426 



NA TURE 



[August 23, 1906 



that chemical substances formed during pregnancy in the 

 tissues of the foetus will, if introduced into the maternal 

 blood, directly evoke the appropriate activities of the re- 

 mote mammary glands.' 



These are only a few instances of a class of mechanisms, 

 strictly chemical in character, by which the activities of 

 remote and dissimilar organs are automatically coordinated ; 

 a further class of such mechanisms, although involving a 

 chemical substance conveyed by the blood, carries out the 

 actual regulation by means of the central nervous system. 

 An example of this class is afforded by the researches of 

 Haldane and Priestley upon the carbonic-acid gas in the 

 pulmonary air. These show that the alveolar pressure of 

 carbonic acid in the lung spaces remains constant even 

 when the atmospheric pressure is considerately altered in 

 amount. The constancy is due to the circumstance that 

 the respiratory nerve centres are exquisitely sensitive to a 

 rise in this carbonic-acid pressure. Any such rise slightly 

 augments the carbonic-acid tension of the pulmonary 

 blood, which, on being conveyed to the nerve centres, 

 arouses their greater activity, and the increased eflRciency 

 of the respiratory ventilation, thus produced, rapidly re- 

 duces the amount of the very agent which is its exciting 

 cause.- The researches of Hill and Greenwood, with air 

 pressures up to seven atmospheres, bear out the con- 

 clusion that by this automatic mechanism the air in the 

 lung alveoli has a practically constant pressure of carbonic 

 acid in any given individual.' 



The introduction, in this example, of the respiratory 

 centres and nerves raises the question whether the nervous 

 system, which is in a very special sense the channel for 

 the regulation and coordination of the various activities of 

 the body, may not itself be conceived to be a supreme 

 example of an automatic physico-chemical mechanism, the 

 transference from one part to another taking place, not 

 through the flow of blood containing chemical substances, 

 but through a more subtle physico-chemical flow along the 

 highly differentiated nervous strands of which this system 

 consists. The nervous system is not popularly regarded in 

 this light ; on the contrary it is considered to be the special 

 seat of vital directive forces, and it is held, even by some 

 scientific men, that the nervous energy which it manifests 

 is so transcendental in its essence that it can never be 

 brought into line with those modes of energy prevailing 

 in chemistry and physics. There is, moreover, a wide- 

 spread belief, founded upon conscious volitional power, 

 that nervous energy can be spontaneously created, and 

 that even if its manifestations are bound up with the 

 integrity of certain defirtite nervous structures, these struc- 

 tures only form the material residence of genii, temporarily 

 ' in possession, endowed with the powers of hypothetical 

 homunculi at the bidding of which the manifestations 

 either take place or cease.* 



The complexity of nervous structure and the apparently 

 uncertain character of nervous activities furnished the 

 older writers with plausible reasons for assuming the exist- 

 ence of animal spirits, but the extensive researches of half 

 a century progressively suggest that nervous phenomena 

 may be regarded as the sum of particular physico-chemical 

 processes localised in an intricate differentiated structure, 

 the threads of which are being unravelled by neurological 

 technique. This chapter of physiology still bristles with 

 difficult problems and obscure points, yet the unmistakable 

 trend of the immense advances which have been made in 

 recent years is towards the assimiption that nervous pro- 

 cesses do not in their essence differ from processes occur- 

 ring elsewhere in both the living and non-living worlds. 



As regards structure it is generally assumed by neuro- 

 logists that the whole system is a fabric of interwoven 

 elements termed neurons, each with a nucleated nerve cell 

 and offshoots, one of which may be extended as a nerve 

 fibre, whilst no nerve fibre exists which is not the offshoot 

 ^ Starling and Lane-CIaypon. 



- Haldane and Priestley, " The Regulation of Lung Ventil: 

 o/PhysM.. xxxii. 1905. 



3 Hill and Greenwood. " The Influence of Increased liarometric Pi 

 on Man." Prr:c Roy. Sue. vol. Ixxvii B, 1006, p 442. 



J Lodge, "Life and Matter " (London : Williams and Norgate, T9r6). 

 " Matter is the vehicle of mind, but it is dominated and transcended by it " 

 (p. 123). "Contemplate a hrain-rell, whence originates a certain nerve- 

 process whereby energy is liberated with some resultant effect" (p. 168). 

 "It is intelligence which directs; it is physical energy which is directed 

 and controlled and produces the result in time and space " (p. 169). 



•Jo 



NO 192 1, VOL. 74] 



of one such cell. This neuron theory is based upon 

 developmental history and upon the suggestive fact that 

 each nerve cell forms an independent trophic centre for its 

 own distributed processes. It is undoubted that, like the 

 atomic theory in chemistry, the neuron theory has proved 

 of enormous service, enabling neurologists to disentangle 

 the woven strands of nerve-cell processes even in such an 

 intricate woof as that of the central nervous mass. There 

 are, however, difficulties associated with its full accept- 

 ance in physiology, as indeed there are said to be in con- 

 nection with the full acceptance of the atomic theory in 

 chemistry ; but dismissing these for the moment, 1 pass on 

 to consider the presumable character of such a conception 

 of nervous activities as would be demanded on the sup- 

 position that the nervous system is, as regards all essentials, 

 an automatic physicochemical mechanism. 



In the nerve fibres, which are undoubtedly the offshoots 

 of nerve cells, the only demonstrable changes during the 

 actual passage of nervous impulses are of an electrical 

 type. These resemble the effects which would occur if 

 there were redistributions of such electrolytes as are known 

 to exist within and around the differentiated fibrillated 

 core or a.Kon of each nerve fibre. All the better-known 

 aspects of nerve-fibre activities are in accordance with such 

 an electrolytic conception. The exquisite sensibility of 

 nerve to physical and chemical changes of a sudden 

 character would be associated with the fluctuating and 

 variable character of electrolytic distribution, this instability 

 being characteristic of particular electrolytes in colloidal 

 solutions ; hence physical and chemical alterations primarily 

 affecting the nerve envelope will, by modifying the electro- 

 lytic distribution, produce physico-chemical change in the 

 internal axon itself. Such changes, when once produced 

 at any point in the differentiated fibrillar continuum of the 

 nerve fibre, must in accordance with the conception first 

 propounded by Hermann be propagated or transmitted along 

 this continuum. The redistribution of electrolytes at the 

 seat of the external impression being itself a source of 

 electromotive effects, electrical currents demonstrably flow 

 from this point into the contiguous parts of the fibrillar 

 continuum. Such flow of current must reproduce in this 

 neighbouring continuum that electrolytic redistribution 

 which is the fundamental aspect of nerve-fibre activity. 

 Thus, by this comparatively simple automatic mechanism, 

 the physico-chemical electrolytic change is successively 

 assumed by the various portions which compose the length 

 of the differentiated axon, and the new or active phase is 

 propagated along a nerve fibre as infallibly as a flame 

 speeds along a fuse when one end is ignited ; in this way 

 the conception explains how a so-called nervous impulse 

 is brought into being. Further, the brief duration of the 

 activity of the nerve, its rapid development and slower 

 decline, and the circumstance that a second external change 

 cannot arouse a second activity if it occurs very shortly 

 after an effective predecessor, all have their counterpart 

 on the electrolytic side, and we have convincing evidence 

 that the electrolytic redistribution during activity cannot 

 be again produced until the electrolytic condition has more 

 or less returned to its original resting poise : the real 

 peculiarity of the living tissue is its persistent tendency 

 to re-establish the electrolytic concentration of this resting 

 poise.' Finally experiments show more and more con- 

 vincingly that the capacity of the nerve to respond to 

 external changes, as well as the magnitude and duration 

 of the aroused activities, are particularly susceptible to 

 modification by all those agents which are most potent 

 in affecting electrolytic aggregates, such as temperature, 

 electrolysis, and impregnation with various electrolytes. 



These electrical indications of nerve-fibre activities are 

 fundamentally the same whether the fibres occur in peri- 

 pheral nerve trunks or in the bundles which course through 

 the central masses ; and thus, if the whole system con- 

 sisted of nothing but the united strands of differentiated 

 nerve fibres, nervous phenomena would be merely the ex- 

 pression of the development, along appropriately distributed 

 tracts, of similar electrolytic changes primarily started by 

 some external physical or chemical alteration. But addi- 

 tional complications are introduced by the existence of 

 nerve-fibre endings and by the interposition of the nerve 



^ Gotch and Burch, yc«r«. o/Pltysiol.^vol. xxiv. 1899, p. 410. 



