CHAPTER XIV 

 NERVE 



THE voluntary movements are originated by efferent or outgoing 

 impulses from the brain, which reach the muscles along their 

 motor nerves. The involuntary movements and the secretions 

 are in many cases able to go on in the absence of central connec- 

 tions, but are normally under central control. Afferent impulses 

 are continually ascending to the cord and brain from the skin, 

 joints, bones, muscles, and organs of special sense like the eye and 

 the ear. Everywhere the connection between the nervous centres 

 and the peripheral organs, and between different parts of the 

 central nervous system, is made by nerve- fibres. Those which 

 run outside the brain and cord are called peripheral nerve-fibres 

 to distinguish them from the intracentral fibres of the central 

 nervous system itself. 



In this chapter we propose to consider certain of the general 

 properties of nerve-fibres. Most of our knowledge of these proper- 

 ties has been derived from experiments on the peripheral, and 

 particularly the peripheral motor nerves ; but there is every reason 

 to believe that the main results are true of all nerve-fibres, afferent 

 and efferent, peripheral and central. 



What we call nerve-fibres were known and named, and many im- 

 portant facts in their physiology discovered, long before their true 

 morphological significance was recognized. The researches of recent 

 years have shown that every nerve-fibre is, as regards its essential con- 

 stituent the axis-cylinder, a process of a nerve-cell. The nerve-cells, 

 each of which, including all its processes, may be conveniently termed 

 a neuron, are the essential elements of the nervous system. The cell- 

 bodies of most of the neurons are situated in, or in close relation to, the 

 spinal cord and the brain, and therefore the detailed description of 

 them will be reserved till we come to treat of the central nervous system 

 (see p. 850 and Figs. 328 to 340). It is enough to say here that in 

 general a nerve-cell gives off two kinds of piocesses: (i) one or more 

 dendrites or protoplasmic processes, which repeatedly bifurcate like the 

 branches of a tree into thinner and thinner twigs, and extend only for 

 a relatively short distance from the cell-body; (2) an axis-cylinder 

 process or axon, which as a rule runs for a considerable distance without 

 altering its calibre, and either gives off no branches (as in the peripheral 

 nerves) or only a comparatively small number of lateral twigs (col- 



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