CHAPTER XXXV. 



THE FUNCTIONS OF THE BRAIN AND 

 SPINAL CORD. 



The Special Physiology of Nerve- Centres. We have 



already 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. In some cases the centres are auto- 

 matic; they originate nerve impulses, as illustrated by the 

 heat of the heart under the influence of its ganglia. In 

 other cases a feeble impulse reaching the centre gives rise 

 to a great discharge of energy from it (as when an unex- 

 pected noise produces a violent start, due to many impulses 

 sent out from the excited centre to numerous muscles), so 

 that certain centres are irritable; they contain a store of 

 energy-liberating material which only needs a slight dis- 

 turbance to upset its equilibrium and produce many efferent 

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

 pulses thus liberated are often co-ordinated. When mucus 

 in the windpipe 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 excita- 

 tion 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 are manifested by different parts of the cen- 

 tral cerebro-spinal nervous system; and under what circum- 

 stances and in what degree: what is known of the general 

 functions of the sympathetic and sporadic ganglia has al- 

 ready been stated (p. 183). 



