INTERNAL-COMBUSTION ENGINES 29 



arm. The forearm is the lever on which it acts ; when- 

 ever it is set in motion it lifts up the forearm and hand 

 and thus bends the elbow. But it can also be made to 

 rotate the radius so that the palm of the hand is turned 

 upwards. 



. In the engine of the motor cycle (fig. 1, p. 10) we 

 saw that there was a pipe — the inlet pipe — which con- 

 veyed to the cylinder the explosive mixture made up 

 of fine particles of petrol diffused through eight or nine 



MUSCLE 



NEKVE 

 VEIN 

 ARTEiRY 



TENDON 



Fig. 4. — A drawing of the biceps of the right arm showing its tendon, 

 its vessels, and its nerve. 



times the same weight of air ; then we saw another pipe 

 — the exhaust pipe — which carried away from the cylinder 

 the gases formed when the charge was exploded. Now 

 the muscular engine has corresponding pipes (fig. 4) ; 

 there is not one pipe but several which enter the biceps 

 muscle, only we name them not pipes but arteries, and they 

 convey to the muscle not petrolised air but red arterialised 

 blood. Then there are pipes which issue from the muscle 

 and carry away from it, not waste gases but dark venous 

 blood. Only we do not call these vessels exhaust pipes, 

 but veins. Then, too, we saw that a wire ended in the 



