INTERNAL-COMBUSTION ENGINES 37 



are below they will be warmed by it. That is only 

 one of several ways in which temperature is regulated 

 in muscles. 



There is another structure which we must take into 

 our consideration if we would find out every part of the 

 machinery which is needed to regulate the temperature 

 of muscular engines. We have seen that a cable of 

 nerve fibres goes to every muscle ; through the nerve 

 fibres the will sends messages which set the muscles into 

 motion, can make them contract less or more, or can 



^M^^^Nl^^ lS^Mf ^m - 



Muscle 



culm- < 



ders 



Capillar!? 



Arti 



Fig. 6. — Showing the manner in which capillaries are arranged among 

 muscle cylinders. 



make them cease contracting. But we have not looked 

 at the exact way in which the separate wires or fibres of 

 the nerve cables end. The manner in which they reach 

 the separate cylinders is shown in fig. 5 ; there a nerve 

 fibre is seen entering its " end plate " ; the end plate is 

 applied closely to the muscle cylinder. A nerve message, 

 when it reaches an end plate, gives rise to a contraction 

 in the muscle cylinder, just as the electric current when 

 it reaches the sparking plug of an internal-combustion 

 engine gives rise to an effective stroke. The end plates 

 seem to be a kind of sparking plug, only they can do 

 more than any sparking plug which has been invented. 

 Through them the nerve current or nerve messages 



