EXAMINATION OF SOME SPINAL NERVES 
89 
SECTION II.— THE SEGMENTAL SCHEME OF INNERVATION 
IN THE LIMBS 
It was pointed out in my previous paper* on the innervation of the limbs, that there is, 
between the scheme of distribution of the motor and of the sensory spinal roots, a striking 
difference when the fields of the two roots are examined in the limbs, by, on the one hand, motor 
nerves to muscles, on the other, sensory nerves to skin. 
The motor root of each spinal nerve was found to supply a band of muscular tissue, 
extending as a fairly continuous ray from the trunk and attached base of the limb, laterally, 
outward a greater or less distance toward the limb apex. The motor rays examined in serial 
order from before backward were found to extend further and further laterally, so that the 
hindmost two (or, in some individuals, three) reach and contribute to the extreme apex of the 
limb. Of the motor spinal roots supplying the limb, each, even of those penetrating to the 
extreme apex of the limb, still contributes to the innervation of the muscles of the trunk. That 
is to say, none of the motor segments of the limb are detached from the median plane of the 
trunk. As instance, the 5th muscular ray of the pelvic limb, contributing as it does to the limb's 
extreme apex, nevertheless does not lose its base in the axial muscles of tiie trunk, but gives 
a share to such axial muscles as sacro-cocc\'geus. 
With this arrangement of the motor roots the distribution of the sensory roots to the skin 
does not accord. The fields of distribution of the sensory roots, as examined on the surface of the 
limbs, are for those roots which supply the apex of the limb, widely disjoined from the median 
plane of the body, both dorsally and ventrally. The sensory arrangement is best described by 
imagining that into the proximal part of the limb on its dorsal and ventral surfaces the mid-dorsal 
and mid-ventral lines of the body have each tiirust a spike, a secondary lateral axis, sidewise, 
almost at right angles to their own direction. Granted these side lines, the arrangement of the 
spinal skin-fields of the limb can be accurately stated by saying that they are ranged upon the 
secondary dorsal and ventral lines as though upon folded portions of the axial lines of the trunk 
itself. I gave further, in a previous paper,* reasons besides the above for believing the secondary 
axes to have a real, not merely hypothetical, existence. 
Also, in another respect, the fields of distribution of the motor and sensory spinal roots 
respectively are, as examined in the muscles and in the skin of the limb, widely dissimilar. The 
motor rays segmentally composing the musculature of the limb are set in fore and aft series in 
such a way that the pelvic limb has a sloping anterior side,t because the rays entering the limb 
are, in front, four, of which each extends further into the limb than the immediately precedent 
ray. Along its posterior edge, on the contrary, the limb is composed in its whole length of one 
ray only, the Vlth or most posterior of all those in the musculature of the limb. With the 
distribution of the sensory roots of the limb, as examined in the skin, the arrangement is different. 
The skin-fields of the sensory roots lie along the posterior face of the limb as well as along its 
* ' Phil. Trans.,' B, 1892, vol. 184. 
■\ In other words, the anterior eilge of the limb is not at right angles to the axis of the trunk, hut encloses behinil it witli 
that axis an angle less than a right angle. 
M 
