96 THOMPSON YATES LABORATORIES REPORT 
central station. In fact, they follow up the extension of the metamer, not by the process in 
virtue of which the other tissue of the metamer extends — namely, by cell-division, but by 
individual growth and increase of the length of the already existing cell-branches. This power of 
comparatively unlimited increase in individual size by elongation of cell-processes, enables the 
nerve-elements, with less metamorphosis and less spatial displacement of its nutritional centre than 
occur in any other tissue, to meet the requirements of changed surroundings, — in short, to become 
adapted with relatively slight transfiguration and dislocation. The pieces of the same individual 
cell can be traced, however far they may extend, by the physiological methods mentioned. A 
limitation to the use of the nerve-fibres as guides to the territorial extent of segments, though 
hardly to their indication of the broad position of segments, is however placed by their habit — 
as it appears to be — of somewhat overstepping, in their ultimate growth, the actual limits of the 
parent metamer. This overstepping is not in the skeletal muscles and in the sensory structures 
of the skin greater than to include a half, or generally rather less, of the adjacent segment, and it 
is regular in figure and extent. It certainly does not seriously confuse the picture that it affords, 
of the general segmental plan of a part. 
I have now to attempt an exposition of the general scheme of arrangement I believe 
evidenced by the facts detailed in the preceding sections. Before proceeding to such generalization, 
it is well to obtain, however, some answer to the following questions : — Is the amount of over- 
lapping of the skin-areas of the various spinal nerves equally great in all body-regions ? Is the 
amount of overlapf i ig of the various spinal nerves greater or less than that of the territories of 
peripheral nerve-trunks ? Is the amount of overlapping of the distribution of the sensory spinal- 
roots in skin equalled by that of the distribution of the motor spinal roots in muscles ? What 
functional significance can be attached to the overlapping ? 
Is the amount of overlapping of the skin-fields of adjacent dorsal [sensory) spinal roots equally 
great in all body regions. In my previous paper I stated that *I conclude that the anterior and 
posterior overlaps are extensive enough in the Monkey to provide that the skin taken along any 
line parallel with the plane of the segmentation is supplied by two adjacent posterior roots. It 
further seems certain that in some places the skin is supplied by three adjacent posterior roots,' In 
instance of skin receiving a triple root supply, I mentioned a part of the planta of the Cat, and the 
nipple of the Monkey, To these I will add portions of the skin of the hand, where a triple over- 
lap is clearly demonstrable by the following experiment : — The Vlllth cervical dorsal (sensory) 
root of Macacus having been severed both on left and right sides in the vertebral canal, these are 
further severed on one side [e.g.^ right) the dorsal (sensory) roots of the Vllth, Vlth and Vth 
cervical nerves, on the opposite side [e.g., left) the dorsal roots of the 1st, Ilnd and Ilird thoracic 
nerves. The field of remaining assthesia is then delimited in each hand. The lines of boundary 
will, on the one hand, be those of the posterior border of the Vllth cervical field ; on the other 
hand, those of the anterior border of the 1st thoracic field. Now, as shown above, the field of the 
Vlllth cervical nerve includes every portion of the surface of the whole hand. Further, although 
a certain amount of asymmetry can be detected in some individuals in the root distribution of right 
and left sides, that is of great rarity in the lower extremity, and even more uncommon in the upper 
