THE PRESIDENTIAL ADDRESS. 5 



is the fullest which the nerve-fibre can at the time give. The energy 

 of the impulse comes not from the stimulus, but from the fibre itself. 

 Lucas and Adrian have shown it gradahle in another way. Though the 

 nerve impulse is a quite brief affair — it lasts about ^^i^^ 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 final return to the old resting state. Hence, by appropriately 

 timing the arrival of a second impulse after a first, that second impulse 

 may be extinguished or reduced or increased or transmitted without 

 alteration. This property oi grading impulses promises a. complete key 

 to reflex action if taken along with one other. The nervous system, 

 including its centres, consists ol 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 points the grading of impulses by 

 the interference process just outlined can lead, therefore, to narrow- 

 ing or widening of their further distribution, much as in a railway 

 system the traffic can be blocked or forv,'arded, condensed or scattered. 

 Thus the distribution and quantity of the muscular effect can be regu- 

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

 the same muscle 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 simply interfering, colliding 

 and over-running as they travel along the inter-connected branches of 

 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 combina- 

 tion. Fuller knowledge of the mechanism of the nervous impulse, 

 many of whose physical properties are now known, a reaction open to 

 study in the simplest units of the nervous system, thus leads to a view 

 of nervous function throughout that 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 instance, 

 the motor nerves of muscle and the sensory nerves of the skin. When 

 they are broken the muscle or skin is paralysed. They establish their 

 ties with muscle and skin during embryonic life. These ties they then 

 maintain practically unaltered throughout the individual's existence, 

 and show no further growth. If severed, say, by a wound, they die 



