330 



CONSTRUCTION OF THE NERVES. 



but generally it is only after the white fasciculi have passed through one or more 

 ganglia that they become thoroughly blended with the grey ; and then, too, the 

 nervous cords receive a large accession of grey fibres, while the white fibres are 

 reduced in number. 



Differences are observed among the cerebro-spinal nerves in the proportionate 

 amount of the two kinds of fibres which they respectively contain, and in the size 

 of their white fibres. Volkmann and Bidder showed that nerves going to voluntary 

 muscles have very few small fibres, those going to the integuments a large propor- 

 tion of small fibres, whereas those distributed to involuntary muscles and to the 

 viscera have many more small fibres than large. The circumstances influencing 

 these variations were not, however, understood until it was shown by Gaskell that 

 the very fine medullated fibres pass out from the cerebro-spinal axis in certain 

 regions only, and are distributed exclusively to the viscera and blood-vessels, for the 



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ogcrOea;^. >fc> J^x'-oO, "$ o o /tCc 

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Fig. 385. SECTION OF A WHITE RAMPS COMMUNICANS FROM THE DOG (Gaskell). 



The section is drawn to the same scale as those shown in fig. 382, and the nerve-fibres are stained 

 foy osmic acid. 



most part through the sympathetic system, but partly directly through the cerebro- 

 spinal nerves. In the cranio-cervical region these small fibres pass out with the 

 upper roots of the spinal accessory nerve and partly perhaps with those of the vagus, 

 glosso-pharyngeal and facial ; in the thoracico-lumbar region with the anterior 

 roots of the nerves from the second thoracic to the second lumbar inclusive, and in 

 the sacral region with the anterior roots of the second, third, and perhaps the 

 fourth sacral nerves. 



These fine medullated fibres form the only true communication between the 

 cerebro-spinal centre and the sympathetic, and, in fact, those of the dorso-lumbar 

 outflow give origin to the main sympathetic system. Passing to the nearer 

 ganglia of the sympathetic, many of them lose their medullary sheath on joining 

 with the cells of those ganglia, and are continued as pale fibres : others 



