io8 THOMPSON YATES LABORATORIES REPORT 
As to the relation between delicacy of co-ordination and number of nerve-roots contributing 
to motor innervation, one remembers that no muscles are more delicately adjustable than the 
ocular, although their individual innervation must be considered uni-segmental. 
Individual Variation 
A point that is very necessary to bear in mind in tlie discussion of generalizations 
obtainable from the above data remains to be insisted on before entering on that discussion. 
In the innervation of muscles and of skin-surface in regard to the nerve-root which supplies the 
innervation, a certain degree of latitude of individual variation occurs with quite remarkable 
frequency.* In the fore-limb as in the hind-limb, this is the case, and examples have been 
mentioned in Section II. In the experimental basis of this paper instances have been particularly 
numerous with the Vllth and Vlth cervical nerve ; that may, however, be a fortuitous result. 
It would require a very large number of experiments to ascertain conclusively whether the 
peripheral distribution of those roots is more variable than that of the other brachial nerve-roots. 
I have already given evidencet that individual variation affects not one root alone, but a series of 
consecutive roots ; but it may, perhaps, reacli its maximum at some one root of the series. 
Since writing my previous paper I have met in the skin-fields of the lower limb a 
particularly pronounced example of individual variation in the distribution of the Ilnd lumbar 
nerve of Macacus rhesus. In two Rhesus Monkeys I severed the dorsal (sensory) roots of the Ilird, 
IVth, Vth, Vlth, Vllth, Vlllth, and IXth post-thoracic nerves of the left side inside the vertebral 
canal. It is difficult in the region of the cauda equina to judge at the time of operation as to the 
exact segmental level of tiie nerve-roots exposed, and that has to remain, for the time being, a 
matter of doubt. When the operation wounds had well healed, the field of remaining anaesthesia 
was in each of the two Monkeys determined, not on one occasion only, but on many, indeed 
almost daily for some weeks. In the one animal (A), the ana-sthetic area did not extend up the 
front of the leg quite so high as to tlie patella, tiiat is, the skin of the front of the thigh and over 
the patella (covering the patella) still retained sensation, distinct though impaired. In the other 
animal (B), the field of the anaesthesia extended fully two-thirds up the front of the thigh; 
sensation was retained in a tongue-shaped field of skin covering Scarpa's triangles, and lower down 
sensation was completely wanting. In (A) the skin between the anus and the tuberosity of the 
left ischium was anaesthetic, although between the anus and root of the tail cutaneous sensation 
was distinctly present, though blunt. In (B) the skin lying between anus and left ischial 
tuber was for its half nearer to the anus distinctly sentient, as also above the anus between it and 
the root of the tail. From this I did not hesitate to conclude that in animal (A) the IVth post- 
thoracic root, but not the Ilird, had been included in the series severed ; and that in (B), that Ilird 
post-thoracic had been severed as well as the IVth, and I thought (B) in all probability an 
individual with a pelvic plexus of markedly post-fixed type. I knew I had divided the same 
number of roots in the two individuals. The animals were kept after the operation wound was 
fully healed (lO days) for six weeks, and frequently compared ; no obvious alteration in the extent 
* Sherrington, 'The Lumbo-sacral Plexus.' ' Journ. of Physiol.,' vol. 13, 1892. 
t Ibid. 
