CHAP, in.] GENERAL FEATURES OF NERVOUS TISSUES. 171 



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

 muscles (MI') 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. 



Each spinal nerve arises by two roots. The metamere of the 

 central nervous system C consists, as we shall hereafter see, of 

 grey 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, " the ganglion 

 of the posterior root," or more shortly "the spinal ganglion"; the 

 anterior root does not pass into this ganglion. Beyond the gang- 

 lion 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, as 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 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 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) ramus 

 cmnmunicans, joins one of a 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 



