364 THE HUMAN BODY. 



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 un- 

 twist 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 contract- 

 ing. 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 in inspir- 

 ation, 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 pro- 

 ducing the respiratory movements; and expiration then be- 

 comes, in part, an actively muscular act. The main expira- 

 tory muscles are the internal intercostals which lie beneath 

 the external between each pair of ribs (Fig. 112, B), and 

 have an opposite direction, their fibres running upwards 

 and forward j. 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 sternum, and so diminish the thoracic cavity 

 dorso-ventrally. At the same time, the contracted abdomi- 

 nal muscles press the walls of that cavity against the viscera 

 within it, and pushing these up forcibly against the dia- 

 phragm make it very convex towards the chest, and so 

 diminish the latter in its vertical diameter in very violent 

 expiration many other muscles may co-operate, tending to 

 fix points on which those muscles which can directly dimin- 

 ish the thoracic cavity, pull. In violent inspiration, also, 

 many extra muscles are called into play. The neck is held 

 rigid to give the scalenes a firm attachment; the shoulder- 

 joint is held fixed and muscles going from it to the chest- 

 wall, and commonly serving to move the arm, are then 

 used to elevate the ribs; the head is held firm on the verte- 

 bral column by the muscles going between the two, and 

 then other muscles, which pass from the collar-bone and 



