MUSCLES OF EXPIRATION 105 



intercostals in elevating the ribs in inspiration. Between the ribs these 

 muscles are directly antagonistic to the external intercostals. They 

 are more nearly at right angles to the ribs, particularly in that portion 

 of the thorax where the obliquity of the ribs is greatest. They are 

 elongated when the chest is distended, and are shortened when the 

 chest is retracted. This fact, taken in connection with experiments 

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

 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 

 thorax, 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 inter- 

 costals, 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 cartilages, it acts on 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, 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 diaphragm 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 diminish the antero- 

 posterior and transverse diameters of the chest. In this action, they 

 may be opposed by voluntary contraction of the muscles that raise the 

 ribs, also for the purpose of regulating the force of the expiratory act 



In labored respiration in disease and in the hurried respiration 

 which follows violent exercise, the auxiliary muscles of expiration, as 

 well as of inspiration, are called into action to a considerable extent. 



Obliquus Externus. This muscle, in connection with the obliquus 

 internus and transversalis, is efficient in forced or labored expiration, by 



