September 30, 1922] 



NA TURE 



447 



The matter is discussed in an article on metrology, 

 by Mr. Scars, which will appear in the forthcoming 

 volume of the " Dictionary of Applied Physics." 



R. T. CiLAZEBROOK. 



Coton End, 63 Grange Road, Cambridge, 

 September 9. 



On the Reality of Nerve- Energy. 



If I have understood Prof. Fraser Harris's letter in 

 Nature of September 9, p. 342, it is a plea for the 

 more widespread use of the term " nerve-energy " by 

 physiologists and for the investigation of it as a 

 special manifestation of energv like heat, light, or 

 electricity. It is no doubt quite true that the term 

 has a definite and useful meaning in psychology and 

 psycho-pathology, though "mental energy" would 

 probably do as well in most cases. 



As a physiological concept, however, " nerve- 

 energy " has little to recommend it. Some idea of 

 the difficulties which are likely to attend its use may 

 be seen even in Prof. Harris's letter ; moreover, there 

 is very little need to postulate a special kind of energy 

 to explain the nervous impulse and its conduction, 

 for of all the different activities of living cells that of 

 conduction can be most readily described in terms of 

 phvsics and chemistry. 



The momentarv change which makes the nervous 

 impulse seems to consist in a depolarisation of the 

 surface layers of the nerve fibre, a resulting increase 

 in permeability, and an escape of ions from the 

 interior of the fibre. The movement of ions brings 

 about an increase of permeability in further sections 

 of the fibre and a decrease in the sections previously 

 active, so that the disturbance spreads but does not 

 last for more than a brief time at any one point. The 

 process is so simple in its essentials that Prof. Lillie 

 has been able to construct model nerve fibres of 

 iron wires coated with a film of passive iron and 

 immersed in nitric acid. These models copy the 

 behaviour of a nerve fibre with surprising fidelity. 



For some time past, evidence has accumulated in 

 favour of this explanation ; it would be quite mis- 

 leading to suggest that every detail of the conduction 

 of the impulse is understood, but the broad outlines 

 of the " membrane theory " have not been seriously 

 challenged. The energetics of the process wen- 

 worked out bv Bernstein. The system loses free 

 energy when the ions escape from regions of high to 

 regions of low concentration, and ultimately this 

 must be replaced by the metabolism of foodstuffs 

 in the fibre. The splitting up of a large molecule into 

 a number of smaller ones would suffice to restore the 

 concentration differences upon which the movement 

 of ions depends, and at various stages heat may be 

 given out or absorbed from the surroundings. In no 

 part of this scheme is there any need, or any oppor- 

 tunitv, for the introduction of a special form of 

 energy peculiar to nerves. 



If the term " nerve-energy " is to be retained it 

 might be used to mean the total potential energy 11 1 

 the neurone available for use in the transmission of 

 impulses, but it is doubtful whether much would be 

 gained bv the measurement of this quantity. Prof. 

 A. V. Hill has show n that the energy expended in the 

 passage of a single impulse is extremely small, and the 

 neurone is able to replenish its stores continually from 

 the nutrient fluids which surround it. When failure 

 of conduction occurs it seems to be due more often 

 to a failure of the surface reaction than to an exhaus- 

 tion of the store of potential energy in the fibre. 



E. D. Adrian. 



Trinity College, Cambridge. 



NO. 2761, VOL. I 10] 



Interspecific Sterility. 



Ix lhs letter on this subject (September 2, p. 312), 

 .Mr. Harrison states some interesting facts regarding 

 the chromosome numbers in Salix. This appears to 

 be the first case in plants where tetraploidy is accom- 

 panied by very little external change. Both tetraploid 

 and hexaploid numbers in a genus have long been 

 known, for example in Musa, but the point of my 

 remark regarding interspecific sterility in crosses 

 between diploid and tetraploid forms appears to have 

 been missed. It is not that there is any difficulty in 

 making such crosses in the first place. Usually they 

 are easily made, but the result is a triploid form with 

 an unstable chromosome content. 



If such crosses between a diploid and a tetraploid 

 species occur in Nature, they cannot lead to a 

 permanent, stable form, except by apogamous repro- 

 duction. The hybrid may cros£ back with either 

 parent, but this leads again to new and irregular 

 chromosome numbers, with the result that, in the 

 absence of apogamy, stability will be reached only 

 when the extra chromosomes have been lost and the 

 number has reverted to the diploid, or possibly in 

 some cases to a balanced intermediate number. The 

 two parental species, respectively diploid and tetra- 

 ploid, will in the meantime each have carried on its 

 own line of descent. 



It follows that if a tetraploid form arises from a 

 diploid species in Nature, it will continue to breed 

 true, while its hybrids with the parent species will 

 not give rise to a permanent line of descent unless 

 there is apogamous reproduction. This is, for prac- 

 tical purposes, a condition of physiological isolation. 

 Spiranthes cernua is a probable example of this sort, 

 and there are many others. Once two such independ- 

 ent lines of descent are established, the divergence 

 between them may go on increasing as fresh variations 

 occur in each series. R. Ruggles Gates. 



King's College, London. 



Micro-Chemical Methods in the Practical 

 Teaching of Chemistry. 



Ix view of the strenuous efforts now being made by 

 education authorities in this country to economise on 

 educational expenditure, considerable interest attaches 

 to Prof. Egerton Grey's letter on the application of 

 micro-chemical methods in the teaching of chemistry 

 (Nature, September 2, p. 309). During the War we 

 conducted a course of practical instruction in 

 chemistry in the Internment Camp for Civilian 

 Prisoners of War in Ruhleben, and the difficulty of 

 procuring large quantities of reagents led to the 

 adoption of " Micro " methods wherever possible. 

 Although we had at times forty students preparing 

 for universitv examinations, the consumption of 

 chemical reagents was extremely small in comparison 

 with what would be required in the ordinary way. 

 To give just one example, half a litre of nitric acid — 

 a precious liquid in the camp — was found sufficient 

 to supply the needs of these students for several 

 months. One enterprising student fitted for himself 

 a fully equipped " micro " laboratory in a tiny corner 

 of his loft and undertook interesting research work. 



A further advantage of the method is that the 

 quantities involved are so small that the students 

 can study the chemical properties of many of the 

 rarer elements with great advantage to themselves 

 and small expense to the laboratory. We are in 

 hearty agreement with Prof. Grey as to the economic 

 and educational advantages of the micro-chemical 

 methods. J. W. Blagden. 



A. Wechsler. 



