124 GENERAL FEATURES OF NERVOUS TISSUES. 



ganglion "; 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, 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 composed of afferent and efferent fibres. 



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

 is distributed (N f ) to the skin and the skeletal muscles, some of the fibres 

 (motor) ending in muscular fibres (M\ others (sensory) ending in epithelial 

 cells ($) 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 deter- 

 mined, in tissues associated with the skin and skeletal muscles. Morpholo- 

 gists distinguish the parts which go to form the skin, skeletal muscles, etc., 

 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 jV leaves the spinal canal, it gives off a small 

 branch V, which under the name of (white) ramus communicans, joins one 

 of a 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 develop- 

 ment or expansion of the visceral or splanchnic divisions of certain spinal 

 nerves. By means of this 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 <r, and, it may be, tertiary ganglia. 

 There are, however, as we shall see, certain nerves or fibres which do not run 

 in the sympathetic system, and yet are distributed to the viscera and are 

 "splanchnic" in nature. We cannot, therefore, use the word sympathetic 

 to denote all the fibres which are splanchnic in nature. On the other hand, 

 the " splanchnic nerves " of the anatomist form a part only of the splanchnic 

 system in the above sense, the term thus used is limited to particular nerves 

 of the splanchnic system distributed to the abdomen ; and the double use of 

 the term splanchnic might lead to confusion. The difficulty, may, perhaps, 

 be avoided by calling the splanchnic nerves of the anatomist " abdominal 

 splanchnic." The majority of these splanchnic fibres seem to be efferent in 

 nature, carrying impulses from the central nervous system to the tissues, 

 some ending in plain muscular fibres (m), others in other ways (#) ; but some 

 of the fibres are afferent and convey impulses from the viscera to the central 

 nervous system, and it is probable that some of these begin or end in epithe- 

 lial cells of the viscera (s). 



We shall have occasion in the next chapter to speak of nerves which 

 govern the bloodvessels of the body, the so-called vasomotor nerves. A cer- 

 tain class of these, namely, the vaso-constrictor nerves or fibres are branches 

 of the splanchnic divisions of the cerebro-spinal nerves, and, as we shall see, 

 the vaso-constrictor nerves of the skeletal muscles, skin, and other parts sup- 



