EXAMINATION OF SOME SPINAL NERVES 109 
of the anaesthesia was found to take place, althougli there were the usual small oscillations of 
briskness of reaction from day to day. The boundary remained stationary. On terniinating the 
experiments and dissecting the two animals, it was found that, in reality, in both individuals the 
roots cut had been exactly the same. No spinal complication was detected to explain the 
difference, but in (A) the plexus was extremelv prefixed, in (B), as I had expected, it was 
markedly post-fixed. 
In Monkey (A)— Post-thoracic roots. 
External cutaneous iicrx e was formed fruni . . Ilnd, IlIrd, IVth. 
Anterior crural nerve was formed from . . . IlIrd, IVth, Vth. 
Obturator nerve was formed from . . . Ilird, IVth, Vth. 
In Monkey (B)— 
External cutaneous nerve was from . . Ilird, IVth. 
Anterior crural nerve was from .... IVth, Vth. 
Obturator nerve was from .... IVth, Vtli, Vlth, 
the contribution from Vlth being a very small slip. 
The frequency and degree of this individual variation of root distribution must preseJit 
serious difficulties in clinical determination of the segmental level of the spinal lesion ; the exact 
seat of a spinal lesion will, because of individual variation, it seems to me, always baffle the 
clinician's knowledge so long as he has only the segmental level of skin points and muscles to 
serve as guides. I have not met in the brachial limb with any instance of individual variation 
so extreme in degree as the just-mentioned example from the lower limb. Slight degrees of 
variation appear to me about as common in the brachial as in the lumbo-sacral region. I also 
find them in t4ie upper neck muscles. Does the individual variation aflfect the spinal root supply 
of muscles as much as it influences that of skin r In the case of the lower limb of Monkey 
and Cat I have already pointed out that it does do so ; and the observations on the upper limb 
confirm the information obtained from the lower to the same effect. One and the same muscle, 
just as one and the same skin-point, is, in many individuals, distinctly differently iimervated as 
regards relation to spinal segments, from its segmental innervation in other individuals. For 
example, the extensor carpi radialis brevior receives in some specimens of Macacm rhesm motor 
nerve-fibres from the Vth cervical root ; in many specimens, on the contrary, it does not. The 
extensor longus pollicis in some individuals receives motor fibres from the Vlth cervical root ; in 
many it does not. The extensor carpi radialis brevior in some individuals receives fibres from the 
Vlllth cervical root; in many it does not. And so on. 
Before leaving this subject I will add that one meets certain instances of bilateral 
asymmetry of segmental innervation. Occasionally I have seen the extreme form of post-fixed 
lumbo-sacral plexus of Macacus, in which the IXth post-thoracic root iimervates the muscles of 
the foot, occur on one side of an individual and not upon the other; although usually, of course, 
bilateral. In the Cat I have altogether met with five individuals in which the Ilnd thoracic root 
contributed to the innervation of the palmar muscles upon one side and not upon the other ; and 
it is noteworthy that the post-fixed side was in some left, in some right. 
