188 MUSCLES OF THE ABDOMEN. 



The triangularis sterni, situated upon the inner wall of the front of 

 the chest, arises by a thin aponeurosis from the side of the sternum, 

 ensiform cartilage, and sternal extremities of the costal cartilages ; 

 and is inserted by fleshy digitations into the cartilages of the third, 

 fourth, fifth and sixth ribs, and often into that of the second. 



Relations. By its external surface with the sternum, the ensiform 

 cartilage, the costal cartilages, internal intercostal muscles, and in- 

 ternal mammary vessels. -By its internal surface with the pleura 

 costalis, the cellular tissue of the anterior mediastinum and the dia- 

 phragm. 



Actions. The intercostal muscles raise the ribs when they act 

 from above, and depress them when they take their fixed point from 

 below. They are, therefore, both inspiratory and expiratory muscles. 

 The triangularis sterni draws down the costal cartilages, and is 

 therefore an expiratory muscle. 



Muscles of /the Abdomen. 



The muscles of this region are the 



Obliquus externus (descendens), 

 Obliquus internus (ascendens), 

 Cremaster, 

 Transversalis, 

 Rectus, 

 Pyramidalis, 

 Quadratus lumborum, 

 Psoas parvus, 

 yDiaphragm. 



Dissection. The dissection of the abdominal muscles is to be 

 commenced by making three incisions : The first, vertical, in the 

 middle line, from over the lower part of the sternum to the pubes ; 

 the second, oblique, from the umbilicus, upwards and outwards, to 

 the outer side of the chest, as high as the fifth or sixth rib ; and the 

 third, oblique, from the umbilicus, downwards and outwards, to the 

 middle of the crest of the ilium. The three flaps included by these 

 incisions should then be dissected back in the direction of the fibres 

 of the external oblique muscle, beginning at the angle of each. The 

 integument and superficial fascia should be dissected off together so 

 as to expose the fibres of the muscle at once. 



If the external oblique muscle be dissected on both sides, a white 

 tendinous line will be seen along the middle of the abdomen, extend- 

 ing from the ensiform cartilage to the os pubis ; this is the linea alba. 

 A little external to it, on each side, two curved lines will be observed 

 extending from the sides of the chest to the os pubis, and bounding 

 the recti muscles : these are the linece semilunares. Some transverse 

 lines, linece, transversce, three or four in number, connect the linea? 

 semilunares with the linea alba. 



The external oblique muscle (obliquus externus abdominis, descen- 



