THORACIC CAVITY 55 



by the subclavian artery on each side, and on the left side it 

 presents a cardiac notch at the level of the fifth costal cartilage. 

 The posterior border is rounded and indistinct. It descends 

 from the apex to the base, along the line of the articulations of 

 the heads of the ribs with the bodies of the vertebrae, and 

 it is considerably longer and thicker than the anterior border. 



Lobes of the Lungs. The left lung is divided into two 

 lobes by a long, deep oblique fissure which penetrates its 

 substance to within a short distance of the hilum. The oblique 

 fissure begins above at the posterior border, about 62 mm. 

 (two and a half inches) below the apex, at the level of the 

 vertebral end of the fourth rib, which corresponds with the 

 medial end of the spine of the scapula and the spine of 

 the third thoracic vertebra. It is continued on the 

 lateral surface, in a somewhat spiral direction, downwards 

 and forwards till it cuts the inferior margin opposite the 

 lateral part of the sixth costal cartilage. The upper lobe of 

 the lung lies above and anterior to the oblique fissure. It 

 is conical in form, with an oblique base. The apex and the 

 whole of the anterior border belong to it. The lower lobe^ 

 somewhat quadrangular, is more bulky than the upper, and 

 lies below and posterior to the fissure ; it comprises the entire 

 base and the greater part of the thick posterior border. 



In the right lung there are two fissures subdividing it into 

 three lobes. The oblique fissure is very similar in its position 

 and relations to the fissure in the left lung, but it is more 

 vertical in direction. It separates the lower lobe from the 

 upper and middle lobes. The second cleft, the horizontal 

 fissure^ begins at the anterior border of the lung at the level 

 of the fourth costal cartilage and extends horizontally till it 

 joins the oblique fissure. The middle lobe, thus cut off, 

 is wedge-shaped in outline. It lies between the oblique and 

 horizontal fissures. 



Differences between the two Lungs. The dissectors should 

 particularly note the following differences between the two 

 lungs : (i) The right lung is slightly larger than the left, in 

 the proportion of n to 10. (2) The right lung is shorter 

 and wider than the left lung. This difference is due to 

 the great bulk of the right lobe of the liver, which elevates 

 the right cupola of the diaphragm to a higher level than 

 the left cupola, and also to the heart and pericardium, which 

 project more to the left than to the right, and thus diminish 

 ii 4 a 



