August 23, 1906] 



NA TURE 



427 



.i-c-Us. According to ihe neuron iheory the fibres of different 

 nerve cells end more or less blindly, and, at any rate in 

 vertebrates, do not demonstrably unite at their termini 

 within the central mass; hence gaps exist at the junction 

 unbridged by the ditTcrenliated structural continuum. But 

 since the nervous iinpulse can pass from one set to the 

 other, a physiological continuum undoubtedly exists ; it is 

 necessary, therefore, to assume cither that the electro- 

 lytic change in one neuron can by mere contiguity in space 

 arouse a similar change in a neighbouring neuron process, 

 or that a differentiated connection actually exists, but of 

 such structural delicacy that it cannot be microscopically 

 demonstrated. Recently several physiologists have stated 

 iheir belief in such continuity ; one of these, li. Pfluger, 

 l)ases his view upon the admitted intracellular nature of 

 peripheral nerve endings in muscles, glands, epithelial 

 lells, and electrical organs. .Arguing from analogy, he 

 infers that the central nerve endings of one neuron prob- 

 ably pierce and enter the cell processes of another neuron.' 

 Such a connection can be actually seen, as a pericellular 

 I>lexus, in the ganglia of Crustacea, and has been occasion- 

 ally described as "observed in higher animals. _ Whether 

 'I he central termini of neuron processes are in reality 

 joined by extremely fine fibrillar filaments or whether they 

 end blindly in mere juxtaposition, it is undoubted that the 

 functional synapsis presents peculiar features. The chief 

 peculiarities of synaptic activities as distinct from the 

 activities of the nerve fibres are the following : — Marked 

 retardation in the maximum rate of propagation ; irre- 

 ciprocity of conduction, which is favoured in the natural 

 or honiodromous direction, whilst in the unnatural or 

 helerodromous direction it is obstructed or completely 

 blocked ; susceptibility to fatigue ; special susceptibility to 

 stimulation and impairment by definite chemical substances, 

 by strychnine, absinthe, anxsthetics, &c. ; the presence of 

 a resistance which diminishes rapidly when subjected to 

 the assault of a series of entering or centripetal nervous 

 impulses even when each member of the series is alone 

 quite powerless to force a passage. All these peculiarities 

 are more or less demonstrable in all nerve endings, peri- 

 pheral as well as central, and are presumably, therefore, 

 related to the character of the propagation which occurs 

 in the finely-divided non-medullated twigs or " arborisa- 

 tions " into which the nerve fibres break up in such end- 

 ings, and possibly to some further " receptive " substance 

 Iving beyond the endings. The retarded propagation, 

 showing itself by an apparent delay, occurs in the motor 

 nerve endings of muscles and in the multitudinous nerve 

 ■endings of electrical organs, as well as in the central 

 nervous system. Garten's researches on non-medullated 

 nerves suggest that it may be connected with such slowed 

 <lpve!opment of the electrolytic redistribution and of its 

 accompanying electromotive alterations as is demonstrable 

 in these structures.^ Irreciprocity of conduction occurs 

 where nerve endings are continued into muscle substance, 

 since the activity process passes from nerve to muscle, but 

 not the reverse way. In i8q6 Engelmann succeeded by 

 means of a double muscle-bath in so modifying one end 

 of a muscle fibre that the wave of contraction, whilst it 

 travelled freely along the muscle fibre from the unmodified 

 to the modified portion, would not do so the reverse way.' 

 The particular modification which produced this abnormal 

 result is an interesting one : it is the development of an 

 abnormally sluggish tjpe of mobility, the whole activity 

 of the modified region being greatly prolonged by means 

 of veratria. This suggests that difference in the duration 

 of the active process on the two sides of a central nervous 

 synapsis would, if present, be one factor in producing the 

 well-known central irreciprocitv. The susceptibility to 

 fatigue may be associated with this augmented difficulty of 

 propagation, and it undoubtediv occurs to a marked extent 

 in muscular nerve endings : for, according to the investi- 

 gations of Joteyko, it may be more pronounced in this 

 peripheral ending than it is even in the spinal cord.' Even 

 the so-called summation phenomena — that is, the ease with 



' E. Pflueer, " Ueber den elementaren Bau des Nervensystems," Archiv 

 ■;■/■ GfS. PliysM. cvii., igo6. 



- Oarten, " Beitr.i^e zur Phv.sioIogie der marltlcien Nerven." Jena, T903. 

 ■ F.nfjelmann. " Ver^uche iiber irreciproke Reizleilung in Muskelfasern," 

 Aychh'/. die Ges. Physiol., Ixii. t8q6. p. 400. 



•» Joteyko, " Travaux de I'Institut Solvay," Bruxelles, iii. 2, 1900. 



NO. 192 1, VOL. 74] 



which a succession of centripetal impulses can force a 

 passage as opposed to the difficulty with which a single 

 such impulse does so— is not peculiar to the central mass, 

 but is observed more or less in peripheral nerve endings ; 

 for instance, those of electrical organs, l-inally, the results 

 obtained by Wedenski suggest that anaisthetics have a 

 particular affinity for nerve endings, including the peri- 

 pheral ones in the muscles ; and although the causation is 

 at present iinperfectly known, it does not seem improbable 

 that they may act upon some such specific substance as 

 that which is conceived of by Langley under the term 

 " receptive." ' 



All the phenomena hitherto described are thus not neces- 

 sarily aspects of the activity of that particular mass which 

 constitutes the body of the nerve cell, but of nerve endings 

 with their fine arborisations. As regards direct electrical 

 evidence of electrolytic changes in these finer branches, it 

 so happens that Nature has provided some nerve endings 

 on such a magnificent scale that this evidence is readily 

 obtained. In the electrical organs of fishes the essential 

 structure consists of a pile of numerous discs each invaded 

 bv nerve endings, and the electric shock of the fish is the 

 siim of all the electrical changes in this pile when an 

 efferent nervous impulse reaches each of its component 

 discs. Its potency is due to the number of these com- 

 ponents, but in each single component it is of the same 

 order as the electromotive change in a nerve, and its 

 character is such as might be produced by electrolytic 

 redistribution occurring simultaneously in the immense 

 number of nerve endings which are present in each disc 

 of the electrical organ. Although displaying the peculiari- 

 ties of apparent delav, &c., just referred to, the general 

 character of the shock of the organ is such as to warrant 

 the belief that electrolytic conceptions of nerve-fibre activity 

 can be extended to the activities of nerve endings. 



There remains that special part of the whole neuron 

 which is the effective source both of its development and 

 of its maintenance, the nerve cell. Continuity with a nerve 

 cell is essential for the integrity of both the structure arid 

 the function of a nerve fibre, but it is undoubted that, in 

 its turn, the nerve cell is* also dependent upon the exist- 

 ence of' its processes in an unimpaired state. Thus the 

 cell suffers a change which comes on slowly but with great 

 certainty if any part of the neuron has been mutilated, or 

 if the cell has been shorn of some of its offshoots. That 

 it forms a special part of the conducting path is indicated 

 bv the occurrence of intracellular and nuclear alterations 

 when a prolonged series of impulses travel towards it, and 

 a further more remarkable point is that it also appears to 

 change if the entering nervous impulses with their electro- 

 Ivtic concomitants are no longer able to reach it. Ihis 

 sugaests that nerve cells, far from being spontaneous 

 actors, are in a very real sense dependents ; they form only 

 one possible conducting portion of the whole differentiated 

 tract and atrophy when this tract is broken or is from 

 anv circumstance 'not utilised. That the cell is primarily 

 trophic and only incidentally a conductor is suggested by 

 Bethe's experiments upon crust.acea. Owing to pericellular 

 connections the actual nerve cell may be removed in these 

 animals without severing the whole conducting tract, tor 

 a portion lies around but outside the cell ; and since, even 

 after such removal, the usual reflex movements of the sup- 

 plied antennre are resumed, the cell cannot in this instance 

 be regarded as essential for the discharge of the motor 

 impulses which evoke Ihe antennas movements.- 



In higher animals such removal of the cell body has 

 been imperfectlv carried out by Steinach in the dorsal 

 spinal ganglia, but in the central mass it is impossible to 

 perform a crucial experiment of this kind so as to deter- 

 mine whether or no the substance of nerve cells can create 

 nervous impulses. There are two particular features ot 

 reflex movements which may be cited as indicating that a 

 motor nerve cell has at its call a store of nervous energy 

 which it can spontaneously discharge. The first of these 

 is the well-known fact that the character of reflex move- 

 ments is such as to indicate the rhythmical discharge of 

 groups of centrifugal nerve impulses the periodicity ot 



1 Wedenski, " Erregung, HemmunK und Narkose," jiicJiiv /■ die Ges. 

 Physiol, c, 1Q03. ... 



s Bethe, Altgemeine A>mt. u. Physiol, dcs Ncr-.'c«systems, 1903, p. 99- 



