EXPIRATION 143 



able on their spinal articulations. Contraction of these mus- 

 cles is, therefore, very efficient in elevating the anterior ends 

 of the ribs. 



The action of the diaphragm is the most notable of the 

 muscular phenomena connected with respiration, and it de- 

 serves to be called the "muscle of respiration." 



These are the muscles which are chiefly concerned in ordi- 

 nary 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 in- 

 spiration. Their action is evident from their attachments 

 all operating chiefly to increase the antero-posterior diame- 

 ter. They are the serratus posticus superior, sterno-mastoi- 

 deus, levator anguli scalpula, trapezius, pectoralis minor, 

 pectoralis major (costal portion), serratus magnus, rhom- 

 boidei and erectores spines. It will be noticed that several of 

 these which usually take their point on the chest, as, for ex- 

 ample, the sterno-mastoideus, pectorales, etc., must, in order 

 to aid inspiration, take their fixed points at their other ex- 

 tremities. 



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 inspiration and which are now seek- 

 ing to return to their normal condition, tends to restore the 

 thorax to the dimensions it had previous to the inspiratory 

 act. So far no actual muscular contraction has been brought 

 into play, and it is here assumed that none is usually con- 

 cerned in the expiratory act of ordinary tranquil 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 



