MUSCLES OF EXPIRATION. 125 



greatest. They are elongated when the chest is distended, and are shortened 

 when the chest is collapsed (Sibson). This fact, taken in connection with 

 experiments on living animals, shows that they are muscles of expiration. 

 Theii contraction tends to depress the ribs and consequently to diminish the 

 capacity of the chest. 



Infracostales. These muscles, situated at the posterior part of the tho- 

 rax, are variable in size and number. They are most common at the lower 

 part of the chest. Their fibres arise from the inner surface of one rib to be 

 inserted into the inner surface of the first, second or third rib below. The 

 fibres follow the direction of the internal intercostals, and acting from their 

 lower attachments, their contractions assist these muscles in drawing the ribs 

 downward. 



Triangularis Sterni. There has never been any doubt concerning the 

 expiratory action of the triangularis sterni. From its origin, the ensiform 

 cartilage, lower borders of the sternum, and lower three or four costal carti- 

 lages, it acts upon the cartilages of the second, third, fourth and fifth ribs, 

 to which it is attached, drawing them downward and thus diminishing the 

 capacity of the chest. 



The above-mentioned muscles are called into action in ordinary, tranquil 

 respiration, and their sole office is to diminish the capacity of the chest. In 

 labored or difficult expiration, and in the acts of blowing, phonation etc., 

 other muscles, which are called auxiliaries, play a more or less important part. 

 These muscles all enter into the formation of the walls of the abdomen, and 

 their general action in expiration is to press the abdominal viscera and dia- 

 phragm into the thorax and diminish its vertical diameter. Their action is 

 voluntary ; and by an effort of the will, it may be opposed more or less by the 

 diaphragm, by which means the duration or extent of the expiratory act is 

 regulated. They are also attached to the ribs or costal cartilages, and while 

 they press the diaphragm upward, they depress the ribs and thus dimin- 

 ish the antero-posterior and transverse diameters of the chest. In this 

 action, they may be opposed by the voluntary contraction of the muscles 

 which raise the ribs, also for the purpose of regulating the force of the ex- 

 piratory act. 



In labored respiration in disease and in the hurried respiration which fol- 

 lows violent exercise, the auxiliary muscles of expiration, as well as of inspi- 

 ration, are called into action to a considerable extent. 



Obliquus Externus. This muscle, in connection with the obliquus in- 

 ternus and transversalis, is efficient in forced or labored expiration, by press- 

 ing the abdominal viscera against the diaphragm. Acting from its attach- 

 ments to the linea alba, the crest of the ileum and Poupart's ligament, by its 

 attachment to the eight inferior ribs, it draws the ribs downward. 



Obliquus Internus. This muscle also acts in forced expiration, by com- 

 pressing the abdominal viscera. The direction of its fibres is from below 

 upward and forward. Acting from its attachments to the crest of the ileum, 

 Poupart's ligament and the lumbar fascia, by its attachments to the carti- 

 lages of the four inferior ribs, it draws them downward. The direction of the 

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