122 RESPIRATION RESPIRATORY MOVEMENTS. 



slight and is physiological, as after exercise, certain of the ordinary auxilia- 

 ries act for a time, until the tranquillity of the movements is restored ; but 

 when there is obstruction in the respiratory passages or when respiration is 

 difficult from any cause, threatening suffocation, all the muscles which can 

 by any possibility raise the chest are brought into action. These are put down 

 in the table under the head of extraordinary auxiliaries. Most of these mus- 

 cles can voluntarily be brought into play to raise the chest, and the mechan- 

 ism of their action can 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 can be readily observed whenever respiration 

 is hurried or exaggerated. 



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

 respiration is very difficult or labored : 



Levator Anguli Scapula and Superior Portion of the Trapezins. Move- 

 ments of the scapula have often been observed in labored respiration. Its 

 elevation during inspiration is effected chiefly by the levator anguli scapulas 

 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 capable 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 mus- 

 cle is capable of assisting the pectorals in raising the ribs and becomes a pow- 

 erful auxiliary in difficult inspiration. 



The uses of the principal inspiratory muscles have been considered with- 

 out taking up those which have an insignificant or undetermined action. In 

 many animals, the nares are considerably distended in inspiration ; and in 

 the horse, which does not respire by the mouth, these movements are as es- 

 sential to life as are the respiratory movements of the larynx. In man, as 

 a rule the nares undergo no movements unless respiration be somewhat ex- 

 aggerated. In very difficult respiration the mouth is opened at each inspira- 

 tory act. 



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 con- 

 sequent elevation of the sternum, is commonly very slight or it may be want- 

 ing. In the female the movements of the upper parts of the chest are more 



