A PAIR OF LIVING BELLOWS 129 



in the lower ribs, but they are present in all. Hence 

 when the ribs are raised they not only elevate and push 

 forwards the front wall of the chest, but they also throw 

 the side walls outwards, in this way enlarging the thorax 

 in a transverse direction. 



Thus in every breath we take twelve pairs of bony 

 levers are set in motion ; they are raised by one set of 

 engines arranged as a complicated sheet — the external 

 intercostal sheet of musculature. That sheet has its 

 fixed base in the backbone ; it cannot act unless the 

 muscles which fix and regulate the vertebrae are also in 

 action. The twelve pairs of levers are lowered by another 

 elaborate set of engines — the internal intercostal sheet, 

 which obtains its chief basis from which to pull in the 

 framework of the pelvis. The great muscular sheets 

 which ascend in the belly-wall from the pelvis to the 

 ribs not only take a part in lowering the ribs and com- 

 pressing and emptying the thorax but have also, when 

 we are standing up, to support the weight of the viscera 

 which are contained in the abdomen. When we see how 

 great is the number of structures set in motion with 

 each breath we take — levers, muscular engines, joints, 

 and nerve centres — we are astonished that so complex 

 a mechanism can be carried on with so little effort on 

 our part. 



We have seen that in all pumps of human invention, 

 whether driven by hand, steam, or electricity, the power 

 or engine has to be built as a separate machine. Nature, 

 having that matchless kind of motor-power at her disposal 

 called muscle, is able to build into its wall the engines 

 which drive the pump, as we have seen is the case in the 

 heart. She has the same advantage when she has to 

 build bellows ; the engines which work the respiratory 

 levers, and the levers themselves, are actually built in to 

 form the wall of the thoracic bellows. At least they form 

 the side and front walls ; but there is another wall, a very 

 important one, which we have not yet looked at. This 

 is the floor or diaphragmatic wall. The floor, we are 

 to find, is built to act as a piston — a piston of a very 



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