396 THE HUMAN BODY 



curved rib, and the clasped hands to the breast-bone. If the 

 hands be simply 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 dorsiventral 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 considerably increased 

 from side to side, as the chest cavity is in inspiration by the simi- 

 lar elevation of the most curved part or " angle" of the middle 

 ribs. 



Expiration. To produce an inspiration requires considerable 

 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, no muscu- 

 lar 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 fibers cease contracting. By these means the 

 chest cavity is restored to its original capacity and the air sent 

 out of the lungs, by the elasticity of the parts which were stretched 

 or twisted in inspiration, and not 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 producing the respir- 

 atory movements; and expiration then becomes, in part, an ac- 

 tively muscular act. The main expiratory muscles are the internal 

 intercostals which lie beneath the external between each pair of 

 ribs (Fig. 120, B), and have an opposite direction, their fibers 

 running upwards and forwards. In forced expiration the lower 

 ribs are fixed or pulled down by muscles running in the abdominal 

 wall from the pelvis to them and to the breast-bone. The internal 

 intercostals, contracting, pull down the upper ribs and the ster- 

 num, and so diminish the thoracic cavity dorsiventrally. At 



