196 MYOLOGY. 



PECTORALIS MAGNUS. 



(Sterno-trochineus. ) 



(Pl. III. 19, 19. Fig. 75.//.) 



This muscle is situated on the infero-lateral part of the thorax, 

 and is large, irregularly four-sided, flat, and thick, the fibres 

 extending forwards and outwards. Slightly aponeurotic ante- 

 riorly, elsewhere it is fleshy. 



Origin. — From the inferior border of the last three or four 

 bones of the sternum, the ensiform cartilage, and its fellow of the 

 opposite side ; externally from the cartilages of the false ribs, 

 fascia of the external oblique muscle of the abdomen, and the 

 abdominal tunic. 



Insertion. — To the fascia on the inside of the arm, and to the 

 inner trochanter of the humerus, under the tendon of the coraco- 

 humeralis, and by fascia over the bicipital groove, to the external 

 trochanter. 



Relations. — Externally with the panniculus, skin, and pectoralis 

 transversus ; internally with the fascia of the external oblique, 

 and the ribs ; anteriorly with the pectoralis parvus. 



Action. — To draw the shoulder back and adduct the arm ; in 

 difficult respiration it acts as an inspiratory muscle. 



PECTORALIS ANTICUS. 



{Sterno-humeraiis. ) 



(Fig. 75. e.) 



Situated on the anterior part of the breast, it forms the fleshy 

 prominence seen in front. Short, thick, and fusiform, its fibres 

 extend outwards ; it is tendinous at its attachments, with a fleshy 

 belly. 



Origin. — From the cariniform cartilage and first bone of the 

 €tenium. It is directed somewhat backwards, outwards, and 

 ■downwards. 



Insertion. — To a ridge on the anterior part of the shaft of the 

 humerus by fleshy fibres, and an aponeurotic tendon common also 

 to the levator humeri, and pectoralis transversus. 



Relations. — Externally with the skin ; anteriorly with the 

 levator humeri and panniculus ; posteriorly with the pectoralis 

 magnus and transversus ; internally with the parvus. 



Action. — Its chief action is to adduct the arm. 



