320 A MANUAL OF ANATOMY. 



better be made through them along the anterior axillary line, using a small 

 saw or bone cutters for the purpose) until the seventh rib is reached ; cut the 

 muscles between it and the sixth up to the sternum, and saw through the 

 latter just above the point where the seventh costal cartilage articulates with 

 it. 



Beginning above, raise the sternum, dividing the internal mammary arteries 

 as close to it as possible, and remove the sternum and cartilages (and portions, 

 of ribs if they have been sawn through) from the thoracic contents by careful 

 dissection, so as not to injure them. 



On the Sternum. 



Triangnlaris Sterni. (Continuation upward of the trans- 

 versalis.) 



Origin. From the internal surface of the fifth, sixth, and 

 seventh costal cartilages, the ensiform appendix, and the 

 lateral margins of sternum for its lower third. 



Insertion. Into the inner surface and lower border of 

 the second to the sixth cartilages, close to their costal 

 junction. 



Nerve Supply. The upper intercostals. 



Action. Feeble muscle of expiration, by depressing the 

 inner anterior portions of the ribs to which it is attached. 



DISSECTION. 



Remove the above muscle. Trace the internal mammary artery and its an- 

 terior and lateral branches as far as possible. 



The Internal Mammary Artery. Figs. 18, 68. 



Arises from the under surface of the first portion of the 

 subclavian artery, close to the inner border of the scalenus 

 anticus muscle, and opposite the thyroid axis. See page 

 113. It passes into the chest and descends behind the 

 cartilages of the ribs about half an inch from the external 

 margin of the sternum until the sixth space is reached, 

 where it divides into the superior epigastric and the muscu- 

 lophrenic. 



