Mabch 19, 1915] 



SCIENCE 



413 



artificial means. If, on the other hand, the 

 same axon should send one branch to a 

 sense organ and another to a muscle, or 

 gland, or blood vessel, we would have the 

 possibility of an axon reflex as a normal 

 event. Keeent work suggests that this pos- 

 sibility may be realized. 



In 1901 Bayliss* found that stimulation 

 of the posterior roots of the sacral nerves 

 between the ganglion and the cord produces 

 dilation of the blood-vessels of the hind 

 limb. The natural explanation of the re- 

 sult, that certain vaso-dilator neurones 

 may send their axons out by the unusual 

 path of the posterior instead of the ante- 

 rior nerve roots, was disproved by the fact 

 that if the posterior roots are cut near the 

 cord and degeneration allowed to occur, 

 stimulation of the peripheral stump of the 

 cut root still produces the dilation. In 

 other words, the ganglion of the posterior 

 root is the trophic center for some of the 

 essential fibers concerned. From consid- 

 eration of the known histological possibil- 

 ities Bayliss concluded that the fibers pro- 

 ducing the dilation are the ordinary affer- 

 ent fibers from the pear-shaped cells of the 

 ganglion, the distal axon being supposed to 

 branch at its ending, one branch going to 

 the sense organ, and the other to the blood- 

 vessel. He furthermore supposed that in 

 addition to serving as a trophic center for 

 the afferent fiber, these cells may be reflexly 

 stimulated by other afferent fibers and thus 

 discharge "antidromic" impulses to the 

 periphery; such impulses passing over the 

 branch to the blood-vessel produces the 

 dilation, while the impulse over the branch 

 to the sense organ would be without effect 

 ( " law of irreciprocal conduction " ) . Phys- 

 iologists have, however, looked askance on 

 this conception of antidromic impulses, 

 even as a working hypothesis. 



* Bayliss, Journal of PJi/ysiology, 1901, XXVI., 

 173; ib., 1902, XXVIII., 276. 



The very important experiments of 

 Bruce,^ however, put the matter in a new 

 light. It is well known that when an irri- 

 tant is applied to the skin, a dilation of the 

 arterioles (active congestion) ushers in the 

 inflammatory reaction. Bruce shows that 

 this will not occur if the area to which the 

 irritant is applied is first rendered anes- 

 thetic with cocaine. It will occur immedi- 

 ately after section of the anterior roots or 

 of the posterior roots, either centrally or 

 distally of the ganglion ; hence it is not a 

 reflex through the cord or the ganglion. It 

 will not occur, however, after the comple- 

 tion of the peripheral degeneration conse- 

 quent upon section of the posterior root 

 distally to the ganglion. In other words, it 

 would seem to depend solely upon the integ- 

 rity of the distal limb of the neurones of 

 the posterior roots, and to be independent 

 of any nerve cell whatever. This would 

 seem almost to force^ the conclusion that 

 we are dealing with an axon reflex. The 

 posterior root fiber branches, as Bayliss sup- 

 posed, at its ending, one branch going to 

 the sense organ while the other serves as a 

 dilator of the arterioles. The same fiber, 

 probably one of pain, which carries the 

 afferent impulse giving rise to the sensation 

 of irritation produces also the active con- 

 gestion of the region through its vascular 

 collaterals. 



If these observations prove well founded, 

 the axon reflex becomes a reality in the 

 normal functioning of the organism, in- 

 stead of a laboratory curiosity. Moreover, 

 the facts discovered by Bayliss receive their 



5 A. Ninian Bruce, Quarterly Journal of Exp. 

 Physiology, 1913, VI., p. 339. 



6 The writer can imagine only one other possibil- 

 ity; namely, collaterals given off from afferent 

 fibers distally to the ganglion may enter the 

 sympathetic ganglia, which would thus serve as 

 reflex centers. No such collateral communications 

 in nerve trunks have, however, been described. 

 Indeed it is the usual teaching that nerve fibers 

 branch only in the ganglion or at their endings. 



