NERVE FIBRES OF SYMPATHETIC SYSTEM. 649 



large fibres, and sometimes none ; the few which may be present pro- 

 bably arise from the vagus. In the cat, the sympathetic chain apart 

 from its ganglia contains few non-medullated fibres. Some of these 

 are fibres we have spoken of above (p. 629) as passing from one ganglion 

 to the ramus of a neighbouring ganglion. In the rabbit, non-medullated 

 fibres are more numerous, and in the sacral region they are the chief 

 constituent, very few medullated fibres being present. These non- 

 medullated fibres I take to be largely pre-ganglionic fibres which have 

 lost their medulla (cf. p. 650). 



The non-medullated fibres make up about one-fifth of the transverse 

 section of the great splanchnic nerve (cf. p. 642). There are fewer in the 

 small splanchnics, still fewer in the inferior splanchnics and in the 

 cervical sympathetic. 



In the branches proceeding from the superior cervical ganglion and 

 the pre-vertebral ganglia the non-medullated fibres preponderate. Some 

 have but few medullated fibres, and others have many. 



The internal carotid branches of the superior cervical ganglion contain in the 

 cat several hundred small medullated fibres, in the rabbit there are a few only, 

 and usually a few only in the dog. The external carotid branches of the 

 superior cervical ganglion contain only a few medullated fibres. A small nerve 

 filament, consisting chiefly of medium and large fibres, runs to these branches 

 from the vago-glosso-pharyngeal nerve. The cardiac accelerator nerve, which 

 runs from the ganglion stellatum, has a few medullated fibres up to about 5 JJL 

 in diameter. The branches from the solar ganglia vary greatly in the number 

 of medullated fibres they contain. The hypogastric nerve l in the cat contains 

 350 to 900 medullated fibres ; of these, one or two only, and sometimes none 

 are large. 



In the grey rami the great majority of the nerve fibres are non- 

 medullated. The limit of size of the medullated fibres is about 1/8 to 

 10 (A in diameter ; they are scattered amongst the non-medullated 

 fibres. The number of medullated fibres in the grey rami varies con- 

 siderably in different mammals. In the rabbit there are few, in the cat 

 there are many. Thus in the seventh lumbar grey ramus of the cat 

 there may be more than 300 medullated fibres. 



The variation in the number of medullated fibres in the grey rami 

 affects almost entirely the small ones, and the most constant form is the 

 medium fibre. Most of the small medullated fibres of the grey rami are 

 post-ganglionic fibres (cf. p. 650). Others are pre-ganglionic fibres on 

 their way to aberrant sympathetic cells. Others, again, medium and 

 large, as well as small, are afferent fibres which run in the white rami 

 to the sympathetic chain from the spinal ganglia, and after a course of 

 varying length leave the chain and accompany the post-ganglionic 

 efferent fibres. When the grey ramus divides, part running centrally 

 and part peripherally, medullated fibres, small and medium, accompany 

 both branches. Both, then, on stimulation, should produce some reflex 

 effect. This, however, so far, has not been obtained. 2 There is reason 

 to believe that one or two of the medium and large fibres of the grey 

 rami have their trophic centre in the corresponding spinal ganglia, 3 i.e. 

 that not all the afferent fibres run to the spinal cord by the white rami, 



1 Langley and Anderson, Journ. PhysioL, Cambridge and London, 1894, vol. xvii. 

 p. 177. 



2 Langley, Phil. Trans., London, 1892, vol. clxxxiii. p. 120. 



3 Langley, Journ. PhysioL , Cambridge and London, 1896, vol. xx. p. 55. 



