540 



HUMAN ANATOMY. 



3. Triangularis Sterni (Fig. 538). 



Attachments. — The triangularis sterni (m. transversus thoracis) forms a thin 

 sheet situated upon the posterior surface of the medial portion of the anterior 

 thoracic wall. It arises at one edge by a series of slips from the costal cartilages of 

 the second or third to the sixth or seventh rib ; the upper fibres are directed obliquely 

 downward and medially and the lower ones transversely to be inserted by a thin, flat 

 tendon to the sides of the lower portion of the sternum and to the xiphoid process. 

 The lower fibres of the muscle are practically continuous with those of the transversus 

 abdominis. 



Nerve-Supply. — By the anterior divisions (intercostal nerves') of the second 

 or third to the sixth or seventh thoracic nerve. 



Fig. 538. 



Internal mammary artery 



Triangularis 



sterni 



.\ttach- 

 - J ment of 

 I diaphragm 



Dissection of anterior thoracic wall from behind, showing triangularis sterni and intercostal muscles. 



Action. — To draw downward the anterior portions of the ribs and so assist in 

 expiration. 



Relations. — The internal mammary vessels pass downward upon the anterior 

 surface of the muscle, separating it from the fibres of the internal intercostals. 



4. Levatores CostarUm (Fig. 521). 



Attachments. — The levatores costarum form a series of thin triangular mus- 

 cles which arise from the transverse processes of the seventh cervical and all the 

 thoracic vertebrae except the twelfth. They are directed downward and laterally to 

 be inserted into the posterior surface of the next succeeding rib {levatores costarujn 

 breves) between the tubercle and the angle, some of the fibres of the lower mus- 

 cles passing over a rib to be inserted into the next but one below {levatores costarum 

 longi). 



