130 EESPIRATION. 



Muscles of Expiration. 



Ordinary Respiration. 



Muscle. Attachments. 



Osseous portion of internal intercostal s . .Inner borders of the ribs. 



Infracostales Inner surfaces of the ribs. 



Triangularis sterni Ensiform cartilage, lower borders of sternum, lower three 



or four costal cartilages cartilages of the second, 



third, -fourth, and fifth ribs. 



Auxiliaries. 



Obliquus externus External surface and inferior borders of eight inferior 



ribs anterior half of the crest of the ileum, Pou- 



: . part's ligament, linea alba. 



Obliquus internus Outer half of Poupart's ligament, anterior two-thirds of 



the crest of the iLeum, lumbar fascia cartilages of 



four inferior ribs, linea alba, crest of the pubis, pec- 

 tineal line. 



Transversalis Outer third of Poupart's ligament, anterior two-thirds of 



the crest of the ileum, lumbar vertebrae, inner surface 



of cartilages of six inferior ribs crest of the pubis, 



pectineal line, linea alba. 



Sacro-lumbalis Sacrum angles of six inferior ribs. 



Internal Intercostals. The internal intercostals have different functions in different 

 parts of the thorax. They are attached to the inner borders of the ribs and costal carti- 

 lages. Between the ribs, they are covered by the external intercostals, but, between the 

 costal cartilages, they are covered simply by aponeurosis. Their direction is from above 

 downward and backward, nearly at right angles to the external intercostals. The function 

 of that portion of the internal intercostals situated between the costal cartilages has al- 

 ready been noted. They assist the internal 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 tho- 

 rax where the obliquity of the ribs is greatest. The observations of Sibson have shown 

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

 collapsed. 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. If we bring an animal, a dog for 

 example, completely under the influence of ether, expose the walls of the chest, dissect off 

 the fascia from some of the external intercostals, and then remove carefully a portion of 

 one or two of these muscles so as to expose the fibres of the internal intercostals, it is 

 not difficult, on close examination, to observe the antagonism between the two sets of 

 muscles ; one being brought into action in inspiration and the other, in expiration. 



Infracostales. These muscles, situated at the posterior part of the thorax, are vari- 

 able 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 intercos- 

 tals, 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 func- 

 tion of the triangularis sterni. From its origin, the ensiform cartilage, lower borders of 

 the sternum, and lower three or four costal cartilages, 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. 



