CHAPTER X 

 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 con- 

 nections, 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 

 properties 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. 748 and Figs. 300 to 311). 

 It is enough to say here that in general a nerve-cell gives off two 

 kinds of processes : (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, 



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