HOW THE BACKBONE IS BALANCED 25 



Now we begin to understand that the human body is a 

 very intricate machine, and the more we know about it 

 the more we marvel that every little child, as it learns to 

 walk, is also able to master the machinery of its backbone. 



We have not yet finished with all the engines which 

 are set in motion when we walk. We have counted 104 

 of them in the lower limbs and 144 in the back. But we 

 have not thought of the head, which is balanced on the 

 top vertebra or atlas. Twenty muscles are required for 

 that purpose. Then there are the shoulders ; if you look 

 at the skeleton you will find that the shoulder-blades 

 (Plate I.) are fastened on by pins or wires, otherwise they 

 would fall off". In the living body they are kept up by 

 muscles which are active and busy so long as we are 

 walking, standing, or sitting. They, too, will become 

 tired in time and then the shoulders ache. Besides, there 

 are also the muscles of the arms ; when we are walking 

 we find that if the arms are swung we feel easier and 

 better balanced. 



We now begin to see why we are tired at the end of a 

 long journey on foot. 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. But why is it that, when we have stood 

 still at attention or sat bolt upright for half an hour, 

 we feel more tired than after a long journey ? That is 

 because, if we are walking, running, jumping, or turning 

 about, our muscles work alternately ; when one is doing 

 a turn, another is having a spell oflF. But if we stand at * 

 attention or sit bolt upright there is no alternation of 

 work and rest ; the muscular engines are kept steadily at 

 work all the time. The drill sergeant knows how soon 

 men become tired if they have to stand at attention ; he 

 relieves them by giving the order " stand at ease," because 

 when we stand easy we make certain muscles keep the 

 body upright and allow the others to rest ; then we 

 change our position, so that the rested muscles have in 

 their turn to do the work of keeping the body upright. 

 It is good for boys and girls to have to sit upright for 



