AUTOMATIC TELEPHONE EXCHANGES 261 



placed in the sole of the foot. We are now to compare 

 the methods of dealing with trunk calls. We shall sup- 

 pose an official from the Ministry of Health has had 

 reason to visit the town in which our local exchange is 

 situated because of the outbreak of an obscure disease. 

 He wishes to communicate with headquarters in order to 

 report what he has found and to have certain orders sent 

 out from the Ministry which will waken up a lethargic 

 local authority. He rings up the local exchange and asks 

 to be " put through " to the Ministry. He has to wait 

 some time, for the local operator has to get in touch with 

 the exchange in the nearest county town ; the county town 

 exchange has to get through to another in London ; two 

 others have to be negotiated before the provincial message 

 has a straight run through to the official quarters of the 

 Ministry. The message thus transmitted by the aid of 

 operators placed in five different exchanges sets into opera- 

 tion the official machinery of the Ministry, with the result 

 that a peremptory order is dispatched to waken up the 

 local authority of the provincial town. 



Let us see how trunk calls are managed in the nerve 

 system of the human body. A sharp fragment of stone falls 

 within a shoe and presently works its way under the foot. 

 With each step it presses against some of the transmitters 

 in the sole and sets up messages, at first giving a mere 

 feeling of discomfort, but afterwards becoming more 

 urgent, giving a sense of intolerable pain. The brain is 

 set into operation, with the result that the movements 

 necessary for unlatching the shoe are undertaken and the 

 offending body is discovered and removed. The instance 

 is not unlike that which we have been following in con- 

 nection with an epidemic in a country town. In the 

 human body, however, the wire which carries messages 

 from the sole of the foot does not pass through the local 

 exchange in the spinal cord ; it sends oft merely a side 

 branch to one, two, or more spinal exchanges and passes 

 up the spinal cord until it reaches the first great exchange 

 of the body— one which is situated where the spinal cord 

 becomes continuous with the brain stem. Here the 



