FUNCTIONS OF NERVE-CENTRES. 483 



the same fact that, when a motor nerve enters a plexus 

 and contributes with other nerves to the formation of a 

 nervous trunk proceeding from the plexus, it does not 

 impart motor power to the whole of that trunk, but only 

 retains it isolated in the fibres which form its continuation 

 in the branches of that trunk. 



Functions of Nerve-centres. 



As already observed (p. 473), the term nerve-centre is 

 applied to all those parts of the nervous system which 

 contain ganglion- corpuscles, or vesicular nerve-substance, 

 i. e., the brain, spinal cord, and the several ganglia which 

 belong to the cerebro-spinal and the sympathetic systems. 

 Each of these nervous centres has a proper range of 

 functions, the extent of which bears a direct proportion to 

 the number of nerve-fibres that connect it with the various 

 organs of the body, and with other nervous centres ; but 

 they all have certain general properties and modes of action 

 common to the^n as nervous centres. 



It is generally regarded as the property of nervous 

 centres that they originate the impulses by which muscles 

 may be excited to action, and by which the several functions 

 of organic life may be maintained. Hence, they are often 

 called sources or originators of nervous power or force. But 

 the instances in which these expressions can be used are 

 very few, and, strictly speaking, do not exist at all. The 

 brain does not issue any force, except when itself im- 

 pressed by some force from within, or stimulated by an 

 impression from without; neither without such previous 

 impressions do the other nerve-centres produce or issue 

 motor impulses. The intestinal ganglia, for example, do 

 not give out the nervous force necessary to the contractions 

 of the intestines, except when they receive, through their 

 centripetal nerves, the stimuli of substances in the intestinal 



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