EESPIEATION 301 



A forced inspiration may be made voluntarily, often it is pro- 

 duced involuntarily. In it every muscle which can act from 

 the fixed spine, head and shoulder girdle upon the thorax, is 

 brought into play. Normally, these act from the thorax upon 

 the parts into which they are inserted ; now they act from their 

 insertion upon their point of origin. The sterno-mastoids, 

 sterno-thyroids, and sterno-hyoids assist in elevating the 

 thorax. The serratus magnus, pectoralis minor, and anterior 

 fibres of the pectoralis major, and the part of the latissimus 

 dorsi which passes from the humerus to the three last ribs, 

 also pull these structures upwards. The facial and laryngeal 

 movements also become exaggerated. 



B. In Expiration the various muscles of inspiration cease 

 to act, and the forces against which they contended again 

 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 inwards (see p, 

 296). 



The elasticity of the costal cartilages causes the ribs again 

 to fall back, and finally the elasticity of the abdominal wall 

 drives the abdominal viscera against the relaxed diaphragm 

 and again arches it towards the thorax, squeezing its marginal 

 portion against the ribs and occluding the complemental pleura. 



Experimental evidence shows that the internal intercostals 

 contract with each expiration, and help to draw the ribs 

 downwards. 



Ordinary expiration is thus 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 then partly 

 due to the above factors, and partly due to the action of 

 muscles. Every muscle which can in any way dimmish the 

 size of the thorax comes into play. 



Chief of these are the abdominal muscles, which by com- 

 pressing the viscera push them upwards and press the dia- 

 phragm further up into the thorax. At the same time, by 

 acting from the pelvis to pull down the ribs, they decrease 

 the thorax from side to side and from before backwards. 



The serratus posticus inferior and part of the sacro-lumbalis 



