EXPIRATION 143 



^These are the muscles which are chiefly concerned in ordinary 

 inspiration. Their combined action also increases slightly 

 the transverse diameter of the chest. But there are certain 

 others, known as auxiliary muscles of inspiration, which are called 

 into play during profound or forced inspiration. Their action 

 is evident from their attachments all operating chiefly to in- 

 crease the antero-posterior diameter. They are the serratus 

 posticus superior, sterno-mastoideus, levator anguli scapula, tra- 

 pezius, pectoralis minor, pectoralis major (costal portion), serratus 

 magnus, rhomboidei and electores spince. It will be noticed that 

 several of these which usually take their point on the chest, as, 

 for example, the sterno-mastoideus, pectorales, etc., must, in 

 order to aid inspiration, take their fixed points at their other 

 extremities. 



Expiration. When the force which expands the chest during 

 inspiration ceases to operate, expiration follows. Not only does 

 the elastic (i) lung tissue force out the air, but the (2) thoracic 

 walls, by their costal cartilages and their intercostal tissues, are 

 themselves elastic, and this elasticity, aided by the (3) "tone" of 

 the muscles which have been put upon the stretch during inspira- 

 tion and which are now seeking to return to their normal con- 

 dition, tends to restore the thorax to the dimensions it had pre- 

 vious to the inspiratory act. So far no actual muscular con- 

 traction has been brought into play, and it is here assumed that 

 none is usually concerned in the expiratory act of ordinary tran- 

 quil respiration. 



Some maintain that the costal portions of the intercostales 

 interni particularly are expiratory in quiet breathing; they do 

 contract and the ribs approach each other during the act, but it 

 is probable that they serve only to maintain the proper degree of 

 tension of the intercostal tissues. 



Although the elastic reaction of the lung tissue during expira- 

 tion operates together with the elasticity of the thoracic wall in 

 diminishing the antero-posterior diameter of the chest, it is 



