

1893.] Correlation of Action of Antagonistic Muscles. 557 



of the vastus internus and part of the crureus divisions of the great 

 quadriceps extensor muscle of the thigh. The spinal centre was found 

 located in the 5th and 4th lumbar segments of the cord of the Rhesus 

 Monkey (4th and 3rd lumbar of Man). The efferent path was found 

 in the anterior roots of the 5th and 4th lumbar nerves, and was trace- 

 able along the anterior crural nerve into those of the muscular 

 branches of that trunk which supply the above-mentioned portions 

 of the quadriceps extensor max. The efferent side of the path corre- 

 sponds accurately with the course of the motor nerve fibres to the 

 muscles in question, and there is little reason to doubt that it con- 

 sists of nothing more or less than of those motor fibres themselves. 

 The afferent path was found to lie in the posterior root of the 5th 

 lumbar of Rhesus (4th of Man, 6th of Cat), and was not usually 

 demonstrable at all in the posterior root of the 4th lumbar, but a 

 small portion of it may, perhaps, lie within that root. 



The posterior root in which exists the afferent path on which the 

 jerk is dependent receives afferent fibres from the obturator and 

 anterior crural nerves, and from the external and internal popliteal 

 nerves, and sometimes from the division of the great sciatic which 

 may be called the hamstring nerve, because distributed to the ham- 

 string muscles. Of the fibres entering the root from these various 

 sources, those on which the " jerk " depends are not from any 

 except the anterior crural nerve. Further, in the anterior crural 

 nerve, they are those fibres of the nerve which issue from the vastus 

 internus and crureus muscles. Thus the afferent fibres on which the 

 jerk depends seem to arise within muscles, and from exactly those 

 muscles, to which belong the efferent fibres with which the " jerk " 

 is concerned. 



The rapid abolition of the jerk produced by severing the posterior 

 root of the 5th lumbar of Rhesus may conceivably be due less to 

 mere interruption of an afferent path than to excitation of an afferent 

 path by the ** current of injury " set up in the injured fibres (" de- 

 marcation current ") of it. This doubt has frequently been strength- 

 ened in my mind by the fact that section of one half the root often 

 suffices to abolish the "jerk," although the remaining half can, 

 when tested, still be shown able to conduct centripetal im- 

 pulses from the skin; and further, by the fact that it appears 

 immaterial whether the anterior or the posterior part of the 

 posterior root be selected for the section. The " jerk " I have 

 seen then abolished in a manner not obviously different from that in 

 which section of the whole root abolishes it. Against such an 

 explanation is, however, the permanence of the effect upon the 

 " jerk " produced by section of the whole root, for the effect con- 

 tinues at least for many days. Regarding the permanence of the 

 effect of section of half the root I have no observation. 



