132 THE ENGINES OF THE HUMAN BODY 



majority of people, act both on piston and ribs, but the 

 stronger spinal fibres have only one action : they cause 

 the piston to descend. The result of combined action 

 by the spinal and costal muscular fibres is to push the 

 piston forwards as well as downwards ; hence we notice 

 that at the pit of the stomach, just below the breast- 

 bone, there is a forward swelling with each breath 

 we take. 



The muscular fibres of the diaphragm cause the piston 

 in the floor of the thorax to descend, and thus enlarge the 

 respiratory bellows on its vertical diameter from floor to 

 apex of the thoracic cone. But how is this heavy piston 

 returned after it has descended ? Which are the muscular 

 engines which must yield as the piston is set in motion, 

 and which will return the piston to start a new inspiratory 

 cycle ? The opponents of the diaphragm are the strong 

 muscular sheets which make up the walls of the abdomen 

 — especially the upper parts of these sheets ; it is they 

 which return the diaphragmatic piston. Thus are the 

 abdominal Viscera used to pack a piston — a piston which 

 the diaphragm and muscles of the belly-wall use as a 

 shuttle-cock with each breath we take and give. 



This, then, is the story of how Nature has constructed 

 the moving walls of the respiratory bellows, which she 

 has fitted to the human machine. She has used the back- 

 bone as the upright or standard for a support ; the side 

 and front wall she has built out of levers and engines, 

 so that when one set of engines are set in motion the side 

 and front walls move outwards as well as upwards ; at 

 the same time a piston packed with viscera and coated 

 with muscular engines moves downwards and forwards. 

 In this manner are the bellows enlarged in every direction, 

 and air is thus drawn into the lungs. Then by setting 

 another elaborate group of engines at work the move- 

 ments of the levers and piston are reversed, and the air 

 is gently expelled from the respiratory chambers. Man 

 has not yet conceived a design which can rival or approach 

 the respiratory bellows. 



