RESPIRATION 521 



magnus, pectoralis minor, and posterior fibres of the pectoralis 

 major, and the part of the latissinius dorsi which passes from 

 the humerus to the posterior ribs, also pull these structures 

 forwards. The facial and laryngeal movements also become 

 exaggerated. 



2. Expiration is a return of the thorax to the position of 

 rest. The various muscles of inspiration cease to act, and the 

 forces against which they contended contract the thorax in its 

 three diameters — 



The elasticity of the lungs is no longer overcome by 

 the muscles of inspiration, and the external atmospheric 

 pressure acting along with it drives the chest wall in- 

 wards. 



The elasticity of the costal cartilages tends to bring the 

 chest back to the position of rest; the elasticity of the abclomninal 

 wall drives the abdominal viscera against the relaxed diaphragm 

 and again arches it towards the thorax, bringing its marginal 

 portion in contact with the ribs and occluding the complemental 

 pleura. By this constriction of the thorax, the air in the lungs is 

 compressed and the pressure is raised above the atmospheric 

 pressure outside, and so the air is driven out. 



Experimental evidence shows that the internal intercostals 

 contract with each expiration, and help to draw the ribs 

 backwards. 



Ordinary expiration is tlius normally mainly a passive act, 

 being simply a return of the thorax to the position of rest. But 

 voluntarily, and, in certain conditions, involuntarily, expiration 

 may be forced. 



Forced expiration is due to the action ot muscles. Every 

 muscle which can, in any way, diminish the size of the thorax 

 comes into play. Chief of these are the abdominal muscles, 

 which, by compressing the viscera, push them upwards and press 

 the diaphragm further up into the thorax. At the same time, 

 by acting from the pelvis to pull back the ribs, they decrease 

 the thorax from side to side and from below upwards. 

 The serratus posticus inferior and part of the sacro-lumbalis 

 pull downwards the lower ribs, and the triangularis sterni also 



assists in this. 



