258 THE ENGINES OF THE HUMAN BODY 



But what of the exchange operators ? Nerve exchanges 

 are automatic. Messages pass from the exchange terminals 

 of a signaller unit straight to the exchange terminals of a 



EXCHANGE 

 TERMINALS 



TO BRAIN 

 FROM. BRAIN 



SPINAL CORP. 

 EXCHANGE 

 STA TION 



DRIVER 

 ..--■CELLS 



TO EARTH 



Fig. 47A. — The arrangement of two 

 units in an exchange of a tele- 

 phone system. 



Fig. 47B. — The arrangement of two 

 units of an exchange in the nerve 

 system of the spinal cord. 



driver unit (fig. 47B). Their terminals are in contact 

 only. There are many exchanges, however, where there 

 are really operators at work — " operative " units, as I 

 prefer to call them. In most of the exchange centres 



