142 SOMATIC AND SPLANCHNIC NERVES. [Book i. 



system splanchnic fibres from the central nervous system are 

 distributed to the tissues of the viscera, some of them on their 

 way passing through secondary ganglia a, and, it may be, tertiary 

 ganglia. The majority of these splanchnic fibres seem to be effer- 

 ent in nature, carrying impulses from the central nervous system 

 to the tissues, some ending in plain muscular fibres (m) others in 

 other ways (x) ; but some of the fibres are afferent (s) and con- 

 vey impulses from the viscera to the central nervous system, and 

 it is possible that some of these begin or end in epithelial cells of 

 the viscera. 



All the fibres issuing from the main sympathetic chain do not 

 however pass to the tissues of the viscera; a certain number of 

 fibres turn back (r. v.) from the ganglion to join the spinal nerve 

 and run for the most part peripherally in the somatic nerve, though 

 some of them pass backwards to the spinal cord, ending probably 

 in the membranes of the cord. In the case of many of the spinal 

 nerves the communicating branch from the spinal nerve consists 

 distinctly of two parts, a ' white ramus ' consisting chiefly of 

 medullated and a ' grey ramus ' consisting chiefly of non-medul- 

 lated fibres ; in these cases these backward turning ' revehent ' 

 fibres run in the grey ramus ; but in the case of some of the spinal 

 nerves it is not possible to distinguish a grey ramus separate from 

 a white ramus. 



We shall have occasion in the next chapter to speak of 

 nerves or rather nerve fibres which by influencing the mus- 

 cles of the blood vessels govern the calibre of those vessels and 

 are spoken of as vaso-motor nerves. Some of these, in their 

 action, constrict or narrow the blood vessels, and are hence called 

 vaso-constrictor nerve fibres. All these vaso-constrictor nerve 

 fibres issuing from the central nervous system pass to the splanch- 

 nic system, to the sympathetic chain ; but while some of them, 

 continuing in the splanchnic system, are distributed to the blood 

 vessels of the viscera, others turning back by revehent branches 

 and running with ordinary somatic fibres in the (somatic) spinal 

 nerves are distributed to the blood vessels certainly of the skin 

 and possibly of other somatic tissues. These are represented 

 in the figure by m'. As we shall see, other nerve fibres, having 

 other functions, take a similar course. 



A nerve fibre is fundamentally a prolongation of a nerve cell ; 

 the axis cylinder which is the essential part of a medullated fibre, 

 and constitutes practically the whole of a non-medullated fibre, is 

 the prolongation of a process, the so-called axis cylinder process of 

 a nerve cell. When we examine a nerve we find that along its 

 course it consists exclusively of nerve fibres bound together by 

 connective tissue carrying blood vessels and lymphatics, the somatic 

 (spinal and cranial) nerves being composed chiefly of medullated 

 fibres, mixed with which are some non-medullated fibres, and 

 the sympathetic nerves being composed chiefly of non-medullated 

 fibres, some of them containing hardly any medullated fibres. 



