2 6o THE ENGINES OF THE HUMAN BODY 



The brigades of driver cells which control the muscles 

 of the body, instead of being placed beside their engines, 

 are brought up and quartered in the precincts of the ex- 

 change from which they receive their orders. Hence the 

 operatives within the exchange can transmit the messages 

 received for the driver units almost directly to them (fig. 

 47B). The driver units having received their information 

 or orders from the signaller cells either directly or through 

 operatives connected with their exchange, dispatch mes- 

 sages to the muscles under their control. The essential 

 differences, then, between a telephone and a nerve system 

 are : (i) That telephone subscribers who carry out orders 

 — who supply our needs — are situated in any part of a 

 town supplied by a telephone system, whereas in a nerve 

 system they are assembled almost within the exchange, 

 and therefore have to be linked to their engines and 

 laboratories by wires or nerve filaments, which in reality 

 represent an extension of the system. (2) The manner 

 in which telephone and nerve messages are carried is 

 different. Telephone wires can carry messages because 

 of the electric current which is made to flow along them. 

 The transmitter into which we speak interrupts the flow 

 of the current, and thus the vibrations of our voices are 

 carried along the wires as pulses or waves of the electric 

 current. In the human body the wires are living and 

 pulsating because of the slow vital combustion which is 

 steadily maintained in their substanee ; the messages 

 which are set going within them when a transmitter is 

 stimulated are carried as self-propagated waves to the 

 exchanges. They travel much slower than electrical 

 messages, their rate being about 4 miles a second. It 

 takes one-hundredth of a second for a touch-impulse 

 which is applied to a transmitter on the pulp of a finger 

 to reach a man's brain, but for the needs of the body that 

 is quick -enough. 



We have made a rapid comparison of the manner in 

 which local calls are dealt with in an exchange situated in 

 a provincial town and another placed in the lower end 

 of the spinal cord — an exchange whose subscribers are 



