, though it does not 

 share, the failure of force of the tired arc playing on it. 

 In regard to the reciprocal innervation of antagonistic 

 muscles W. MacDougall has offered a suggestion of great 

 interest, for which he obtains support from various sensual 

 reactions. He suggests that the neurones of an antagonistic 

 pair are so coupled that when one becomes active it drains 

 energy from its fellow. This takes cognisance of the 

 significant fact that central inhibition seems always accom- 

 panied by heightened activity at some related spot. Yet at 

 certain times both the antagonists can show high con- 

 ternporaneous activity (strychnia, some forms of " willed " 

 action). I think, rather, that in some way the terminal 

 of that arc which for the moment dominates the final 

 common path, disconnects that path from all terminals dis- 

 similar from itself. 



Whatever be the nature of the physiological process in 

 the conflict between the competing'reflexes, the issue of that 

 conflict — namely, the determination of which competing arc 

 shall for the time being reign over the final common path 

 — is largely conditioned by three factors. One of these is 

 the relative intensity of the stimulation of the rival refle.xes. 

 An arc strongly stimulated is caeteris paribus more likely 

 to capture the common path than one which is excited feebly. 

 In the spinal dog, retraction equally induced in both legs 

 mutually excludes the crossed extension of either side, but 

 if unequally induced allows the crossed extension of the 

 stronger reflex to exclude the weaker reflex altogether. The 

 common path is probably never out of the grasp of some 

 one or other reflex. Thus, in the spinal dog even, with 

 Its hmb apparently at rest, this is true. The final common 

 path of the extensor of the knee lies, then, in the hands of 

 a tonic reflex arising in the muscle itself. Given a strong 

 skm stimulus, and it passes under the mastery of the reflex 

 ansmg m the stimulated skin ; but when that is over, the 

 tonus arc immediately repossesses it, and for a short time, 

 as shown by the knee-jerk, more stronglv than before. 



A second main determinant for the issue of the conflict 

 between the rival reflexes is the functional species of those 

 reflexes. Arcs belonging to species of receptors which, con- 

 sidered as sense-organs, provoke strongly affective sensation 

 —e-S- pain, sexual feeling &c.— win the final common path 

 with remarkable facility. Such reflexes override and set 

 aside with peculiar potency reflexes belonging to touch 

 organs, muscular sense-organs, &c. As the sensations 

 evoked by these arcs, e.g. pains, exclude and dominate con- 

 current sensations in consciousness, so do the refle.xes of 

 these arcs prevail in the competition for possession of the 

 common paths. They seem capable of pre-eminent intensity 

 of action. 



A third main factor deciding the conflict between the com- 

 peting reflexes is " fatigue." An arc under long continuous 

 stimulation of its receptor tends, even when it holds the 

 common path, to retain its hold less well. Other arcs can 

 then more readily dispossess it. A stimulus to a fresh arc 

 has, in virtue of its mere freshness, a better chance of 



NO. I 8 19, VOL. 70] 



capturing the common path. The common path does not 

 tire. In the scratch reflex under stimulation of so when 

 the motor discharge becomes slow and irregular from 

 fatigue, it is still perfect for s/3, or L, &c. (Fig. i B). This 

 waning of a reflex under long-maintained excitation is one 

 of the many phenomena that pass in physiology under the 

 name " fatigue." Its place of incidence lies at the synapse. 

 It seems a process elaborated and preserved in the selective 

 evolution of the neural machinery. It prevents long con- 

 tinuous possession of a common path by any one refle.x of 

 considerable intensity. It favours the receptors taking turn 

 about. It helps to ensure serial variety of reaction. The 

 organism, to be successful in a million-sided environment, 

 must in its reactions be many-sided. Were it not for such 

 so-called " fatigue," an organism might, in regard to its 

 receptivity, develop an eye, or an ear, or a mouth, or a hand 

 or leg, but it would hardly develop the marvellous congeries 

 of all those various sense-organs which it actually does. 



But while talking of fatigue in general I forget the fatigue 

 in particular of listeners. The principle I have tried to 

 outline to you has many and wide applications ; it seems 

 fruitful for problems of Pathology and Psychology, as well 

 as for those of Physiology. But I keep you too long. Let 

 me sum up. The reflex arcs (of the synaptic system) con- 

 verge in their course so as to impinge upon links possessed 

 by whole varied groups in common — common paths. This 

 arrangement culminates in the convergence of many 

 separately arising arcs upon the efferent-root neurone. This 

 neurone thus forms a final common path for many different 

 reflex arcs and acts. It is responsive in various rhythm and 

 intensity, and is relatively unfatigable. Of the different arcs 

 which use it in common, each can do so exclusively in due 

 succession, but different arcs cannot use it simultaneously. 

 There is, therefore, interference between the actions of the 

 arcs possessing the common path, some reflexes e.xcluding 

 others and producing inhibitory phenomena, some reflexes 

 reinforcing others and producing phenomena of " bahnung." 

 Intensity of stimulation, species of reflex, fatigue, and fresh- 

 ness, all these are physiological factors influencing this 

 interaction of the arcs — and under pathological conditions 

 there are many others, e.g. " shock," toxins, &c. Hence 

 follows successive interchange of the arcs that dominate one 

 and the same final common path. We commonly hear a 

 muscle — or other effector organ — spoken of as innervated 

 by a certain nerve ; it would be more correct as well as more 

 I luminous to speak of it as innervated by certain receptors ; 

 I thus, the hip flexor, now by this piece of skin, now by that, 

 by its own foot, by the opposite fore-foot, by the labyrinth, 

 I by its own muscle-spindles, by the eye, by the " motor " 

 ; cortex, &c. This temporal variability, wanting to the nerve- 

 net system of medusoid and lower visceral life, in the 

 ' synaptic system provides the organism with a mechanism 

 I for higher integration. It fits that system to synthesise 

 1 from a mere collection of tissues and organs an individual 

 I animal. The animal mechanism is thus given solidarity by 

 this principle which for each effector organ allows and 

 j regulates interchange of the arcs playing upon it, a principle 

 which I would briefly term that of " the interaction of 

 reflexes about their common path." 



\ SECTION K. 



BOT.WV. 



Opening Address by Fr.incis Darwin, F.R.S., Fellow 

 OF Christ's College, President of the Section. 



On the Perception of the Force of Gravity by Plants. 

 When I had the honour of addressing this Association at 

 Cardiff as President of the mother-section from which ours 

 has sprung by fission, I spoke of the mechanism of the 

 curvatures commonly known as tropisms. To-day I pro- 

 pose to summarise the evidence — still far from complete — 

 which may help us to form a conception of the mechanism 

 of the stimulus which calls forth one of these movements — 

 namely, geotropism. I have said that the evidence is in- 

 complete, and perhaps I owe you an apology for devoting 

 the time of this Section to an unsolved problem. But the 

 making of theories is the romance of research ; and I may 

 say, in the words of Diana of the Crossways, who indeed 

 spoke of romance, " The young who avoid that region 

 escape the title of fool at the cost of a celestial crown." I 

 am prepared for the risk in the hope that in not avoiding 



