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THE MEDIASTINUM. 



completely covering this latter muscle, and expanding out- 

 ward from the spine over the ribs and intercostal muscles, 

 forward to the sternum, where the sac was opened and the 

 tracing begun, thus showing one continuous whole through- 

 out. It is thus seen that each pleura forms a vertical sep- 

 tum from the sternum in front to the spine behind, and 

 that the space between these almost parallel partitions is, 

 as just stated, the mediastinum. 



The anterior mediastinum is immediately behind the ster- 

 num and in front of the pericardium. Its form is triangu- 

 lar, the base being the sternum, the sides are formed by the 

 two pleura, which, approaching each other very closely on 

 the top of the pericardium, constitute the apex. It con- 

 tains much fine cellular structure, some lymphatic glands, 

 and at the superior portion the origins of the sterno-hyoid 

 and sterno-thyroid muscles, with the remains of the thymus 

 gland. This space is exposed by passing up one or two of 

 the fingers behind the sternum from the abdomen, so as to 

 break down the cellular connections between the pleurae, 

 then sawing the sternum longitudinally along the median 

 line, and dividing the cartilages near the sternum. 



The middle mediastinum contains the pericardium and 

 heart, ascending aorta, superior vena cava, pulmonary ar- 

 teries and veins, and division of the trachea, and is, as its 

 name implies, between the anterior and the next division. 



The posterior mediastinum, directly behind the middle, 

 and in front of the spine, is exposed by dividing the right 

 pleura in a longitudinal direction behind the root of the 

 lung, and turning the latter over to the left side, when 

 this space will be seen to contain the oesophagus and eighth 

 pair of nerves, the descending aorta, vena azygos, thoracic 

 duct, splanchnic nerves, a quantity of cellular tissue, and 

 several lymphatic glands. 



A reflection of the pleura from the root of the lung to 

 the diaphragm is called ligamentum latum pulmonis. 



Structure. The pleura3 belong to the class of serous 

 membranes, and are thin and transparent, with their 

 internal surface smooth, polished, and free. Their exter- 



