CHAP, in.] FEATURES OF NEKVOUS TISSUES. 141 



with the spinal nerve, to supply vasomotor (constrictor) fibres to the muscles (w') of 

 blood vessels in certain parts, for example, in the limbs. 



Si/, the sympathetic chain uniting the ganglia of the series 2. The terminations 

 of the other nerves arising from 2, a, a' 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 G y " 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 exclusively (or nearly so), occupied in carry- 

 ing 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 ner- 

 vous 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 ner- 

 vous system may produce effects other than movements) efferent 

 fibres. The nerve trunk N is consequently a mixed nerve com- 

 posed 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 (S) connected with the 

 skin, which we shall consider hereafter under the name of sen- 

 sory 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 branch V, which under the name of (white) ramus 

 communicans, runs into the longitudinal series of ganglia (2) 

 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 this 



