398 



THE MUSCLES 



4. TRIANGULARIS STERNI 



As the external and internal ol)li(|ue nuiscles of the aljdonien are rej^resented 

 1)y the external and internal intercostals in the thorax, so also the transversalis 

 abdominis has its counterpart in the thin stratum of muscular fibre at the sides of 

 the sternum called the triangularis sterni, and the still thinner expansion behind 

 of the infracostales. 



The triangularis sterni — named from its shape and its connection with the 

 sternum — is a thin, musculo-membranous, triangular sheet, with the apex below 

 and directed internally, Avhile the serrated base is external. 



Origin. — (1) The side of the lower third of the back of the sternum; (2) the 

 upi)er and lateral part of the back of the ensiform cartilage; (3) the back of the 

 inner ends of the fifth, sixth, and seventh costal cartilages. 



Fig. 301. — The Muscles attached to the Back of the Steexum. 



Sterno-hyoid 



Sterno-thyroid 



Sternal origin 

 of diaphragm 



Costal origin 

 of diaphragm 



Triangularis 

 Bterni 



Transversalis abdominis 



Insertion. — The outer ends of the posterior surfaces and lower borders of the 

 cartilages of the second or third to the sixth ribs, and occasionally the tips of the 

 ribs also. 



Structure. — The muscle is membranous at its origin and insertion, and it con- 

 tains maiiy Ixuids of fibrous tissue. Its fibres diverge fanwise, the lower ones 

 being horizontal, and in serial continuation with the ui)pcr digitations of the trans- 

 versalis abdominis, while its higher fibres run ol:)li(iucly ujiwards and outwards. 



Nerve-supply. — The upper intercostals, which send filaments to its anterior 

 aspect. 



Action. — To depress the anterior extremities of the ribs to Avhich it is attached, 

 and so to help in ex])iration. 



Relations. — In front, the internal intercostals and internal mammary vessels; 

 behind, the pleura and pericardium. 



