.MUSCLES OF THE ABDOMEN. 211 



and inferior, scalenus posticus ; sacro-lumbalis, and longissimus dorsi, 

 with their continuations, the cervicalis ascendens and transversalis 

 colli; the levatores costarum, and the obliquus externus abdominis. 

 By their internal surface with the internal intercostals, the intercostal 

 vessels and nerves, and a thin aponeurosis, and posteriorly with the 

 pleura. The internal intercostals, by their external surface with the 

 external intercostals, and intercostal vessels and nerves ; by their in- 

 ternal surface with the pleura costalis, the triangularis sterni and 

 diaphragm. 



Connected with the internal intercostals are a variable number of 

 muscular fasciculi which pass from the inner surface of one rib near 

 its middle to the next or next but one below ; these are the subcostal 

 or more correctly the intracostal muscles. 



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 car- 

 tilages ; 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 areolar tissue of the anterior mediastinum and the 

 diaphragm. The lower fibres of the triangularis sterni are continuous 

 with those of the diaphragm. 



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, 



Pyramidal! s, 



Quadratus lumborum, 



Psoas parrus, 



Diaphragm. 



Dissection. The dissection of the abdominal muscles is to be com- 

 menced by making three incisions: The first, vertical, in the middle 



