254 THE ENGINES OF THE HUMAN BODY 



automatically dispatched. These pressure transmitters 

 work very quietly as long as we merely press on them, 

 but if we touch them lightly — if we tickle them — then 

 they dispatch messages of the most imperious kind. 

 These messages stream into the usual exchange centre, one 

 situated almost at the lower end of the spinal cord. In 

 the exchange they are immediately "put through" to the 

 units which control and drive the great muscular engines 

 of the thigh and leg. The muscular engines spring into 

 activity and the foot is withdrawn immediately from the 

 source of irritation. Try as we will, we cannot prevent 

 these tickling messages from forcing a way through the 

 exchange centres. And yet the will, as we shall see, has 

 free access to the spinal centres and can use them for 

 carrying out planned and deliberate movements. 



Another example will suffice to illustrate how extensive 

 the telephone service of the body is. A particle of dust 

 is blown into the eye ; its arrival is immediately announced 

 to two exchanges ; through one of these the incoming 

 messages are transmitted to a central station some dis- 

 tance off, occupied by the driver units which can shut the 

 eyelids. These driver units keep the eyelids shut as 

 long as irritant messages come through from the eye ; 

 when the foreign particle is removed the messages cease 

 and the driver cells sink into their usual state of wakeful 

 quiescence. The particle of dust also sets up another 

 group of messages, one which is transmitted to a small 

 local exchange in communication with the lachrymal gland. 

 The messages thus transmitted send the lachrymal gland 

 into a state of activity ; tears stream across the eye with 

 the intention of washing away the offending particle of 

 dust. Even when that has been got rid of, if damage 

 has been done, the flow of tears continues as long as the 

 irritation lasts. Thus Nature employs a kind of tele- 

 phonic system for many and diverse purposes in carrying- 

 on the work of the human machine. 



Having just seen the kind of office served by telephone 

 systems in a provincial town and in the human body, we 

 now turn to compare the units or elements of which they 



