FEATURES OF SYMPATHETIC NERVE SUPPLY. 637 



presents certain difficulties in considering the whole question. We need 

 not, however, dwell on these. It is sufficient at present to point out 

 that a bundle of pre-ganglionic fibres may run to a single vertebral 

 ganglion, and restrict its connections with cells which supply a given 

 region, entirely passing over the cells which supply fibres to other 

 regions. 



The external generative organs and a portion of the ano-genital skin 

 receive efferent fibres from the sacral nerves by way of the pelvic nerve 

 (nervus erigens), but these fibres do not belong to the sympathetic 

 system, and will be considered later. 



Some General Features of the Sympathetic Nerve Supply 



to the Skin. 



White and grey rami as paths for fibres from the spinal cord. — 

 It appears from the preceding account that in the cat, the skin of the 

 body receives in all cases sympathetic fibres from the first thoracic to 

 the fourth lumbar nerves inclusive, and from no others, 1 except in those 

 cases in which the nerves have a more posterior arrangement, when 

 the skin receives some sympathetic fibres from the fifth lumbar nerve 

 also. These nerves are the only ones which, on anatomical and micro- 

 scopical investigation, are found to have white rami connecting them 

 with the sympathetic chain. Since every nerve is connected with a 

 vertebral ganglion by a grey ramus, the conclusion may fairly be drawn 

 that all the sympathetic nerve fibres which run to the skin from the 

 spinal cord, run to the sympathetic chain, on their way to the skin, by 

 the white rami. And the conclusion is confirmed by the entire absence 

 of sympathetic effect, when the roots of a lower thoracic or upper lumbar 

 nerve 2 are stimulated after section of its white ramus, its grey ramus 

 remaining intact. 



The adoption of the view that the efferent fibres of the sympathetic 

 pass to it by the white rami, is due to Gaskell. 3 He arrived at his conclusions 

 chiefly from a histological investigation of the white and grey rami and of the 

 nerve roots. He pointed out that since the nerve roots contained very few 

 non-medullated fibres, and since . the few present might reasonably be sup- 

 posed to end in the vessels of the spinal cord, it must be concluded that all 

 the fibres running from the cord to the sympathetic are medullated fibres. 

 Further, since medullated fibres are numerous in the white rami, and very 

 scanty in the grey rami, it must be concluded that the white rami form the 

 channel of communication between the spinal cord and the sympathetic. 



This line of argument is satisfactory up to a certain point, but it is only 

 conclusive as regards the great majority of fibres. It does not exclude the 

 possibility of some efferent fibres passing by the grey rami. In fact, some 

 grey rami contain a considerable number of medullated fibres (cf. p. 649). 



If efferent fibres run to the sympathetic chain by way of the grey rami, 

 they would degenerate on section of the nerve roots. Some experiments 4 

 have been made on the lines thus suggested. As a rule, section of the nerve 

 roots caused no degeneration of medullated fibres in the grey rami. In some 



1 A considerable number of observers have described sympathetic effects as occurring on 

 stimulation of nerves which have grey rami only. As I have been unable to obtain any 

 such effects, I attribute them to experimental errors. 



- The uppermost thoracic white and grey rami are too closely associated to be cut 

 separately. 



3 Journ. Physiol., Cambridge and London, 1886, vol. vii. p. 1. 



4 Langley, ibid., 1896, vol. xx. p. 60. 



