28 THOMPSON YATES LABORATORIES REPORT 
recently such work has vigorously been resumed. An example of its fruitfulness for the present 
problem is furnished by the observations of Hyde* on Limulus made in the Laboratory of 
Professor Loeb, of Chicago. 
When in Limulus all that is analogous to the brain has been ablated, and indeed only the 
abdominal region of the spinal cord remains, the rhythmic respiratory movements of the abdominal 
segments still proceed regularly and co-ordinately. Even when a fraction of the nerve-cord, 
separated by transections in front and behind, is left corresponding with a single abdominal 
segment, the musculature of that segment continues its rhythmic action. Its rhythm is then no 
longer timed to that of the adjacent segments ; its co-ordination with the rest is destroyed, but its 
activity is maintained. Its activity ceases only if the segmental fraction of the spinal cord is 
itself destroyed. This instance is paralleled by the sexual ' clasp ' reflex of the brachial segments 
of the male frog, maintained when all the rest of the central nervous system is destroyed. t 
Similarly, in the cat and monkey, the reflex wagging of the tail persists when behind the spinal 
transection only the sacral region of the cord is left intact. 
To judge how far the reactions of the spinal organ can be really considered as segmentally 
arranged, it is important to have a conception as clear as possible of the spatial relations of the 
spinal nerve-cells. The delineation of the spinal segment usually given presents its true extension 
very imperfectly. 
The edifice of the whole nervous system is based, as upon two pillars, upon two nerve- 
cells, the afferent root-cell, and the efferent root-cell. These form a fundamental spinal arch upon 
which all other neural arcs are superposed and functionally rest, immediately or mediately — even 
those of the hcmispheral cortex. The afferent root-cells of the spinal axis may be arranged in 
three great groups : (i) 'cutaneous,' from the sense organs of the skin ; (2) 'muscular,' from the 
sense organs of tlie musculo-articular apparatus ; (3) ' sympathetic,' from the viscera. Each spinal 
afferent root consists typically of three constituent roots — a cutaneous, a muscular, and a visceral. 
The efferent root-cells are conveniently grouped in two sets, one supplying skeletal muscles (i), 
the other entering the sympathetic chain (2) to innervate the musculature of the blood-vessels, of 
the skin, and of the viscera, including some secretory apparatus in the two latter. 
To deal with the afferent root-cells first. The cutaneous afferent root-cells have their 
perikarya or cell bodies in the spinal ganglion. Probably in each one of the spinal ganglia the 
majority of the cells belong to nerve-fibres afferent from skin. The peripheral distribution of 
the collection of cutaneous nerve-cells of each spinal ganglion occupies a semi-zonal field of body 
surface. X The zone is relatively wide, and invests a little more than the entire width of one 
lateral half of the body, trespassing slightly across the middle line both ventrally and dorsally. 
In the regions of the trunk and neck and perinaeum this zonal arrangement is quite obvious, but 
in the regions of the limbs it is less so, and at first sight appears departed from. The skin fields of 
the last three cervical and the first two thoracic, and of the last two lumbar and the first two 
sacral ganglia, are entirely confined to the limb, and do not meet the middle line of the body either 
'Journ. of Morphology,' 1894, vol. ix. t Goltz, ' Ccntralb. f. meii. Wissensch.,' 1870. 
\ Sherrington, ' Phil. Trans.,' 1892, London. 
