EXAMINATION OF SOME SPINAL NERVES 
U5 
Thoracic. Post-thoracic. 
Number of spinal nerve— . . XI. XII. I. II. III. IV. V. VI. VII. VIII. IX. X 
which evokes reflex movement of limb . + + + + + + + + 
which supplies sense-endings in skin of 
limb . . . . +++++ + + + 
which supplies motor innervation in 
muscle of limb . . . + -i- + + + + + 
By the above it is seen that centripetal impulses entering the spinal cord in the 
neighbourhood of the limb regions tend to be led into those regions ; in other words, irradiation 
is largely directed into those regions, and this both from above and from below. On the other 
hand, impulses entering the limb region by roots belonging to the sensory nerves of the limb do 
not so tend to irradiate beyond the confines of the limb region. Thus, impulses arriving vi& the 
sensory root of Ilird cervical usually evoke action of supinator longus more readily than impulses 
arriving by the sensory root of Vlth cervical evoke action in any part of the musculature belonging 
to Ilird cervical. Again, impulses poured into the sensory root of the ist post-thoracic usually 
evoke action of the flexors of the knee more readily than do impulses poured into the sensory root 
of the Vlth post-thoracic evoke contraction of the musculature innervated by the ist post-thoracic. 
I have given above as a broad rule of ' reflex spinal spread ' that, taken generally, there 
exists for each afferent root a reflex motor park or arc in its own spinal segment offering as little 
resistance as any open to it anywhere. Although that is so, there usually exist in each spinal 
segment certain groups of motor nerve-cells which are only with great difficulty reached or excited 
by excitor impulses conveyed into the segment by the fibres of its own afferent root. 
In a spinal segment of the limb region, among the entire collection of its motor nerve-cells 
certain sub-groups are far less excitable in local spinal reflexes than are others. In other words, of 
the entire contraction produced by direct excitation of a whole motor root certain parts are 
elicitable by spinal reflex much less easily than other parts. Whereas, between its own afferent 
root fibres and some of the motor neurons of the segment connection is facile, resistance low ; 
between its afferents and others of its motor neurons connection is difficult, resistance high. 
Hence, under the conditions maintained in my experiments, the centrifugal discharge, although 
occurring contemporaneously in several segments, in each of the several segments engaged certain 
only among the motor neurons. The motor discharge although plurisegmental is in each segment 
only fractional : certain neurons in the segment are selected, certain neglected. The nature of 
the selection recurs with a degree of constancy altogether remarkable, although not invariable. 
Examples : — Good contraction of supinator longus group in arm of Macacus, by excitation of dorsal 
(afferent) root Vth cervical nerve with Kronecker secondary coil at 15 k.s. ; to evoke contemporaneous 
biceps action as well as supinator, secondary had to be brought up to 90 k.s. No contraction of triceps 
could be provoked contemporaneously with supinator longus. 
In the Cat, cerebrum and cerebellum above their crura having been ablated, and the cord transected 
above Ilnd cervical nerve, a touch on the roof of the mouth causes wide opening of the mouth, i.e., 
depression of the mandible. The elevators of the jaw being much more powerful than the depressors, 
the afferent path (Vth cranial) must have selected the motor neurons of digastric, mylo-hyoid, &c., and 
neglected comparatively, or absolutely, or inhibited those of the elevator muscles. 
The intercostal muscles cannot at all readily be reflexly excited via the lateral cutaneous branches of 
the corresponding spinal nerves, but the superficial muscles of the chest can be. 
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