144 THOMPSON YATES LABORATORIES REPORT 
geneities as adductors of great toe, protrusors of anus, and vaso-dilators of the penis. Similarly 
with the skin, the median nerve-trunk supplies a patch of the palm that has obviously functional 
unity, but 1st thoracic nerve-root supplies such incongruities as the back and front of the little 
finger and of half the annulus and the tip of the olecranon process. It is the formation of func- 
tional collections of nerve-fibres [peripheral nerve-trunks) out of morphological collections [nerve-roots], 
which is the explanation — the meaning — of the existence of limh-plexuses. The reply to the oft-asked 
question what is the meaning and explanation of the distribution of the brachial and pelvic limb 
spinal nerves by plexuses, while the spinal nerves of the trunk region are not distributed by 
plexuses is, in my opinion, as follows : — In the trunk region, the innervation of the muscles of 
the skin is, as regards the distribution in them of the segmental nerves, a system of comparatively 
slight overlap : the peripheral territory of each segmental nerve — especially each motor territory — 
is confluent with but does not mingle nearly so widely with the neighbour territories as in the 
limb regions. That is to say, in other words, each several area of skin and of muscle, especially 
of the latter, has in either of the limbs a more pluri-segmental spinal innervation than a com- 
parable area in the trunk. The anatomical mode of innervating a definite area of tissue is, as we 
know, by means of collecting the nerve-fibres for the region into a nerve-trunk ; where the 
innervation is pluri-segmental, the nerve-trunk will therefore naturally be combined from compo- 
nents of several segmental nerves ; where several such areas co-exist, several pluri-segmental nerve- 
trunks will be formed, and the seperate segmental nerves will be split up into components, which 
become redistributed in the combinations which constitute the pluri-segmental nerve-trunks of the 
region — as, for instance, in the brachial region. The brachial and lumbo-sacral plexuses are an 
anatomical result of the greater degree of overlap, especially in the distribution of the motor part 
of their spinal nerves, obtaining in the limbs as compared with other, e.g., the trunk region of the 
body. 
Just as the motor spinal roots (apart from sympathetic) to the limb form a rather less 
numerous series* than do the sensory distributed to it, so the series of the latter is again rather 
less numerous, less extensive, than of those which by excitation approximately minimal will evoke 
reflexly a movement of the limb. The spinal apparatus of the limb may therefore be likened to 
a funnel, the wide entrant mouth of which is represented by sensory nerves, the narrow end of 
exit by the spinal motor roots to the nmsculature. In the upper limb the sensori-motor funnel 
has the following segmental extension : — 
Cervical. Thoracic. 
Number of spinal nerve— . . II. III. IV. V. VI. VII. VIII. I. II. III. IV. V. 
which evokes reflex movement of limb . + + ++ + + + + + +f 
which supplies sense-endings in skin 
limb . . . ^ +++++ + + + 
which supplies motor innervation to 
muscle of limb . . . ++++ + + 
and in the lower limb the following : — 
* Sherrington, ' Phil. Trans.,' B, vol. 184. 
t Not always. 
