II. Minute Localisation within the Excitable Area.
Repeatedly it was found that with a minimal stimulus it was possible to evoke movement either in the tail (and anus) only, or in the hamstrings, or in the hip or in the side only, and whenever this was obtained it was an invariable rule that the point for producing movement in the tail was placed in the cord mesially of that point, stimulation of which gave movement in the hamstrings, and that this latter point was mesial of that for the hip, while most external of all was the point from which movement of the side of the trunk was elicited. This lateral arrangement has been in part foreshadowed by the observations of Mott on the relation between the coccygeal nerves and Goll’s column.
(a) Latency of Effect.—Of course, in accordance with all previous investigations, the delay in passing through the spinal cord was well marked.
(b) Character of Movement Elicited from the Respective Roots.— Stimulation of the peripheral end of an anterior root gave, on the whole, a quick powerful extension of the whole limb, the latency, of course, being extremely short; on the other hand, excitation of the corresponding posterior root resulted in a slower, though strong, flexion of the whole limb with a well-marked latency. This remarkable functional distinction between the roots, viz., anterior giving extension and posterior flwTas quite constant, and was obtained in every animal in which the experiment was made. Of course, the movement which took place was a resultant effect, and was produced by the contraction of many muscles, each muscle contracting in whole, or in part, in combination with other muscles to produce the extension or flexion respectively.
The results with each root are given in the tables.
Further, stimulation of a posterior root (say the 5th) produced flexion of a joint or joints even when all the neighbouring anterior roots but one were divided. Hence this flexion can only be due to the stimulus passing from the posterior root thixmgh the spinal cord along a particular anterior root to the muscles (differentiation of function in the nerve centre of that root), yet stimulation of this same anterior root produces extension.
And this agrees entirely with the results obtained in a different way by Dr. Risien Russell (‘Phil. Trans.,’ 1893).
The above experiment also goes to show that stimulation of one posterior root causes impulses to pass out along many anterior roots.
