GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 189 



brain, the excitation of which is accompanied in us by various 

 states of consciousness, as sensations, emotions, and the will; 

 concerning these centres of consciousness our physiological 

 knowledge is still very incomplete; what we know about them 

 is based rather on psychological than physiological observa- 

 tion. The brain also contains a great many important reflex 

 centres, as that for the muscles of swallowing, an act which 

 goes on perfectly without our consciousness 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, as is 

 illustrated by the difficulty many persons experience on trying 

 to swallow a pill. To complete the statement of the functions 

 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 conveying impulses 

 to them, but in other ways. For example the breathing 

 movements go on independently of our consciousness, being 

 dependent on stimulation of a nerve-centre in the brain by 

 the blood which flows through it (see Chap. XXVII); and 

 the beat of the heart is also much dependent (Chap. XVIII) 

 upon nerve-centres, the excitant of which is unknown. 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 conscious in function. They 

 may be called relay and junction centres, since in them prob- 

 ably an impulse entering by one nerve-fibre excites a cell, 

 which by its communicating branches arouses many others, 

 and these then send out impulses by the many nerve-fibres 

 connected with them. By such means a single nerve-fibre can 

 act upon an extended region of the Body. In other cases it 

 seems likely that a feeble nervous impulse reaching an irri- 

 table nerve-cell excites changes in this comparable to those 

 produced in a muscle when it is stimulated; the cell is in 

 fact a store of readily decomposable material which breaks 

 down when stimulated through one branch, with the liberation 

 of energy, the discharge of which takes the form of reinforced 

 nerve impulses sent along other branches or one of them. 



That nerve-cells are the seats of considerable metabolic 

 changes is indicated by the abundant supply of blood always 

 sent to regions where they are numerous: and that some of 

 their material is used up, or undergoes katabolism, during 

 their activity and is replaced by anabolic processes during 



