124 ANATOMY OF UPPER EXTREMITY, ETC 



casionally empties into the vena azygos major, that being 

 its normal destination in some mammalia. The duct has 

 been found also on the left side of the aorta. 



The intercostal spaces are filled by two muscular layers 

 called external and internal intercostal muscles. 



The EXTERNAL INTERCOSTAL MUSCLES are eleven in mini 

 ber on each side ; they arise from the outer lip of the lowe 

 border of the rib, and are inserted into the correspondin 

 part of the rib next below ; the fibres run forward and 

 downward, and extend from the tubercle of each rib nearly 

 to its cartilage. 



The INTERNAL INTERCOSTAL MUSCLES, also eleven in 

 number on each side, arise from the inner lip of the lower 

 border of the rib, and are inserted into the upper border 

 of the rib next below ; they extend from the sternum to 

 the angles of the ribs, and their fibres are directed back- 

 ward and downward. The internal muscular layer is 

 covered by the pleura, and the intercostal vessels and 

 nerves ramify between the two muscles. 



The INFRA-COSTALES MUSCLES are bundles of muscular 

 fibres on the inner surface of the ribs, having the same 

 direction as the internal intercostals : they stretch across 

 two or three spaces, and vary in size and number. They 

 are most constant on the lower ribs. 



LUNGS. 



Dissecting-room subjects rarely afford a good opportu- 

 nity for the examination of the lungs ; riddled with tuber- 

 cular cavities, bound down by old pleuritic adhesions, they 

 are apt to be mutilated in their removal, or rendered 

 puzzling to the student, by their unnatural condition. 



If uninjured, the lungs should be inflated for their examination. 



The LUNG is conical in shape, and presents a rounded 

 apex which extends above the first rib, and a concave base, 

 or diaphragmatic surface, the concavity being greatest in 

 the right lung, owing to the position of the liver; the sharp 

 border of this surface penetrating the space between the 

 diaphragm and ribs posteriorly, makes the posterior longi- 

 tudinal measurement of the lung greater than the anterior. 

 The sides of the lung, with the exception of the medias- 

 tinal, present a smooth convex surface, covered with 

 the pleura, and conforming to the shape of the thoracic 

 cavity. The mediastinal surface is concave, the position 



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