THE PLEUKAL CAVITY. 441 



the chest the artery lies on the pleura and the triangularis stern i, 



and is crossed by the intercostal nerves. It is accompanied by 



two veins, and by the chain of sternal lymphatic glands. Its Branches: 



thoracic branches are numerous but small : 



a. A long, slender branch (comes nervi phrenici) arises as the superior 

 artery enters the chest, and descends to the diaphragm with the p mc> 

 phrenic nerve : this branch is seen in the dissection of the thorax. 



b. Small sternal branches supply the triangularis sterni, and small pos- 

 raniify over the back of the sternum. From these, mediastinal 



twigs pass backwards to be distributed to the remains of the 

 t hymns gland and the pericardium. 



c. Two anterior intercostal branches run outwards in each space, intercostal, 

 lying along the borders of the costal cartilages, and terminate by 

 anastomosing with the aortic and superior intercostal branches. 



d. Perforating branches, one or two opposite each space, pierce perforating, 

 the internal intercostal and large pectoral muscles, and are dis- 

 tributed on the surface of the thorax with the anterior cutaneous 



nerves : the lower branches supply the mamma in the female. 



e. The musculo-phrenic branch courses outwards beneath the muscuio- 

 cartilages of the seventh and eighth ribs, and enters the diaphragm : P 1 ""' 1110 - 

 it supplies anterior branches to the lower intercostal spaces. Its 

 termination has been seen in the dissection of the abdomen. 



Two veins accompany the artery ; these join into one trunk, Veins, 

 which opens into the innominate vein. 



SECTION IT. 



THE CAVITY OF THE THORAX. 



The cavity of the thorax is the space included by the spinal Definition, 

 column, the sternum, and ribs, and by certain muscles in the 

 intervals of the bony framework. In it the organs of respiration, Contents of 

 and the heart with its great vessels are lodged ; and through it the cavit y- 

 gullet, and some vessels and nerves are transmitted. 



Dissection. The soft parts should now be cleared away from Dissection 

 between the ribs and cartilages on both sides, and the parietal Jj^? 

 layer of the pleura will then be seen adherent to their inner 

 surfaces. Care should be taken, however, below the ninth rib not 

 to remove portions of the diaphragm, as it lies here close to the 

 chest wall, and the cavity of the thorax is there very narrow. The Remove 

 second, third, fourth, fifth and sixth ribs on the left side should "j^ 011 left 

 then be divided as far back as possible, and taken away without 

 opening the pleura beneath. 



A longitudinal incision will then be made down the whole 

 length of the exposed pleura, about midway between the vertebrae Open 

 and the sternum, and small cross cuts will be made above and p 



