AUTOMATIC TELEPHONE EXCHANGES 259 



of the human brain and spinal cord " operatives " or 

 " shunt " units will be seen linking terminals in one 

 exchange to terminals of driver units in an adjoining 

 exchange. Such operatives might well be named relay 

 units. It is in the manner just outlined that the hundreds 

 of exchanges within the spinal cord are built up. 



From the head or body of one of these connecting or 

 " operative " units is thrown out a series of branching 

 antennae or dendrites ; from its other end emerges a nerve 

 filament which soon branches out into a series of terminals. 

 The antennae and terminals represent the switches or plugs 

 by which the operatives " put through " messages which 

 pour in upon an exchange when the body is in action. 

 In fig. 47 b only two of the thousands of wires which 

 reach the exchange are represented. One of these will 

 be seen to spring from the main wire or fibre we have 

 followed through the posterior root ganglion of a spinal 

 nerve to the spinal cord. That wire, having given off a 

 side branch to the small exchange centre we are consider- 

 ing, passes along the cord for the purpose of carrying 

 trunk calls to the brain. In the exchange of a provincial 

 town all trunk calls have to pass through the local ex- 

 change ; here we see Nature making an improvement by 

 giving each subscriber access to metropolitan as well as 

 to local exchanges. In this spinal exchange which we are 

 now considering a wire from the main executive exchange 

 of the body — the seat of the will — is seen to arrive and 

 terminate. The will also uses local exchanges for the 

 distribution of orders. The operatives in a local ex- 

 change of the spinal cord receive messages coming both 

 from local subscribers and also from the government 

 or brain, and it is their business to put such messages 

 through to their proper destinations. In the instance 

 cited a few pages back the local operator put a call 

 through to a jobmaster who lived some miles from his 

 exchange. Nature has arranged her system quite differ- 

 ently. She has brought the nerve units which control 

 her conveyances — her muscular engines — almost within 

 the exchange itself. 



