CHAP. HI.] GENERAL FEATURES OF NERVOUS TISSUES. 173 



nection with the spinal nerve, to supply vasomotor (constrictor) fibres to the 

 muscles (m 1 ) of blood vessels in certain parts, for example, in the limbs. 



Sy, the sympathetic chain uniting the ganglia of the series S. The terminations 

 of the other nerves arising from S, <r, <r' are not shewn. 



The figure is necessarily schematic, and must not be taken to shew that the 

 visceral branch joins only the ganglion belonging to the same segment as the spinal 

 nerve ; the visceral branch joins the sympathetic chain, passing to other ganglia 

 besides the one of the same segment, indeed in some cases does not join this at all. 



matter Gr in the interior and white matter W on the outside. 

 From the anterior part of grey matter is given off the anterior 

 nerve root A and from the posterior part the posterior nerve 

 root P. The latter passes into a swelling or ganglion 6r, "the 

 ganglion of the posterior root,'*' or more shortly " the spinal gan- 

 glion" ; the anterior root does not pass into this ganglion. Beyond 

 the ganglion the roots join to form the nerve trunk N. We shall 

 later on give the evidence that the nerve fibres composing the 

 posterior root P are, so far as we know at present, exclusively 

 occupied in carrying nervous impulses from the tissues of the body 

 to the central nervous system, and that the fibres composing the 

 anterior root A are similarly occupied in carrying impulses from 

 the central nervous system to the several tissues ; that is to say 

 the former is made up of sensory fibres, or, (since the impulses 

 passing along them to the central system may give rise to effects 

 other than sensations) afferent fibres, while the latter is made up 

 of motor, or, (since the impulses passing along them from the 

 central nervous system may produce effects other than movements) 

 efferent fibres. The nerve trunk N is consequently a mixed nerve 

 composed of afferent and efferent fibres. 



By far the greater part of this mixed nerve, dividing into 

 various branches, is distributed (N') to the skin and the skeletal 

 muscles, some of the fibres (motor) ending in muscular fibres (M ), 

 others (sensory) ending in epithelial cells (8) connected with the 

 skin, which we shall consider hereafter under the name of sensory 

 epithelial cells, while others, X, after dividing into minute 

 branches and forming plexuses end, in ways not yet definitely 

 determined, in tissues associated with the skin or skeletal muscles. 

 Morphologists distinguish the parts which go to form the skin, 

 skeletal muscles, &c. as somatic, from the splanchnic parts which 

 go to form the viscera. We may accordingly call this main part 

 of the spinal nerve the somatic division of the nerve. 



Soon after the mixed nerve N leaves the spinal canal it gives 

 off a small branch V, which under the name of (white) ramies 

 communicans, runs into the longitudinal series of ganglia (S) 

 conspicuous in the thorax as the main sympathetic chain. This 

 branch is destined to supply the viscera, and might therefore be 

 called the splanchnic division of the spinal nerve. We may say 

 at once, without entering into details, that the whole of the 

 sympathetic system with its ganglia, plexuses and nerves is to 

 be regarded as a development or expansion of the visceral or 

 splanchnic divisions of certain spinal nerves. By means of 



