THE SPINAL ANIMAL 29 
ventrally or dorsally. But when tlie skin fields of the ganglia of the limb* region had been 
carefully ascertained, the results collated showed clearly that both in the brachial and in the pelvic 
limbs the zonal form of the fields still obtains, each semi-zone being wrapped half round the limb 
instead of half around the body. Hence exist what I have termed the ventral and dorsal axial 
lines of the limbs ; these, forming, as it were, lines of watershed between the systems of semi-zones 
(PI. I), meet at their one end the mid-ventral and mid-dorsal lines of the body, and may be 
looked upon as lateral extensions thereof. They are not hypothetical, for they are exhibited in the 
striping of animals. The dorsal axial line of the hind-limb is particularly well seen in the tiger, 
the stripes in that limb starting from it ; the dorsal axial line of the fore-limb is in the same way 
well seen in the zebra. The dorsal axial lines of the fore-limbs, diverging from the dorsal median 
line of the body, are in the ass usually marked by heavier pigmentation of the coat, thus making 
the ' sign of the Cross,' well known on the ass's shoulders. These axial lines of the limbs are of 
much clinical importance, for they are the boundaries observed by the upper limits of the anaesthesia 
accompanying injuries to the spinal cord or spinal roots in the regions of the lower and upper 
limbs respectively. 
The skin fields belonging to the spinal ganglia are wide, and that for each ganglion largely 
overlaps its neighbours. In the monkey I have not been able to satisfy myself that there is any 
patch of skin in the neck, trunk, or limbs that is not in receipt of sensory supply from each of two 
adjacent spinal ganglia. In certain regions {e.g. the hand, the foot, the pinna of the ear) the skin 
receives sensory fibres from each of three adjacent ganglia.t This explains how it is that the limb 
plexuses exist. The peripheral nerves of the limbs have to obtain components from more than 
one spinal nerve root. The innervation of the limb musculature is similarly plurisegmental. If, 
therefore, the spinal ganglion be considered a segmental collection of nerve-cells, those nerve-cells 
at their peripheral endings impinge on the body-surface over a zonal area which overlaps slightly 
with the contra-lateral zonal areas across the ventral and dorsal lines, and overlaps greatly with 
the collateral zonal areas next in front and next behind (headward and tailward) of itself. This 
zonal skin area may be considered a 'segmental field' of skin. 
As to which of the cell-bodies in the spinal ganglion belong to the skin-fibres, the 
probability is that the size of the nerve-fibres is a guide to the size of the cells, and that the 'skin' 
cells are neither the largest nor the majority ot the very smallest ; they are probably the majority 
of the medium-sized cells. 
The visceral constituent of the spinal ganglion is probably in all the ganglia numerically 
the weakest. It can be considered as practically, if not absolutely, wanting in the cervical ganglia 
and in the ganglia of the lower lumbar nerves. In order to estimate what number of the cells in 
a thoracic ganglion belong to its afferent path from the viscera, all that is necessary is to divide the 
spinal roots afferent and eflferent proximal to the ganglion, and subsequently, when time has 
been allowed for degeneration, to count the number of myelinate fibres remaining sound in the 
white ramus communicans. In this way I found the sensory ganglion of the tenth thoracic 
* Sherrington, ' Phil. Trans. Roy. Soc.,' London, 1892. 
t //«■</. 
