262 THE ENGINES OF THE HUMAN BODY 



messages from the sole of the foot are automatically 

 switched on to main trunk cables which carry them to a 

 second exchange centre — one situated in the great masses 

 of grey matter placed within the cerebral hemisphere (see 

 p. 239). It is in these grey masses that the messages from 

 the sole rise up into the field of consciousness as painful 

 sensations. The executive exchanges or departments of 

 the brain lie still further on ; they are spread out in the 

 grey matter which forms the cortex or rind of the cerebral 

 hemispheres. Hence the messages which have reached 

 the basal masses and given rise to the sensation of pain 

 have to be transmitted by a third relay of wires to the 

 cortex of the brain before steps can be taken by the 

 executive departments. To obtain relief the " driver " 

 cells of the cortex have to be set in motion. We have 

 already seen that the " driver" units, which exercise a direct 

 control over muscular engines, are grouped round the 

 local exchanges of the spinal cord. These driver cells we 

 meet with in the cortex of the brain are " master drivers " ; 

 they control the driver units in the local exchanges, and 

 combine their actions so that the muscular engines carry 

 out the movements which are determined on by opera- 

 tions effected within the exchange systems of the cortex. 

 The cortex of the human brain is by far the most elaborate 

 central exchange in creation. 



It is into this exchange, then, that messages dispatched 

 from the sole of the foot pass, after being switched through 

 two great exchanges and having travelled over three relays 

 of nerve cables. Arriving in the cortical exchange they 

 set going a machinery which leads to the master driver 

 cells taking charge of the movements of the body. It 

 was they which controlled the driver units of the local 

 centres during the body movements which led to the 

 removal of an offending stone from the shoe. 



The reader will probably be surprised to know that the 

 bulk of the messages which stream through the exchanges 

 of the nervous system have nothing to do with the kind 

 of message he is familiar with — messages which give the 

 sensation of touch, pressure, pain, heat, or cold. They 



