THORACIC CAVITY 41 



marked contrast with each other. The anterior is compara- 

 tively short and thin and it extends medially into the costo- 

 mediastinal sinus of the pleura, which lies posterior to the 

 sternum and the costal cartilages. It commences at the apex, 

 curves downwards, anteriorly and medially, posterior to the 

 sterno-clavicular articulation, to the lower border of the 

 manubrium sterni, and then it descends vertically to the base. 

 Immediately below the highest point of the apex it is grooved 

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

 presents a cardiac notch at the levelof 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 much longer 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 hilus. This fissure 

 begins above at the posterior border, about two and a half 

 inches below the apex, at the level of the vertebral end of 

 the third rib, which corresponds with the medial end of the 

 spine of the scapula. It is continued on the lateral surface, 

 in a somewhat spiral direction, downwards and anteriorly till it 

 cuts the inferior margin opposite the lateral part of the costal 

 cartilage. The upper lobe of the lung lies above and anterior 

 to this cleft. 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 or intermediate 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 



