CHAPTER X. 



si . ';..,. ;?] I'j . NERVE. 



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THE .voluntary movements are .p.rigijnated by efferent or outgoing' 

 impulses fi;om the brain, which reach the muscles along their motor 

 nerves. : The involuntary movements and the secretions are in many 

 cases able td'goon in the absence ! of central connections, 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 pro- 

 perties 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 

 important facts in their physiology discovered, long before their true 

 morphological significance was recognised. The researches of recent 

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

 constituent 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. 66 1 and Figs. 227 to 231). It is enough 

 to say here that in general a nerve-cell gives off two kinds of pro- 

 cesses : (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 (collaterals). Ultimately 



