172 SOMATIC AND SPLANCHNIC NERVES. [BOOK i. 



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 o; 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 

 (x) ; but some of the fibres are afferent and convey impulses 

 from the viscera to the central nervous system, and it is pro- 

 bable that some of these begin or end in epithelial cells of the 

 viscera (s}. 



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

 which govern the blood vessels of the body, the so-called vaso- 

 motor nerves. A certain class of these, namely the vaso-constrictor 

 nerves or fibres are branches of the splanchnic divisions of the 

 cerebrospinal nerves, and as we shall see the vaso-constrictor 

 nerves of the skeletal muscles, skin, and other parts supplied by 

 somatic nerves, after running for some distance in the splanchnic 

 division (F), turn aside (r.v and v. m) and join the somatic 

 division, the fibres of which they accompany on their way to the 

 tissues whose blood vessels (m) they supply. 



We have seen ( 68) that a nerve going to a muscle is 

 composed of nerve-fibres, chiefly medullated, some however being 

 non-medullated, bound together by connective tissue. The same 

 description holds good for the whole somatic division of each of 

 the spinal nerves. The splanchnic division also consists of 

 medullated and non-medullated fibres bound together by con- 

 nective tissue, but in it the non-medullated fibres preponderate, 

 some branches appearing to contain hardly any medullated fibres 

 at all. The non-medullated fibres which are found in the somatic 

 division appear to be fibres which have joined that division from 

 the splanchnic division. So prominent are non-medullated fibres 

 in splanchnic nerves and hence in the sympathetic system that 

 they are sometimes called sympathetic fibres. 



We have said that the axis-cylinder, whether of a medullated or 

 non-medullated fibre, is to be considered as a long drawn out process 

 of a nerve cell. Nerve cells are found in three main situations. 

 1. In the central nervous system, the brain and spinal cord. 



