THE THORAX. Ill 



abscess, is usually performed between the fifth and sixth 

 ribs, just behind their middle, and never behind the 

 angle, because of the size of the intercostal arteries at 

 this point, and moreover on account of the thickness of 

 the layers of muscle. 



Some prefer a point situated an inch or more below 

 the angle of the scapula, between the seventh and eighth, 

 or eighth and ninth ribs, the instrument being passed 

 just above the upper border of the rib, so that the inter- 

 costal vessels may be avoided. On the right side, it is 

 advised to puncture through an interspace higher, on 

 account of the position of the liver and diaphragm on 

 that side. 



The internal mammary artery belongs properly to the 

 cavity of the chest, lying on the left side in the anterior 

 mediastinum, whilst on the right it is so overlapped by 

 the lung as to be excluded from the space. It is best 

 exposed from the surface, by raising the cartilages of the 

 ribs and the sternum, and is then seen lying upon the 

 pleura at a short distance from the margin of the sternum. 

 It is given off from the subclavian opposite the verte- 

 bral, and passes into the thorax in relation with the 

 phrenic nerve, which crosses it anteriorly and then de- 

 scends internal to it. Two veins, which usually unite 

 to form a common trunk, accompany it. In the upper 

 part of the chest it is covered in by the costal cartilages 

 and internal intercostals, whilst below it lies between the 

 triangularis sterni and the pleura. Its chief inoscula- 

 ting branches are given off at the interval between the 

 sixth and seventh cartilages, and are musculo-phremc 

 and superior epigastric. 



Wounds of this vessel, especially if situated where it 

 has any bulk, i. e.. between the first and seventh rib, are 



