146 Anatomy Applied to Medicine and Suigety. 



the lower part of the body of the fourth dorsal vertebra, 

 posteriorly. This imaginary plane is not exactly horizon- 

 tal, since, a horizontal plane from the junction of the 

 manubrium and gladiolus in front, would touch the 

 middle of the fifth dorsal vertebra behind. That portion 

 of the mediastinal column which lies above this imagin- 

 ary plane, is termed the superior mediastinum, while that 

 below it is the inferior mediastinum. The principal part 

 of the latter is that which contains the heart and the peri- 

 cardium, and is styled the middle mediastinum ; the small 

 space in front of the pericardium, and between it and the 

 sternum and left cartilages, is the anterior mediastinum; 

 while the portion of the column behind the pericardium, 

 and between it and the vertebrae, is the posterior media- 

 stinum. All the structures, that enter or leave the thorax 

 through the inlet, pass through the superior mediastinum,, 

 but, below this, they may pass through either the anterior, 

 the middle, or the inferior mediastinum. In the anterior 

 mediastinum are (1) the remains of the thymus gland, 

 (2) lymphatics ascending from the liver with two or 

 three lymphatic glands, and (3) the left internal mam- 

 mary artery. In the middle mediastinum are (1) the 

 heart enclosed in the pericardium with (2) the phrenic 

 nerve on either side of it, (3) the pulmonary arteries and 

 veins, (4) lower half of the superior vena cava, (5) the 

 termination of the vena azygos major, (6) the ascending 

 aorta, and (7) the roots of the lungs. In the posterior 

 are (1) the oesophagus, (2) the pneumogastric, and (3) 

 the sympathetic nerves, (4) the thoracic aorta, (5) the 

 azygos veins, (6) the thoracic duct, and (7) lymphatic 

 glands. Of the above structures we will first consider (1) 

 the arteries. The ascending aorta is two inches in 

 length, and passes from the left ventricle upwards, for- 

 wards and to the right. It ascends as high as the upper 



