September 8, 1904] 



NA TURE 



461 



If we regard the nervous system of any higher organism 

 from the broad point of view, a salient feature in its archi- 

 tecture is the following. At the commencement of every 

 reflex arc is a receptive neurone, extending from the 

 receptive surface to the central nervous organ. That 

 neurone forms the sole avenue which impulses generated at 

 its receptive point can use whithersoever may be their 

 distant destination. That neurone is therefore a path 

 exclusive to the impulses generated at its own receptive 

 points, and other receptive points than its own cannot 

 employ it. 



But at the termination of every reflex arc we find a final 

 neurone, the ultimate conductive link to an effector organ, 

 gland or muscle. This last link in the chain, e.g. the motor 

 neurone, differs obviously in one important respect from the 

 first link of the chain. It does not subserve exclusively 

 impulses^ generated at one single receptive source alone, 

 but receives impulses from many 

 receptive sources situate in manv 

 and various regions of the body. 

 It is the sole path which all 

 impulses, no matter whence thev 

 come, must travel if they would 

 reach the muscle-fibres which it 

 joins. Therefore, while the re- 

 ceptive neurone forms a private 

 path e.xclusive for impulses of one 

 source only, the final or efferent 

 neurone is, so to say, a public 

 path, common to impulses arising 

 at any of many sources in a 

 variety of receptive regions of the 

 body. The same effector organ 

 stands in reflex connection not 

 only with many individual re- 

 ceptive points, but even with 

 many various receptive fields. 

 Reflex arcs arising in manifold 

 sense-organs can pour their in- 

 fluence into one and the same 

 muscle. A limb-muscle is the 

 terminus ad quern of nervous arcs 

 arising not only in the right eye 

 but in the left, not only in the 

 eyes but in the organs of smell 

 and hearing ; not only in these, 

 but in the geotropie labyrinth, in 

 the skin, and in the muscles and 

 joints of the limb itself and of the 

 other limbs as well. Its motor 

 nerve is a path common to all 

 these. 



Reflex arcs show therefore the 

 general feature that the initial 

 neurone is a private path exclusive 

 for a single receptive point ; and 

 that finally the' arcs embouch into 

 a path leading to an effector 

 organ, and that this final path is 

 common to all receptive points 

 wheresoever they may lie in the 

 body, so long as they have any 

 connection at all with the effector 

 organ in question. Before finally 

 converging upon the motor 

 neurone arcs usually converge to 

 some degree by their private 



paths embouching upon internuncial paths common in 

 various degree to groups of private paths. The terminal 

 path may, to distinguish it from internuncial common paths, 

 be called the final common path. The motor nerve to a 

 muscle is a collection of such final common paths. 



Certain results flow from this arrangement. One seems 

 the preclusion of qualitative differences betw-een nerve- 

 impulses arising in different afferent nerves. If two con- 

 ductors have a tract in common, there can hardly be quali- 

 tative difference between their modes of conduction. 



.\ second result is that each receptor being dependent for 

 communication with its effector organ upon a path not 

 exclusively its own but common to it w^ith certain other 

 receptors, that nexus necessitates successive and not simul- 



taneous use of the common path by various receptors using 

 it to different effect. 



Let us consider this for a moment. Take the primary 

 retinal reflex, which moves the eye so as to bring the fovea 

 to the situation of the stimulating image. From all the 

 receptors in each lateral retinal half rise reflex arcs with a 

 final common path in the nerve of the opposite rectus 

 lateralis. Suppose simultaneous stimulation of two of these 

 retinal points, one nearer to, one farther from, the fovea. 

 If the arcs of both points pour their impulses into the final 

 common path together, the effect must be a resultant of 

 the two discharges. If these sum, the shortening of the 

 muscle will be too great and the fovea swing too far for 

 either point. If the resultant be a compromise between the 

 two individual effects, the fovea will come to lie between 

 the two points of stimulation. In both cases the result 

 obtained would be useless for the purposes of either. Were 



, I. — The Scratch Reflex. A. — The "receptive field," as revealed after low cervical transection, a 

 saddle-shaped area of dorsal skin, whence the scratch reflex of the left hind limb can be evoked. 

 /r marks the position of the last rib. B. — Diagram of the spinal arcs involved. L, receptive or 

 afferent nerve-palh from the left foot ; R, receptive nerve-path from the opposite foot : sa, s^, recep- 

 live nerve-paths from hairs in the dorsal skin of the left side ; FC, the final common path, in this case 

 the motor neurone to a flexor muscle of the hip ; pa, P)3, proprio-sptnal i 



there to occur at the final common path summation of the 

 impulses received from two unlike receptors, there would 

 result in the effector organ an action useless for the purposes 

 of either. 



When two stimuli are applied simultaneously which would 

 evoke reflex actions that employ the same final common 

 path in different ways, in my experience one reflex appears 

 without the other. The result is this reflex or that reflex, 

 but not the two together. Excitation of the afferent root of 

 the eighth or seventh cervical nerve of the monkey evokes 

 reflexly in the same individual animal, sometimes flexion 

 at elbow, sometimes extension. If the excitation be pre- 

 ceded by excitation of the first thoracic root, the result is 

 almost always extension ; if preceded by excitation of the 



NO. 1 8 19, VOL. 70] 



