AUTOMATIC TELEPHONE EXCHANGES 263 



are mostly silent muscle messages sent up to the driver 

 cells and to the master drivers to keep them informed of 

 the state of the muscular engines under their control. 

 From start to finish of a body movement the driver cells 

 have to be deluged with information. An enormous 

 special exchange has been built for dealing with muscle 

 messages — the cerebellum. If the cerebellum is injured, 

 then the movements of the muscles and of limbs are no 

 longer timed and co-ordinated. Engineers have designed 

 and applied automatons to take the place of exchange 

 operators. Such exchanges are called automatic, but they 

 are really not so, for the automaton in the exchange has to 

 be worked by a subscriber before he can get his message 

 through to its right destination. On the other hand, 

 Nature's exchanges are automatic. 



In one respect the telephone system invented by man 

 has an advantage over Nature's system. As long as the 

 batteries keep up an electric current along the wires they 

 will carry messages hour after hour or year after year 

 without sleep or rest. Nature's wires are different. 

 Messages are propagated along them at the expense of 

 their own substance and of the substance of the living 

 units of which they form a part. If worked hard the 

 available supply in nerve cells and fibres runs low. So 

 that rest and sleep are necessities for them ; it is during 

 rest and sleep that our brain and nerves recuperate. 



We have frankly admitted, three chapters back, that 

 the traces of a nervous system in a motor cycle are miser- 

 able travesties. The wire which carries the electric 

 current to the sparking plug serves the same purpose 

 as the nerve which carries motor impulses to a muscle. 

 The gearing mechanism which times the opening and 

 shutting of valves and the firing of the electric spark 

 does serve a purpose, comparable to that carried on 

 by the cerebellum, which has to time over 200 pairs of 

 muscular engines. 



To find any system which is comparable to that repre- 

 sented by the human brain, spinal cord, and nerves, we 

 have to study the great machines which are composed 



