MUSCLES OF THE ABDOMEN. 211 



the abdomen. The two muscles cross each other in the direction of their 

 fibres. 



In structure the intercostal muscles consist of an admixture of muscular 

 and tendinous fibres. They arise from the two lips of the lower border 

 of the ribs, the external from the outer lip, the internal from the inner, 

 and are inserted into the upper border. 



Relations. The external intercostals, by their external surface, with the 

 muscles which immediately invest the chest, viz. the pectoralis major and 

 minor, the serratus magnus, serratus posticus superior 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 aponeu- 

 rosis, and posteriorly with the pleura. The internal intercostals, by their 

 external surface with the external intercostals, and intercostal vessels and 

 nerves ; by their internal surface with the pleura costalis, the triangularis 

 sterni and diaphragm. 



Connected with the internal intercostals are a variable number of mus- 

 cular 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 cor- 

 rectly the infracostal 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, ensi- 

 form cartilage, and sternal extremities of the costal cartilages ; and is in- 

 serted 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 carti- 

 lage, the costal cartilages, internal intercostal muscles, and internal mam- 

 mary 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 trian- 

 gularis sterni draws down the costal cartilages, and is, therefore, an expi- 

 ratory 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, 



Diaphragm. 



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, frorr. 



