632 SYMPATHETIC AND OTHER SYSTEMS OF NERVES. 



We may now return for a moment to the stimulation of the spinal 

 nerves in the vertebral canal. I have said that in certain ways the 

 pilo-motor nerve fibres of the grey ramus of the nerve can be stimulated. 

 But these are post-ganglionic fibres, and they are not paralysed by nicotin. 

 So that, after observing the area of the skin in which the hairs move on 

 stimulation of each spinal nerve, we have only to inject nicotin into 

 the blood, and to stimulate the nerves a little past the posterior root 

 ganglia, or to stimulate it with a strong current, to observe also the 

 area supplied by each grey ramus. 



Thus, to take an example from an experiment. The thirteenth thoracic 

 nerve roots caused the hairs to become erect from a spot a little below the 

 third lumbar vertebra (fourth spine) to a little below the first coccygeal vertebra 

 (cf. long bracket XIII. in Fig. 311). In the middle part the hairs were strongly 

 erected ; in the ends, especially the lower end, they moved slightly only. Then 

 nicotin was injected. After this, stimulation of the thirteenth thoracic nerve 

 had no effect, unless the nerve was stimulated peripherally of the ganglion or 

 with strong currents, in which case a small patch of hairs just above the 



A m p 



Fig. 312. — Diagram to show variation in nerve-origin. M = median arrangement. 

 The upper nerve roots of M pass into the nerve above in the anterior arrange- 

 ment (A), and the lower nerve roots of M pass into the nerve below in the 

 posterior arrangement (P). 



previous limit moved sharply and strongly ; and on stimulating similarly 

 each of the following nerves, small successive patches were obtained, as 

 indicated in the lower line of brackets in the figure. In this case the 

 pre-ganglionic fibres of the thirteenth thoracic sent fibres to all the lumbar 

 sympathetic ganglia, sending most to the second, third, fourtb, and fifth 

 ganglia. It will be noticed that no area is given for the sixth lumbar grey 

 ramus ; as we have said above, this runs entirely to the limb. 



On investigation in this fashion, it becomes clear that there are 

 individual variations. In part these can be referred to a difference in 

 the functional activity of the tissues in different individuals ; in some, 

 for example, the contractile muscular tissue of the hairs is much better 

 developed than in others, and thus spinal nerves which produce a weak 

 erection of hairs in the one, may have no observable effect in the other. 

 In part, however, they are due to structural variations, which are 

 correlated with a number of other structural differences. The roots of 



