348 



SCIENCE 



[Vol. LVI, No. 1448 



impulse is not gradable by grading the strength 

 of the stimulus. The energy of the impulse 

 eomes not from the stimulus, but from the 

 fiber itself. But Lucas and Adrian have shown, 

 however, that it is gradable in another way. 

 Though the nerve impulse is a very brief af- 

 fair — it lasts about one thousandths second at 

 any one point of the nerve — it leaves behind 

 it in the nerve-fiber a short phase during which 

 the fiber cannot develop a second impulse. 

 Then follows rapid but gradual recovery of the 

 strength of impulse obtainable from the fiber. 

 That recovery may swing past normal to super- 

 noi-mal before returning finally to the old rest- 

 ing 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 centers, consists of nothing but chains of 

 cells and fibers. 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 gi-ading of im- 

 pulses by >the interference process just outlined 

 can lead, therefore, to narrowing or widening 

 their further distribution, much as in a rail- 

 way system the traffic can be blocked or for- 

 warded, condensed or scattered. Thus the dis- 

 tribution 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 sub- 

 tracting from the number of fibers activated 

 within that muscle. As pointed out by Pro- 

 fessor Alexander Forbes, it may be, therefore, 

 that the nerve impulse is the one and only re- 

 action 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 coordination of reflexes. The 

 nerve-center seems nothing more than a meet- 

 ing-place of nerve-fibers, 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 func- 

 tion throughout the system much simpler than 

 formerly obtained. 



Yet for some aspects of nervous mechanism 

 the nerve impulse offers little or no clue. The 

 fibers 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 prac- 

 tically unaltered throughout the individual's 

 existence, growing no further. If severed, say, 

 by a wound, they die for their whole length be- 

 tween the point of severance and the muscle or 

 skin they go to. Then at once the cut ends of 

 the nerve-fibers start regrowing from the point 

 of severance, although for years they have 

 given no sign of growth. The fiber, so to say, 

 tries to grow out to reach to its old far-distant 

 muscle. There are difficulties in its way. A 

 multitude of non-nervous repair cells growing 

 in the wound spin sear tissue across the new 

 fiber's path. Between these alien cells the new 

 nerve-fiber threads a tortuous way, avoiding 

 and never joining any of them. This olbstruc- 

 tion it may take many days to traverse. Then 

 it reaches a region where the sheath-cells of the 

 old dead nerve-fibers lie altered beyond ordi- 

 nary recognition. But the growing fiber recog- 

 nizes them. It joints them and, tunneling 

 through endless chains of them, arrives finally, 

 after weeks or months, at the wasted muscle- 

 fibers which seem to have been its goal, for it 

 connects with them at once. It pierces their 

 covering membranes and reforms with their 

 substance junctions of characteristic pattern re- 

 sembling the original that had died weeks or 

 months before. Then its growth ceases, 

 abruptly, as it began, and the wasted muscle 

 recovers and the lost function is restored. 



Can we trace the causes of this beneficent 

 yet so unaccountable reaction? How is it that 

 severance can start the nerve re-growing. How 

 does the nerve-fiber find its lost muscle micro- 

 scopically miles away? What is that mechan- 

 ism that drives and guides it? Is it a chemo- 

 taxis like that of the antherozooid in the botan- 

 ical experiment drawn towards the focus of the 



