390 THE HUMAN BODY. 



curved part lies below the level of a straight line drawn from 

 the vertebral to the sternal attachment of the bone. By the 

 rotation of the rib, during inspiration, this curved part is 

 raised and turned out, and the chest widened. The mech- 

 anism can be understood by clasping the hands opposite the 

 lower end of the sternum and a few inches in front of it, 

 with the elbows bent and pointing downwards. Each arm 

 will then answer, in an exaggerated way, to a curved rib, and 

 the clasped hands to the breast-bone. If the hands be sim- 

 ply raised a few inches by movement at the shoulder-joints 

 only, they will be separated farther from the front of the 

 Body, and rib elevation and the consequent dorso-ventral en- 

 largement of the cavity surrounded will be represented. But 

 if, simultaneously, the arms be rotated at the shoulder-joints 

 so as to raise the elbows and turn them out a little, it will be 

 seen that the space surrounded by the two arms is consider- 

 ably increased from side to side, as the chest cavity is in in- 

 spiration by the similar elevation of the most curved part or 

 '*' angle " of the middle ribs. 



Expiration. To produce an inspiration requires consid- 

 erable muscular effort. The ribs and sternum have to be 

 raised; the elastic rib cartilages bent and somewhat twisted; 

 the abdominal viscera pushed down ; and the abdominal wall 

 pushed out to make room for them. In expiration, on the 

 contrary, but little, if any, muscular effort is needed. As 

 soon as the muscles which have raised the ribs and sternum 

 relax, these tend to return to their natural unconstrained 

 position, and the rib cartilages, also, to untwist themselves 

 and bring the ribs back to their position of rest; the elastic 

 abdominal wall presses the contained viscera against the 

 under side of the diaphragm, and pushes that up again as 

 soon as its muscular fibres cease contracting. By these means 

 the chest cavity is restored to its original capacity and the 

 air sent out of the lungs, rather by the elasticity of the parts 

 which were stretched or twisted in inspiration, than by any 

 special expiratory muscles. 



Forced Respiration. When a very deep breath is drawn 

 or expelled, or when there is some impediment to the entry 

 or exit of the air, a great many muscles take part in produc- 

 ing the respiratory movements; and expiration then becomes, 

 in part, an actively muscular act. The main expiratory mus- 

 cles are the internal intercastals which lie beneath the exter- 



