AUTOMATIC TELEPHONE EXCHANGES 257 



as pulses or waves of an electric current. The waves 

 have to be turned into sound waves and words before 

 they can serve as human messages. We shall see that the 

 subscribers to nerve exchanges can read the messages in the 

 form in which they are transmitted over the wires ; they 

 need no receiver to make them audible and intelligible. 



Having thus contrasted the elements which make up a 

 unit in a telephone and in a nerve system of communica- 

 tion, we now proceed to see how they are built up to 

 form exchanges. In figs. 47A and 47 b only two units of 

 an exchange are represented ; whereas even in a local 

 exchange, whether it be one in a provincial town or its 

 counterpart in the spinal cord, there are really hundreds 

 of subscribers and units. We have chosen only two 

 units for simplicity's sake. In the telephone exchange 

 two units are seen to be joined within the exchange ; 

 over these two units messages can be given or received 

 by their respective subscribers. In the nerve system two 

 units are also shown. The unit which is furnished with 

 a transmitter can only carry ingoing messages ; it has no 

 receiver at the outer or calling end. The unit to which 

 it is connected has no transmitter, only a receiver of a 

 peculiar kind, one which changes nerve messages into 

 orders or stimuli which set muscles in motion. It will 

 thus be seen why I did not wish to look upon the bodies 

 of the nerve cells as mere batteries. The two nerve units 

 represented in fig. 47 b are of different kinds. The nerve 

 cell in connection with the transmitter is a " signaller " 

 cell ; its unit has to do with messages which pass to the 

 exchange. The nerve cell placed within the spinal cord 

 and connected with the receiver or end plate of a muscle 

 cylinder is a "driver" cell. It has to do only with out- 

 going messages. The messages which reach it are turned 

 into orders which start, regulate, or stop muscular engines. 

 The telephone units can convey ingoing as well as out- 

 going messages, but it will be seen that nerve units serve 

 either the one purpose or the other. We must look on 

 the nerve cells as more than mere batteries ; they have 

 in them also the function of subscribers. 



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