September 14, 191 1] 



NATURE 



03 



tained in a condition of enforced rest. How then does 

 the change transmitted into the several branches of the 

 afferent neurone, having the same character as it invades 

 every branch, succeed in causing diametrically opposite 

 conditions in two groups of perfectly similar efferent 

 neurones? There is but one answer to this question, 

 namely, that transmission into the second group must be 

 through some intermediate mechanism which reverses the 

 character of the change. Now I have no hesitation in 

 naming definite structures in the nervous system as being 

 alone those to which we can impute this reversal, namely, 

 certain intermediate neurones which have a way of being 

 interpolated between afferent and efferent neurones. Such 

 neurones are seen in the cord sometimes sending their 

 main nerve-fibre towards efferent neurones placed on the 

 other side of the cord, and in the cerebellum the large cells 

 of Purkinje are seen lo be approached by afferent nerve- 

 fibres in this double fashion ; one set reaching them 

 feectly, the other set indirectly through intermediate 

 neurones. We shall then picture neurones with short nerve- 

 fibre processes as placed in these paths that are inhibited, 

 and as sometimes responsible for this singular reversal of 

 the transmitted excitation. 



In this connection, too, we must deal briefly with another 

 fact observed by Sherrington, that certain drugs, tetanc- 

 roxin and strychnine, affect these intermediate mechanisms 

 in such a way that they lose their power of reversing the 

 character of change transmitted through them. When 

 these drugs are applied to any part of the nervous system 

 action and antagonistic action are simultaneous conse- 

 quences, and the stronger wins. Of the greatest interest, 

 too, is the fact that this disturbance of the process of 

 reversal may be obtained in a graduated manner by the 

 application of such drugs in varied strengths of solution. 

 It is thus clear that there is nothing peculiar about the 

 nerve-fibre portion of these intermediate neurones, since 

 when given excitations to transmit they transmit them, 

 although it is so frequently their normal business to 

 transmit inhibitions. Clear, too, that their cell-bodies fre- 

 quently inhibited like those of the efferent neurones mav 

 also with a slight modification of condition tend towards 

 excitation, or, as a matter of fact, be excited, again like 

 the efferent neurones. There is no difference discoverable 

 hero between these two sets of cells other than a difference 

 of degree. The one salient fact demanding explanation is 

 this difference under normal conditions in which the efferent 

 neurones are seen as excited by identically the same 

 character of transmitted change that inhibits the inter- 

 mediate neurones. 



Now it would be a simple matter to show that all these 

 points might be dealt with adequately on the assumption 

 that nerve-cells invariably contained a mixture of two 

 materials, existing in different proportions in different cells, 

 each of which was forced into a diametrically opposite 

 plusical state to the other as the result of changes in 

 physical conditions of the kind transmitted bv nerve-fibres. 



It is of interest then that there is definite reason to 

 suppose that within nerve-cells there are always two sub- 

 stances which seem to have their states diversely affected 

 by different conditions. One of them is the characteristic 

 constituent of what I have been irreverently terming the 

 cuticle, the nerve-fibre : and the other a complex material 

 which apparently represents the primary product of nuclear 

 activity, and is spoken of as the material of Nissl. It mav 

 seem a weak point in my use of the term that this 

 cuticle-stuff is found within the cell-bodv. Perhaps so, 

 but perhaps also not so ; the point is not worth discussing. 



The point really worth discussion is as to whether it is 

 true that these substances are affected in diametrically 

 opposite ways by the same change, just as if, for example, 

 one of them was possessed of acid and the other of basic 

 characters ; so that the basic was precipitated, and the 

 acid dissolved by the addition of an alkali : since if they 

 exhibit any opposite behaviour in the presence of the trans- 

 mitted excitation, then it is indeed probable that their 

 admixture is responsible for many of the orderly vagaries 

 of transmission through nerve-cells. I am proceeding as if 

 Shis is really true to a consideration of its influence "on the 

 development of nerve-cells. 



Imagine a developing afferent neurone in contact with 

 two other neurones, but by different extents of its surface, 

 NO. 2185, VOL. 87] 



so that it transmits a larger quantity of change to the one 

 than to the other. In both cases it affects an algebraical 

 sum of opposing properties, and we might think of it as 

 effecting a compression and an expansion. Now let there 

 be the slightest difference in the forces required to compress 

 and to expand, and it might readily happen that the effect 

 of a minimal dose might be to produce an algebraical sum 

 in favour of compression, whilst a maximal created a 

 general effect of expansion. One of these cells trien 

 might be habitually excited and grow a cuticle traversing 

 considerable distances in the central nervous system ; 

 whereas the other is inhibited until the accumulation of 

 charges previously received add up to the dose required to 

 tip the algebraical sum in favour of excitation, and then 

 first commences the growth of a short nerve-fibre. 



This, however, involves the assumption that these cells 

 of both classes store up all the transmitted energy they 

 receive, that they do not leak, do not transmit, and thus 

 grow their nerve-fibres from the effects of accumulation. 

 Within certain limits this supposition is sound, since we 

 are familiar with that summation which is a leading 

 feature in nerve-cell conduction. Below a certain definite 

 quantity of charge they do not leak, and are found by a 

 second impulse arriving some little time after an apparently 

 ineffectual predecessor in a new state, so that the new- 

 comer is effectual. Now if no new-comer arrives in time 

 we must suppose the energy due to the first as having 

 affected the growth of the cell in one direction or another — 

 that is to say, in one direction if it produced the change 

 characteristic of excitation, and the other if producing to a 

 minimal degree the change characteristic of inhibition. It 

 is legitimate, too, to suppose these limits as set by the 

 capacity and extent of excitable contacts. The larger the 

 extent of contact the sooner and the more effectual must 

 be the leakage. Thus we may readily picture the excited 

 neurone as growing more and more cuticle until this 

 growth is checked by the number, extent, and capacity of 

 the excitable contacts made in course of growth. When a 

 certain measure of growth has occuned we may suppose 

 that residual charges below the margin of leakage are now 

 onlv just sufficient to maintain the district of cuticle that 

 has been laid down. We have therefore encountered the 

 limits of growth of the nerve-fibre. 



As for the second cell, which we have considered as 

 mainly inhibited. In it the mass-action of the products of 

 nuclear change is diminished and we must think of it as 

 enlarging its cell-body by an increased nuclear activity ; 

 possessed of a short cuticle but an extending cell-body, 

 possessed of no more than a short nerve-fibre and an 

 extensive set of dendritic processes. As each new dendritic 

 process makes contact with a new branch of the excited 

 afferent neurone its growth will be more and more limited. 

 We have here, then, encountered the limits of growth of 

 the nerve-cell. 



There is no difficulty other than that due to the short 

 time at mv disposal in compounding these statements so as 

 to cover the whole scale of differential cell-growth, and 

 within each cell of the relative growth of its several parts, 

 that is observed within the nervous system. I may perhaps 

 be permitted this abrupt closure to a development of the 

 probabilities underlying the following expression of opinion. 



I hold it as probable that all the individual structures of 

 the nervous system, and so in the brain, have just so much 

 difference from one another in size, in shape, and in 

 function, as is the outcome of that measure of purely 

 physical experience to which each one of them has been 

 subjected : and that the physiological function of each one 

 of them is of the simplest kind. The magnificent utility 

 of the whole system, where the individual units have^ such 

 simplicity, is due to the physically developed peculiarities of 

 their arrangement in relation to one another, and to the 

 receptive surfaces and motor-organs of the body. 



To relieve the monotony of this discussion, let us turn 

 awav for a moment to the consideration of certain physical 

 mechanisms found in the body, external to the central 

 nervous system ; mechanisms that are placed, so to speak, 

 upon the front of that system so that they are capable 

 rather of affecting it than of being affected by it, and this 

 to such a degree that we must suppose them as rather 

 assisting in the development of the central nervous system 

 than as being assisted to their development by the central 



