F UNCTIONS OF NER VE- CENTRES. 183 



moreover do not act, as a rule, indifferently and casually, 

 but rearrange the impulses reaching them, so as to pro- 

 duce a protective or in some way advantageous result. In 

 other words, these centres, acting in health, commonly co- 

 ordinate the incoming impulses and give rise to outward- 

 going impulses which produce an apparently purposive 

 result. The burnt hand or the tickled foot, in the absence 

 of all consciousness, are snatched away from the irritant; 

 and food chewed in the mouth excites nerves there which 

 act on a centre which causes certain cells in the salivary 

 glands to form and pour into the mouth more saliva. In 

 addition tp the reflex centres we have others, placed in the 

 brain, which, when excited, cause in us various states oi 

 consciousness, as sensations, emotions, and the will ; con- 

 cerning these centres of consciousness our physiological 

 knowledge is still very incomplete; what we know about 

 them is based rather on psychological than physiological 

 observation. The brain also contains a great many im- 

 portant reflex centres, as that for the muscles of swallow- 

 ing, an act which goes on perfectly without our conscious- 

 ness at all. In fact if we pay attention to our swallowing 

 we fail to perform it as well as if we let the nervo-muscular 

 apparatus alone. To complete the statement of the func- 

 tions of the nerve-centres we must probably add two other 

 groups. The first of these is that of the automatic centres, 

 which are centres excited not directly by nerve-fibres con- 

 veying impulses to them, but in other ways. For example 

 the breathing movements go on independently of our con- 

 sciousness, being dependent on stimulation of a nerve-centre 

 in the brain by the blood which flows through it (see Chap. 

 XXVI.); and tlie beat of the heart is also dependent (Chap. 

 XVII.) upon nerve-centres the excitant of which is un- 

 known. The final group of nerve-centres is represented 

 by certain sporadic sympathetic and cerebro-spinal ganglia 

 which are not known to be either reflex, automatic, or con- 

 scious in function. They may be called relay and junc- 

 tion centres, since in them probably an impulse entering 

 by one nerve-fibre excites a cell, which by its communi- 

 cating branches arouses many others, and these then send 



