102 RESPIRATION 



cause, threatening suffocation, all the muscles that can by any possi- 

 bility raise the chest are brought into action. These are classed in the 

 table as extraordinary auxiliaries. Most of these muscles can volunta- 

 rily be brought into play to raise the chest, and the mechanism of their 

 action may in this way be demonstrated. 



Serratus Posticus Superior. This muscle, by reversing its ordinary 

 action, is capable of increasing the capacity of the thorax. 



Sterno-mastoideus. That portion of the muscle which is attached 

 to the mastoid process of the temporal bone and the sternum, when the 

 head is fixed, is capable of acting as a muscle of inspiration. It does 

 not act in ordinary respiration, but its contractions may be observed 

 whenever respiration is hurried or exaggerated. 



The following muscles as a rule act as muscles of inspiration only 

 when respiration is difficult or labored : 



Levator Anguli Scapula and Superior Portion of the Trapezius. 

 Movements of the scapula have often been observed in labored res- 

 piration. Its elevation during inspiration is effected chiefly by the 

 levator anguli scapulae and the upper portion of the trapezius. 



Pectoralis Minor and Inferior Portion of the Pectoralis Major. 

 These muscles act together to raise the ribs in difficult respiration. 

 The pectoralis minor is the more efficient. With the coracoid process 

 as the fixed point, this muscle is capable of powerfully assisting in the 

 elevation of the ribs. That portion of the pectoralis major which is 

 attached to the lower part of the sternum and costal cartilages is capa- 

 ble of acting from its insertion into the bicipital groove of the humerus, 

 when the shoulders are fixed, in concert with the pectoralis minor. 



Serratus Magnus. Acting from the scapula as the fixed point, this 

 muscle is capable of assisting the pectorals in raising the ribs and be- 

 comes a powerful auxiliary in difficult inspiration. 



The division into muscles of ordinary inspiration, ordinary auxiliaries 

 and extraordinary auxiliaries, must not be taken as absolute. In the 

 male, in ordinary respiration, the diaphragm, intercostals and levatores 

 costarum are the principal inspiratory muscles, and the action of the 

 scaleni, with the consequent elevation of the sternum, is commonly very 

 slight or it may be wanting. In the female, the movements of the 

 upper parts of the chest are more prominent, and the scaleni, the ser- 

 ratus posticus superior, and sometimes the sterno-mastoid, are brought 

 into action in ordinary respiration. In the different types of respiration, 

 the action of the muscles necessarily presents considerable variations. 



Expiration. The air is expelled from the lungs, in ordinary expira- 

 tion, by a simple and comparatively passive process. The lungs contain 

 a large number of elastic fibres surrounding the air-cells and the small- 



