24 LANDMARKS MEDICAL AND SURGICAL. 



From the sternal ends of the clavicles the lungs converge, so 

 that their thin edges almost meet in the mesial line on a level 

 with the second costal cartilage. Thus there is little or 

 no lung behind the first bone of the sternum. From the 

 level of the second costal cartilage to the level of the fourth, 

 the margins of the lungs run parallel, or nearly so, close be- 

 hind the middle of the sternum : consequently their thin edges 

 overlap the great vessels and valves at the base of the heart. 



Below the level of the fourth costal cartilage the margins 

 of the lungs diverge, but not in an equal degree. The margin of 

 the right corresponds with the direction of the cartilage of the 

 sixth rib : the margin of the left, being notched for the heart, 

 runs behind the cartilage of the fourth. A line drawn per- 

 pendicularly from the nipple would find the lung margin 

 about the lowest part of the sixth rib. Laterally, i.e. in the 

 axillary line, the lung margin comes down as low as the 

 eighth rib : posteriorly, i.e. in the dorsal or scapular line, it 

 descends as low as the tenth. 



It should be remembered that, in a deep inspiration, the 

 lung margins descend about one inch and a half. 



In children the lungs are separated in front by the thymus 

 gland. Allowance should be made for this. About the 

 approach of puberty the thymus disappears. 



46. Anterior mediastinum. The direction of the anterior 

 mediastinum is not straight down the middle of the sternum, 

 but slants a little to the left, owing to the position of the heart. 

 The right pleural sac generally encroaches a little upon the 

 left, behind the middle of the sternum. A needle introduced 

 through the middle of the sternum opposite the third or the 

 fourth rib would go through the right pleura. 



47. Reflection of pleura. The reflection of the pleura 

 from the wall of the chest on to the diaphragm corresponds 

 with a sloping line drawn from the bottom of the sternum 

 over the cartilages of the ribs down to the lower border of the 

 last rib. 



Since the pleura lines the inside of the last rib, a musket 

 ball or other foreign body, loose in the pleural sac, and rolling 

 on the diaphragm, might fall to the lowest part of the sac, 



