NA TURE 



[August 23, 1906 



whieh bears no relation to that of the centripetal ones. 

 But it must be remembered that even in nerve fibres it is 

 possible for a succession of stimuli to evoke a different 

 succession of electrolytic changes and of nerve impulses, 

 provided that some of the successive stimuli fall within 

 the period of inexcitability which occurs during the estab- 

 lishment of each new electrolytic poise.' We have, there- 

 fore, only to assume, as is very probable, that in the 

 central portion of the nervous path this poise is prolonged 

 in its development, and numbers of centripetal impulses 

 must necessarily fail ; hence the emergent ones will have a 

 special periodicity indicative of the duration of the swing 

 of the electrolytic rearrangement which occurs when the 

 synapses plus the cells are traversed by the entering 

 impulse. 



The second feature which more particularly suggests 

 spontaneous cellular activity is the well-known fact that 

 reflex centrifugal discharges may continue after the obvious 

 centripetal ones have ceased. This is preeminently the case 

 when the central mass is rendered extremely unstable by 

 certain chemical compounds, such as strychnine, &c. 

 I here are, however, suggestive indications in connection 

 with such persistent discharges. The more completely all 

 the centripetal paths are blocked by severance and other 

 means, the less perceptible is such persistent discharge, and 

 since nervous impulses are continually streaming into the 

 central mass from all parts, even from those in apparent 

 repose, it would seem that could we completely isolate 

 nerve cells, their discharge would probably altogether 

 cease. In this connection a suggestive e.xperiment was 

 carried out some years ago upon the spinal cord of the 

 mammal.- A portion was isolated in situ by two cross- 

 sections, and a part of this isolated cord was split longi- 

 tudinally into a ventral half containing the motor or centri- 

 fugal nerve cells and a dorsal half containing the breaking 

 up of the centripetal nerves ; each half was then examined 

 for those electrolytic changes which indicate the presence 

 of nervous impulses. It was found that, even in the 

 strychnised animal, no electrical effects could be detected 

 in the ventral half of the cord or its issuing roots, although 

 such effects were marked in the whole cord, and occurred 

 in the dorsal half which contained the centripetal nerve 

 fibres. 



This experiment indicates that even in the hyper-excitable 

 condition produced by strychnine the spinal motor nerve 

 cells did not discharge centrifugal impulses when cut off 

 from their centripetal connections. It is corroborated by 

 the results obtained by Baglioni in the frog and smaU 

 mammal," and, taken in connection with those previously 

 mentioned, it affords considerable foundation for assert- 

 ing that the chief rSle of the nerve cell is trophic, and 

 that, as regards issuing nerve impulses, it only forms a 

 modified part of the conducting path. The more we 

 investigate the physiology of the nervous system, the 

 stronger becomes our belief that for centrifugal' discharges 

 to occur centripetal impulses must be primarily started 

 either in the peripheral sensory surfaces by chaiiges of a 

 physical or chemical type occurring in the external world, 

 or at some point in the nerve continuum by local chemical 

 or physical changes within the body, especially those due 

 to the chemical condition of the blood. Having been thus 

 started they course along definite structural paths, and the 

 only direct indications of this passage consist of such 

 phenomena as would be produced by the redistribution of 

 concentrated groups of electrolyte's — a purely physico- 

 chemical process. 



This conception places the propagation of the nervous 

 excitatory state as the sole determining factor of nerve 

 activities, central or peripheral. It derives additional sup- 

 port from the circumstance that it is in harmony with that 

 aspect of these activities which is comprised' under the 

 term, inhibition. .Any effective regulating system must be 

 able to bring into play both incentive and restraint — the 

 whip and the reins. The possession by the central nervous 



('f Physiol., vol. xxiv. 1800, p. 410; Povrotf, 

 1S95, p. 144 ; Buchanan, /oKra. of Physiol., 



1 Gotch and Burch, /i 

 Journ. or Physiol., vol. 

 vol. XXvii TQOT, p. qR, &f 



" Gotch and Horsley, Phil. Trans., vol. clxxxii". pp. .367-526. (Londi 



:< Baelioni, Archiv /. die Ges. Physiol., tqoo, Supplement, pp. 19S-2 

 (Leipzig.) 



NO. 192 1, VOL. 74] 



mechanism of inhibitory powers is remarkable both for its 

 extent and its delicacy. It appears more and more prob- 

 able that this is achieved by the propagation of nervous 

 impulses of the ordinary type. Thus, recent researches by 

 Sherrington show that the propagated impulses from a 

 given central mass may, although normally inhibitory to 

 the centrifugal discharge of another mass, become directly 

 incentive if the second controlling centre has its excitability 

 abnormally augmented by strychnine, tetanus to.xin, &c.' 

 .■\s regards their fundamental characters it thus appears 

 that both augmenting and inhibiting impulses belong to 

 the same category. Moreover, such theories of central 

 inhibition as embrace all the phenomena involve as their 

 essential basis the rutting-olf of the potent centripetal 

 supply to the inhibited centre. In the interference theory 

 this cutting-off is assumed to be caused by the arrival of 

 other nerve impulses which, breaking into the path of 

 normal centripetal flow, obstruct and run counter to this 

 potent stream. In the ingenious drainage theory, pro- 

 pounded by McDougall, the cutting-off is an indirect one, 

 ii being assumed that the new stream enters other side- 

 channels, and thereby opens up a short circuit through 

 which the potent ones drain away without reaching the 

 centrifugal centre. Even Langley's conception of receptive 

 substances played upon by impulses must be associated 

 with a check in the efficiency of the continuous centripetal 

 supply. 



From the foregoing it appears that the physiologist has 

 definite grounds for believing that, as far as present know- 

 ledge goes, both the production and cessation of central 

 nervous discharges are the expression of propagated 

 changes, and that these changes reveal themselves as 

 physico-chemical alterations of an electrolytic character. 

 The nervous process, which rightly seems to us so re- 

 condite, does not, in the light of this conception, owe its 

 physiological mystery to a new forin of energy, but to the 

 circumstance that a mode of energy displayed in the non- 

 living world occurs in colloidal electrolytic structures of 

 great chemical complexity. There is a natural prejudice 

 against the adoption of this view, but such prejudice should 

 surely be mitigated by the consideration that this full 

 admission of physiology into the realm of natural science, 

 by forcing a more comprehensive recognition of the 

 harmony of Nature, is invested with intellectual grandeur. 



With such questions as the essential meaning of con- 

 sciousness and the interpretation of the various aspects of 

 mind revealed by introspective methods, the physiologist, 

 as such, has no direct concern. For his purpose states of 

 consciousness are regarded merely as signs that certain 

 nervous structures are in a state of physiological activity ; 

 and he thus limits the scope of physiology to the objective 

 world. This limitation of physiology does not prohibit a 

 treatment of the subjective world along lines calculated 

 to display that intellectual causative array which character- 

 ises science ; it merely indicates that this particular appli- 

 cation of scientific method is not physiology, but that 

 something else, still more profound, which is now termed 

 psychophysics. 



But if objective phenomena form the subject-matter of 

 the physiologist, then " the legitimate materialism of 

 science" must constitute his working hypothesis; and his 

 " well-defined purpose " must be to adapt and apply the 

 methods of physics and chemistry for the analysis of such 

 phenomena as he can detect in all physiological tissues, 

 including the nervous system. The trend of such a strictly 

 ohysiological analysis is towards a conception in which the 

 highest animal appears as an automaton composed of differ- 

 entiated structures exquisitely sensitive to the play of 

 physical and chemical surroundings." The various parts of 

 the animal body are linked by circulating fluids and by 

 one special structure, the nervous system ; in this linking 

 of parts the physiologist detects the working of automatic 

 chemical mechanisms of great delicacy which, once de- 

 veloped, are retained and perfected in proportion as they 

 efficiently regulate the various bodily activities and co- 

 ordinate them for the welfare of the whole organism. The 



1 Shcr ;ngto 



Roy. Sor,, vol. Ixxvi B, pp. 269-297. (Londo 



2 See H.i.tlev, "On the Hvpothesis Ihaf .\n'<r<,\^ are Anton-an," 

 ''venin? ;Addres=. Brit. A«soc , Belfast, 1874. Re-published in " Collecled 

 Essays," vol. i. (Macmillan, 1904.) 



