238 DISSECTION OF THE UPPER LIMB. 



costal nerve and artery will appear. A branch of the nerve to the surface 

 (lateral cutaneous of the thorax) should be followed through the external 

 muscle ; and the trunk of the nerve is to be traced forwards in one or 

 more spaces to the sternum, and the surface of the thorax. 



The hinder part of these muscles will be seen in the dissection of the 

 Back and thorax. 



The internal intercostal muscle, attached to the inner border of the ribs 

 bounding the intercostal space, begins in front at the extremity of the 

 ribs, and ceases behind near their angles. Posteriorly they do not end at 

 the same distance from the spine, for the upper and lower approach nearer 

 than the middle ; and, anteriorly, in the two lowest spaces, the muscular 

 fibres are continuous with the internal oblique of the abdomen. One sur- 

 face is covered by the external muscle and in part by the intercostal ves- 

 sels and nerve ; and the opposite surface is in contact with the pleura. 



Action. By the alternate action of the intercostal muscles the ribs are 

 moved in respiration. 



The external intercostal elevate the ribs and evert the lower edges, so 

 as to enlarge the thorax in the antero-posterior and transverse directions : 

 they come into play during inspiration. 



The internal intercostals act in a different way at the side and fore part 

 of the chest. 



Between the osseous parts of the ribs they depress and turn in those 

 bones, diminishing the size of the thorax ; and they are brought into use 

 in expiration. 



Between the rib-cartilages they raise the ribs, and become muscles of 

 inspiration like the outer layer. 



If both stt.s of muscles contract simultaneously, the motion of the ribs 

 will be arrested ; or if two or more ribs are broken near the spinal column, 

 the muscles of the space or spaces injured will be unable to move these 

 bones. 



Dissection. To bring into view the triangularis sterni muscle and the 

 internal mammary vessels, the cartilages of the true ribs, except the first 

 and seventh, are to be taken away with the intervening njuscles on the 

 right side of the body ;* but the two ribs menti )ned are to be left un- 

 touched for the benefit of the dissectors of the abdomen and head and 

 neck. Small arteries to each intercostal space and the surface of the 

 thorax, and the intercostal nerves, are to be preserved. The surface 

 of the triangularis sterni will be apparent when the loose tissue and fat 

 are removed. 



The TRIANGULARIS STERNI (fig. 72, A) is a thin muscle beneath the 

 costal cartilages. It arises internally from the side of the xiphoid carti- 

 lage, from the side of the sternum as high as the third costal cartilage, 

 and usually from the inner ends of the lower three true costal cartilages. 

 Its fibres are directed outwards, the upper being most oblique, and are 

 inserted by fleshy fasciculi into the true ribs except the last two and the 

 first, at the junction of the bone and cartilage, and into an aponeurosis in 

 the intercostal spaces. 



The muscle is covered by the ribs and the internal intercostals, and by 

 the internal mammary vessels and the intercostal nerves. It lies on the 

 pleura. Its lower fibres touch those of the transversalis abdominis. 



1 On the left side the vessels and the muscle will have been destroyed by the 

 injection of the body. 



