628 SYMPATHETIC AND OTHER SYSTEMS OF NERVES. 



fibres can cause some contraction in the large arteries. In this way a 

 modification of blood supply to the skin, and to the muscles, may take 

 place, without any direct effect upon the peripheral vessels. 



Distribution of grey rami. — Broadly speaking, the successive spinal 

 nerves to the neck and the trunk are found anatomically to supply 

 successive bands of skin encircling the deeper lying tissues. These 

 rings are broken where the nerves pass to the fore- and hind-limbs; 

 there the several nerves cease to run separately, but join to form the 

 brachial and the lumbo-sacral plexus. One or more of the nerves in the 

 middle of the series of nerves forming the plexus send no cutaneous fibres 

 to the trunk, but run entirely to the limb. Since each spinal nerve has 

 its grey ramus, and since, as we have seen, the grey ramus contains sym- 

 pathetic fibres running to the periphery with the spinal nerve, the pro- 

 bability is great, that the distribution of the fibres of the successive grey 

 rami is the same in its general plan as that of the cutaneous branches of 

 the successive spinal nerves. 



The question is easily tested as regards pilo-motor fibres, by stimulating 

 the successive grey rami in the cat, and observing the areas of skin in which 

 erection of hairs occur. In this way, it is found 1 that the area of skin 

 innervated by successive effective grey rami are successive, overlapping by a 

 millimetre or two only, and that the areas correspond to the anatomical areas of 

 the successive spinal nerves as nearly as could be expected from the lack of 

 accuracy of the anatomical method. A similar result is obtained by observing 

 the contraction of the small arteries to the skin, although the result is less 

 constant and precise, on account of the variability on the course of the 

 arteries. 



I have said that the grey rami which are effective supply successive strips 

 of skin with pilo-motor nerves. There are one or more grey rami about the 

 middle of each of the series which form the brachial and lumbo-sacral plexuses, 

 which have no pilo-motor nerves. These rami run to nerves which have no 

 dorsal cutaneous branches. In the cat these nerves are the eighth cervical, often 

 with either the seventh cervical or the first thoracic in the region of the 

 brachial plexus ; and the sixth lumbar, often with either the fifth lumbar or the 

 seventh lumbar in the region of the lumbo-sacral plexus. When the sixth 

 and seventh lumbar nerves have no dorsal cutaneous branches, the sixth and 

 seventh lumbar grey rami have no pilo-motor nerve fibres, and send no fibres 

 to the skin over the vertebrae, the dorsal cutaneous branch of the first 

 sacral nerve follows 'immediately on that of the fifth lumbar, the first sacral 

 grey ramus causes erection of hairs in a strip of skin immediately posterior to 

 that affected by the fifth lumbar grey ramus. 



From these and other experiments, it is clear, as regards the skin of 

 the neck and trunk, that the grey ramus running from the sympathetic 

 to a spinal nerve may be regarded as having broadly the same skin 

 distribution as the cutaneous branches of the nerve to which it runs. 

 These branches contain sensory fibres of the skin. Sherrington 2 has 

 shown that the successive spinal nerves which send sensory fibres to the 

 skin of the neck and trunk supply successive areas, but that these areas 

 overlap considerably, so that in general each area includes a half of the 

 area above, and a half of the area below. On this plan it will be noticed 

 each portion of the skin sends afferent fibres to two nerve roots. As 

 we have said, the efferent fibres of the grey rami do not overlap to this 



1 Langley, Jonrn. Physiol., Cambridge and London, 1893, vol. xv. p. 199. 

 2 Phil. Trans., London, 1893, vol. clxxxiv. p. 641. 



