MEDIASTINUM. 499 



include a space which is nearly circular, the vacuity between 

 these laminae is necessarily of that form, at its commencement 

 above. 



Here, therefore, is a space between them above, (Superior 

 Mediastinum) which is occupied by the transverse vein that 

 carries the blood of the left subclavian and the left internal 

 jugular to the superior cava ; by the trachea ; by the oesopha- 

 gus ; and by the subclavian and carotid arteries, as they rise 

 from the curve of the aorta. This space is bounded below by 

 the above mentioned curve of the aorta. 



The heart and pericardium are so placed that there is a 

 small distance between them and the sternum : in this space 

 the two laminae of the mediastinum are very near to each 

 other ; and cellular substance intervenes between them. This 

 portion of the mediastinum is called the Anterior Medias- 

 tinum.* 



Posteriorly, the heart and pericardium are also at a small 

 distance from the spine ; and here the lamina of the mediasti- 

 num are at a greater distance from each other, and form a 

 long narrow cavity which extends down the thorax in front of 

 the vertebrae : this is called the Posterior Mediastinum. It 

 contains a considerable portion of the aorta as it descends from 

 its curve, the oesophagus, the thoracic duct, and the vena azygos. 

 The aorta is in contact with the left lamen, and can often be 

 seen, through it when the left lung is lifted up. 

 The posterior and anterior mediastina are separated from 

 each other by the pericardium which encloses the heart. But as 

 the serous layers of the anterior, are reflected one on each side 

 of the pericardium, to meet the posterior mediastinum, it 

 appears to -me to render the study of this part more easy, to 



* This mediastinum, being placed in front of the longitudinal diameter of 

 the pericardium is found at its lower part inclined to the left of the middle line. 

 The cellular tissue between its layers, communicates indirectly with the cellular 

 tissue on the outer side of the peritoneum, in the notch formed by the origin of 

 the greater muscle of the diaphragm, under the xiphoid appendix of the 

 sternum. By this channel, abscesses of the anterior mediastinum, may make 

 their way externally upon the abdomen. p. 



