CHAPTER XXXVI. 

 THE SPINAL CORD AND REFLEX ACTIONS. 



The Special Physiology of Nerve-Centres. We have al- 

 ready studied the general physiological properties of nerves 

 and nerve-centres (Chap. XIII) and found that while the 

 former are mere transmitters of nervous impulses, the latter 

 do much more than merely conduct. In some cases the centres 

 are automatic ; they originate nerve impulses, as illustrated by 

 the rhythmic impulses starting from the respiratory centre. 

 In other cases a feeble impulse reaching the centre gives rise to 

 a great discharge of energy from it (as when an unexpected 

 noise produces a violent start, due to many impulses sent out 

 from the excited centre to numerous muscles), so that certain 

 centres are irritable. Such nerve-centres contain a store of 

 energy-liberating material which only needs a slight disturb- 

 ance to upset its equilibrium and initiate powerful efferent 

 impulses as the result of one feeble afferent. Further, the im- 

 pulses thus liberated are co-ordinated. When mucus in the 

 larynx tickles the throat and excites afferent nerve impulses, 

 these, reaching a centre, cause discharges along many efferent 

 fibres, so combined in sequence and power as to produce, not 

 a mere aimless spasm, but a cough which clears the passage. 

 In very many cases the excitation of centres, with or without 

 at the same time some of the above phenomena, is associated 

 with sensations or other states of consciousness. 



We have now to study which of these powers is manifested 

 by different parts of the central cerebro-spinal nervous system, 

 and under what circumstances and in what degree : and also 

 some of the phenomena of conduction in spinal cord and 

 brain. 



Conduction in the Spinal Cord. The spinal cord, form- 

 ing (except the slender sympathetic) the only direct com- 

 munication between the brain and most of the nerves of the 

 Body, was considered by the older physiologists as merely a 

 huge nerve-trunk, into which the various spinal nerves were 



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