A PAIR OF LIVING BELLOWS 131 



cone is occupied by a piston which is as remarkable for 

 its construction as for its effectiveness. The cone-shaped 

 cylinder in which it works alters in dimension with every 

 breath, yet the fit of the piston within the cylinder is 

 perfect in every phase. It is a packed piston, packed 

 with viscera — with great workshops which we shall 

 examine later — the liver, the stomach, spleen, and other 

 soft structures, all of them kept tense and turgid by the 

 blood pumped into them by the cardiac pump. Indeed 

 the diaphragmatic piston is packed by the top of the 

 elastic column of viscera which fills the abdominal cavity. 

 When we stand up the great muscular walls of the 

 abdomen keep the top of the visceral column pressed 

 within the diaphragmatic piston ; and when we lie down 

 the viscera, by their own weight, keep it distended. 



How is the visceral piston set in motion ? Nature 

 builds her pistons as she builds her pumps and bellows ; 

 the engine which moves the piston forms part of it. 

 The piston has a muscular hood or covering called the 

 diaphragm. The strongest part of this muscular hood 

 is fixed to the backbone, just below the part to which 

 the ribs are attached ; from this spinal basis the muscular 

 fibres ascend over the back of the piston, ending on the 

 fibrous caps or domes of the diaphragmatic piston. This 

 constitutes the spinal part of the diaphragm. Then the 

 muscle which covers the sides and front of the visceral 

 piston obtains a fixed basis from which to act on the 

 margins of the thorax — from the costal levers and costal 

 cartilages forming the right and left margins. These 

 fibres make up the costal part of the muscular diaphragm. 

 The costal fibres are awkwardly situated in this sense : 

 at one end they are attached to movable levers — the 

 ribs ; at the other end they are attached to a movable 

 piston. If the piston is fixed by being pressed up by 

 the abdominal muscles, then the costal fibres will help 

 to elevate the ribs ; if the ribs are fixed and the piston 

 is movable, then they bring the piston down ; in either 

 case they help to enlarge the thorax and draw in the 

 breath. In quiet breathing the costal fibres, in the 



