186 THE HUMAN BODY. 



several groups. 1. Motor fibres, which are distributed to 

 the muscles and govern their contractions. 2. Vaso-motor 

 fibres. These are not logically separable from other motor 

 fibres; but they are distributed to the muscles of the blood- 

 vessels and by governing the blood-supply of various parts, 

 indirectly produce such secondary results as entirely over- 

 shadow their primary effect as merely producing muscular 

 contractions. 3. Secretory fibres. These are distributed 

 to the cells of the Body which form various liquids used in it, 

 as the saliva and the gastric juice, and arouse them to ac- 

 tivity. The salivary glands for instance may be made ta 

 form saliva by stimulating nerves going to them, and the 

 same is true of the cells which form the sweat poured out. 

 upon the surface of the Body. 4. Trophic nerve-fibres. 

 Under this head are included nerve-fibres which have been 

 supposed to govern the nutrition of the various tissues, and 

 so to control their healthy life. It is very doubtful, how- 

 ever, if any such nerve-fibres exist, most of the facts cited 

 to prove their existence being otherwise explicable. For 

 instance shingles is a disease characterized by an eruption 

 on the skin along the line of certain nerves which run be- 

 tween the ribs; but it may be dependent upon disease of 

 the vaso-motor nerves which control the blood-supply of 

 the part. In other cases diseases ascribed to injury of 

 trophic nerves have been shown to be due to injury 

 of the sensory nerves of the part, which having lost its 

 feeling, is exposed to injuries from which it would other- 

 wise have been protected. On the other hand it may be 

 said that secretory nerves are trophic nerves in the true 

 sense of the word, since when excited they cause the se- 

 cretory cells to live in a special way (p. 269) and produce 

 substances which when unacted upon by their nerves they 

 do not form. But if we call secretory nerves trophic we 

 must include also under that name all other efferent 

 nerves; the nutritive processes going on in a muscular 

 fibre when at work are different from those in the sama 

 fibre when at rest, and the same is true of all other cells 

 the activity of which is governed by nerve-fibres. 5. Peri- 

 pherally-acting inhibitory nerves. 



