September 9, 1922J 



NA TURE 



347 



is the retina, has to be postured normally, and the pair 

 of little gravity-bags in the skull, which serve to restore 

 the head posture, act also on the eyeball muscles. 

 Whichever way the head turns, slopes, or is tilted, they 

 adjust the eyeball's posture compensatingly, so that the 

 retina still looks out upon its world from an approxi- 

 mately normal posture, retaining its old verticals and 

 horizontals. As the head twists to the right the eye- 

 ball's visual axis untwists from the right. These 

 reactions of head, eyes and body unconsciously take 

 place when a bird wheels or slants in flight or a pilot 

 stalls or banks his aeroplane ; and all this works itself 

 involuntarily as a pure mechanism. 



True, in such a glimpse of mechanism what we see 

 mainly is how the machinery starts and what finally 

 comes out of it ; of the intermediate elements of the 

 process we know less. Each insight into mechanism 

 reveals more mechanism still to know. Thus, scarcely 

 was the animal's energy balance in its bearing upon 

 food intake shown comfortably to conform with 

 thermodynamics than came evidence of the so-called 

 " vitamins " — evidence showing an unsuspected influ- 

 ence on nutrition by elements of diet taken in quantities 

 so small as to make their mere calorie value quite 

 negligible ; thus, for the growing rat, to quote Prof. 

 Harden, a quantity of vitamin A of the order of - ? i ff 

 milligram a day has potent effect. Again, as regards 

 sex determination, the valued discovery of a visible 

 distinction between the nuclear threads of male and 

 female brings the further complexity that, in such cases, 

 sex extends throughout the whole body to every dividing 

 cell. Again, the association of hereditary unit-factors, 

 such as body colour or shape of wing, to visible details 

 in the segmenting nucleus seemed to simplify by 

 epitomising. But further insight tends to trace the 

 inherited unit character not to the chromosome itself, 

 but to balance of action between the chromosome group. 

 As with the atom in this heroic age of physicists, the 

 elementarv unit once assumed simple proves, under 

 further analysis, to be itself complex. Analysis opens 

 a vista of further analysis required. Knowledge of 

 muscle contraction has, from the work of Fletcher and 

 Hopkins on to Hill, Hartree. Meyerhof, and others, 

 advanced recently more than in many decades hereto- 

 fore. The engineer would find it difficult to make a 

 motive machine out of white of egg, some dissolved 

 salts, and thin membrane. Yet this is practically 

 what Nature has done in muscle, and obtained a 

 machine of high mechanical efficiency. Perhaps 

 human ingenuity can learn from it. One feature in 

 the device is alternate development and removal of 

 acidity. The cycle of contraction and relaxation is 

 traced to the production of lactic acid from glycogen 

 and its neutralisation chiefly by alkaline proteins ; and 

 physically to an admirably direct transition from 

 chemical to mechanical effect. What new steps of 

 mechanism all this now opens ! 



But knowledge, while making for complexity, makes 

 also for simplification. There seems promise of simpli- 

 fication of the mechanism of reflex action. Reflex 

 action with surprising nicety calls into play just the 

 appropriate muscles, and adjusts them in time and in 

 the suitable grading of their strength of pull. The 

 moderating as well as the driving of muscles is involved. 

 Also the muscles have to pass from the behest of one 



NO. 2758, VOL. I 10] 



stimulus to that of another, even though the former 

 stimulus still persist. For these gradings, coadjust- 

 ments, restraints, and shifts, various separate kinds of 

 mechanism were assumed to exist in the nerve-centres, 

 although of the nature of such mechanisms little could 

 be said. Their processes were regarded as peculiar to 

 the nerve-centres and different from anything that the 

 simple fibres of nerve-trunks outside the centres can 

 produce. We owe to Lucas and Adrian the demon- 

 stration that, without any nerve-centre whatever, an 

 excised nerve-trunk with its muscle attached can be 

 brought to yield, besides conduction of nerve impulses, 

 the grading of them. That is remarkable, because the 

 impulse is not gradable by grading the strength of the 

 stimulus. The energy of the impulse comes not from 

 the stimulus, but from the fibre itself. But Lucas and 

 Adrian have shown, however, that it is gradable in 

 another way. Though the nerve impulse is a very brief 

 affair — it lasts about yi^ second at any one point of 

 the nerve — it leaves behind it in the nerve-fibre a short 

 phase during which the fibre cannot develop a second 

 impulse. Then follows rapid but gradual recovery of 

 the strength of impulse obtainable from the fibre. That 

 recovery may swing past normal to super-normal before 

 returning finally to the old resting state. Hence, by 

 appropriately timing the arrival of a second impulse 

 after a first, that second impulse may be extinguished, 

 reduced, increased or transmitted without alteration. 

 This property of grading impulses promises a complete 

 key to reflex action if taken along with one other. The 

 nervous system, including its centres, consists of nothing 

 but chains of cells and fibres. In these chains the 

 junctions of the links appear to be points across which 

 a large impulse can pass, though a weak one will fail. 

 At these poiats the grading of impulses by the interfer- 

 ence process just outlined can lead, therefore, to narrow- 

 ing or widening their further distribution, much as in a 

 railway system the traffic can be blocked or forwarded, 

 condensed or scattered. Thus the distribution and 

 quantity of the muscular effect can be regulated and 

 shifted not only from one muscle to another, but in one 

 and the same muscle it can be graded by adding to or 

 subtracting from the number of fibres activated within 

 that muscle. As pointed out by Prof. Alexander 

 Forbes, it may be, therefore, that the nerve impulse 

 is the one and only reaction throughout the whole 

 nervous system, central and peripheral, — trains of 

 impulses colliding and over-running as they travel along 

 the conductive network. In this may lie the secret of 

 the co-ordination of reflexes. The nerve-centre seems 

 nothing more than a meeting-place of nerve-fibres, its 

 properties but those of impulses in combination. Fuller 

 knowledge of the mechanism of the nervous impulse, 

 many of the physical properties of which are now- 

 known, a reaction which can be studied in the simplest 

 units of the nervous system, thus leads to a view of 

 nervous function throughout the system much simpler 

 than formerly obtained. 



Yet for some aspects of nervous mechanism the nerve 

 impulse offers little or no clue. The fibres of nerve- 

 trunks are, perhaps, of all nerve-structures those that 

 are best known. They constitute, for example, the 

 motor nerves of muscle and the sensory nerves of the 

 skin. They establish their ties with muscle and skin 

 during embryonic life and maintain them practi- 



