THORACIC CAVITY 29 



the inferior angle of the scapula when the arm is by the side. 

 The lower margin of the lung lies at the sixth rib in the para- 

 sternal line, the eighth rib in the mid-axillary line, and the 

 tenth rib in the scapular line. The lower margin of the 

 pleura is roughly about one rib lower; it lies at the seventh 

 rib in the parasternal line, the ninth rib in the mid-axillary 

 line, and the eleventh rib in the scapular line. The points 

 given are only approximate, but they are sufficient for most 

 practical purposes, and are easily remembered. It should be 

 noted that, on account of the obliquity of the ribs, a horizontal 

 line drawn round the thorax at the level of the point where 

 the parasternal line cuts the sixth rib will cut the eighth rib 

 in the mid-axillary line and the tenth rib in the scapular line ; 

 but the inferior margin of the lung is not quite horizontal; 

 on the contrary, it is slightly convex downwards ; therefore, 

 when the line indicating its position is drawn the convexity 

 must be allowed for. Further, the student must not forget 

 that the line of the lower margin of the pleura is fixed, but 

 the lower margin of the lung varies in position, being highest 

 at the end of expiration and lowest at the end of inspiration, 

 and the amount of its possible excursion varies in different 

 persons. 



Keeping the above-mentioned points in mind, the student 

 should mark out the margins of the pleural sacs on the living 

 body, using himself and his friends for the purpose, until he 

 can indicate them correctly, judging from the contour of the 

 body alone and without feeling for the skeletal points. 



After the dissector has made himself thoroughly conversant 

 with the limits of the pleural sacs, he should examine the cut 

 section of the root of the lung, and should endeavour to 

 recognise, through the mediastinal part of the parietal pleura, 

 the positions of the main constituent parts of the mediastinum. 

 As these vary on the opposite sides, each side must be con- 

 sidered separately, and each dissector must make himself well 

 acquainted with the conditions on both sides. 



On the right side, in the posterior part of the face of the section 

 of the lung root, at least two parts of the bronchial tube will 

 be seen ; an upper, which is the so-called eparterial bronchus, 

 and a lower, the main stem of the right bronchus. Anterior 

 to and between the two bronchi is the pulmonary artery, and 

 more anteriorly, and at a slightly lower level, the upper pul- 

 monary vein. The lower pulmonary vein lies in the lowest 



