EXAMINATION OF SOME SPINAL NERVES 
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and limb between overlying skin innervation and underlying muscular innervation. The former 
being segmentally anterior to the latter, one might have expected that the sensory root of a spinal 
nerve would have evoked motor reply primarily from a segment a couple of metameres lower 
down in the segmental chain rather than from the motor neurons of its own segmental level. 
I have, indeed, given evidence above, that in some regions the direction taken by ' spread ' does 
actually, to some extent, bear out the supposition suggested by the former paper ; in other words, 
that, contrary to Pfluger's 4th law, the spread does not occur with greater facility in direction 
towards the brain, but rather, in some cases, with greater facility in direction from the brain. This 
downward tendency supports, not overrides, the rule ; so that, indeed, from the rule of segmental 
proximity, another, which is its logical corollary, may be proceeded to. This is : taken generally^ 
for each afferent root there exists in its own spinal segment a reflex motor path of as little resistance as 
any open to it anywhere. 
Examples : — The adduction flexion of hallux elicitable with minimal stimuli from the Vlllth 
post-thoracic afferent root is obtainable with the same or even a lesser stimulus when the Vlth, Vllth, and 
IXth post-thoracic motor roots are severed. (Monkey.) 
Contraction of the hamstrings is elicitable from the Vlth post-thoracic afferent root via the Vlth 
post-thoracic motor root with an absolutely minimal stimulus. (Monkey.) 
The afferent roots of the Vth, Vlth, and Vllth cervical, and 1st thoracic nerves having been 
severed, excitation of the skin of the palm evokes flexion and adduction of the thumb ; this response is 
not altered in character by subsequent section of the Vlth and Vllth cervical and 1st and Ilnd thoracic 
motor roots. (Monkey.) A similar result to this is obtainable from the Vllth post-thoracic and the 
hallux. (Monkey.) 
By minimal excitation of the long saphenous nerve flexion of the hip, due to fascialis and rectus 
internus, is elicited, and it is usually only by increasing the strength of stimulation that other muscles {e.g., 
psoas and pectineus) co-operate with the above. The same result is seen when the Vth and IVth post- 
thoracic afferent roots have been severed. Now, I have pointed out in my previous paper that the fascialis 
is the only muscle in the lower limb supplied by one root only ; that root is the Vlth post-thoracic. 
Hence the internal saphenous, by its component from the Vlth post-thoracic root, elicits contraction at 
least as easily as from any other, from a muscle whose motor innervation is through the corresponding {i.e.^ 
Vlth) motor root. (Cat.) 
By minimal excitation of the IXth post-thoracic afferent root protrusion of the anus is usually 
more easily obtained than flexion of the digits, and this is so in prefixed plexuses in which the IXth motor 
root does not innervate any muscles of the limb. The anal reflex persists after section of the Vllth, 
Vlllth, and Xth motor roots, i.e.., must have had a component through the IXth motor : the IXth afferent 
root, therefore, shows a slight preference for discharging into a motor centre present in its own segment, as 
compared with another which is also open to it, but outside its own segment. 
Minimal excitation of the Vlth cervical afferent root elicits contraction of the supinator longus, 
while minimal excitation of Vlllth cervical afferent root elicits flexion and adduction of the hallux : 
motor fibres of the Vlth cervical innervate the supinator longus, while motor fibres of the Vlllth cervical 
innervate the long and short muscles of the thumb. 
Excitation (mechanical) of the 1st thoracic afferent root evokes usually flexion and adduction of 
thumb, but no dilatation of pupil ; similar excitation of the Ilird thoracic afferent root evokes usually 
well-marked dilatation of the pupil (almost exclusively homonymous), but no movement of thumb. The 
motor root of the 1st thoracic innervates thumb muscles but not the iris, while that of the Ilird 
innervates the iris but not the thumb muscles ( Macacus). 
In the monkey adduction-flexion of pollex and hallux, which are the most facile cutaneous reflexes 
of the whole limb, occurring when all others are absent, is provoked from the ulnar and fibular sides of 
palm and sole and from little digit easier than from elsewhere : ulnar and fibular sides of palm and sole are 
supplied by those spinal ganglia corresponding with the spinal segments possessing motor neurons for pollex 
