THE BIOLOGY OF HEALTH 129 



our backbone does not sway unduly forwards or back- 

 wards, to one side or to the other. But our backbone is 

 made up of many vertebrge, and to these 144 muscles 

 are attached which give our body what we call poise. 

 Indeed, there are about 300 muscles concerned when we 

 walk. As Professor Keith says, " With every step — every 

 half-second — some three hundred engines have been 

 started, regulated, and stopped, and each has done its 

 allotted task in helping the body forward." Were we 

 not right in saying that walking is really a quite wonder- 

 ful achievement ? 



When we draw our lower arm towards our upper 

 arm, e.g. in lifting a teacup to our lips, we use a big 

 muscle called the biceps, which we can feel working 

 if we grasp our right arm just above the elbow with the 

 fingers of our left hand. As the forearm draws nearer 

 the upper arm, we feel the biceps muscle contracting ; 

 it becomes shorter and broader and harder under our 

 fingers. The muscle is fixed above to the shoulder- 

 blade ; it is fixed below by means of its piston-cord or 

 tendon (mainly) to the radius bone of the forearm. 

 When it contracts it lifts up the forearm and bends the 

 elbow. Now this biceps-muscle is a huge collection of 

 muscle-fibres — half a million in the arm of a working- 

 man — each of which is a sort of microscopic engine- 

 cylinder. In the motor-cycle the space inside the 

 cylinder is lengthened when an effective stroke is made ; 

 we are deahng with a ' push ' engine. In the muscle- 

 fibre, the cylinder itself becomes shorter and wider ; 

 we are dealing with a * pull ' engine. In the motor-cycle 



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