MUSCLES RECIPROCAL ENGINES 43 



must move in the closest harmony if effective work is to 

 be done ; at every instant they must be so exactly 

 balanced that the hand is kept perfectly steady for every 

 delicate act in which it is concerned. There must be no 

 opposition or jarring at any point of a movement — how- 

 ever intricate that movement may be. The one engine 

 must feel all the time exactly what its opponent is doing, 

 so that it may make its action respond. Engines which 

 work by explosions, as those of motor cycles do, could 

 not be so harmoniously balanced as to do that. Only 

 the exactly regulated slow-combustion muscular engines 

 invented and elaborated by Nature are capable of acting 

 as perfect reciprocating antagonists. 



Now all the muscles of the body are set as opposing 

 pairs, or antagonistic groups, and Nature had to find out 

 a way of making them work in harmony. She has done 

 it in this way. Mention has already been made of the 

 nerve fibres, which go to the end plates or sparking plugs 

 of the muscle cylinders. Messages reaching the end- 

 plates set the cylinders in motion and also regulate the 

 rate of their combustion and contraction. Besides these 

 ingoing nerve fibres there are also other or outgoing fibres, 

 with which every muscle is supplied. These outgoing 

 fibres begin in curious little structures set amongst the 

 muscle cylinders and also amongst the fibres of the 

 tendon or piston cord. They act as " transmitters " or 

 end organs for taking up nerve messages. When the 

 muscle goes into action, be it ever so slightly, these trans- 

 mitters are squeezed or pressed upon, and automatically 

 dispatch messages which speed along the nerve fibres to the 

 brain. The messages reach the centre or exchange, which 

 controls its fellow or opponent muscle. Thus, through 

 a system of central nerve exchanges or centres, opposing 

 muscles learn the exact state of each other. When a 

 muscle is contracting hard, messages are being dispatched 

 from it which make its opponent or opponents yield at 

 the right rate, and yet offer sufficient resistance to steady 

 or balance the part that is being moved. That is why 

 it is so hard to become a perfect engine-driver — the 



