24 THE ENGINES OF THE HUMAN BODY 



shall see that these levers form part of a great air-pump — 

 the chest or thorax (Plate I.). Their chief use is to 

 enlarge the thorax and draw breath into the lungs, but 

 we must not forget that they are also the most powerful 

 levers of the spine ; all the muscles which are attached to 

 them can and do help in keeping the spine erect. Thus 

 we see that the spine is provided with levers and muscles 

 to prevent it from swaying backwards, forwards, to the 

 right or to the left. We can not only bend our bodies 

 in all of these directions, but we have also the power of 

 twisting or partly turning the vertebrae on each other, so 

 that we can look over the right shoulder or over the left. 



^ , Spinal cord 



era! lever 



S-Uind lever 



(Spinous process) 



I <l -Sheath or Spinal 



Fig. 3. — Plan of vertebrae in the lower or lumbar part of the spinal column. 

 A, A lumbar vertebra looked at from above to show its levers. B, Three 

 lumbar vertebrae showing the discs between them which act as buffers. 



It is clear, then, that there are also muscles which can act 

 on the levers of the vertebrae and produce a twisting 

 movement. The elaborate machinery needed for balancing 

 the vertebrae on each other and for keeping the spine 

 erect is now becoming evident to us. We have seen 

 that there are 24 vertebrae poised one above another ; 

 each has not less than 3 levers to be worked, some of 

 them 5 ; each of the 24 vertebrae has at least 6 effective 

 engines working on it, bending it to the right, left, for- 

 wards, backwards, or twisting it, as occasion requires. 

 With 24 vertebrae and 6 muscles supplied to each, we 

 have a total of 144 — a gross of engines, which we employ 

 to balance the spine in every step we take. They are set 

 in motion, speeded up, and slowed down with every step. 



