DISSECTION OF THE BACK AND THORAX. 105 



The Mediastinum. This, as has already been said, is a septum formed 

 towards the mesial plane of the chest by the approximation of two 

 layers of pleura, one from each sac. At some points the right and left 

 layers are in close contact, as, for example, in front of the heart in a 

 lean subject ; but at other points the layers are pushed apart by organs 

 included between them. The largest of these organs is the heart, 

 opposite which the right and left layers of the mediastinum are distant 

 four or five inches from one another. In the foetus of the horse, and 

 throughout adult life in some animals, the mediastinum is a complete 

 imperforate septum, there being no communication between the right 

 and left pleural sacs; but in the adult horse the mediastinum immedi- 

 ately behind the heart is cribriform or lace-like, and through the aper- 

 tures which exist here a pleural effusion formed on one side passes 

 readily through to the other. 



The heart, contained within its pericardial sac, is, as has already been 

 stated, the largest organ in the mediastinum, and it is situated about 

 the centre of that septum. For convenience of description, this division 

 of the mediastinum and the part vertically over it may be termed the 

 middle mediastinum; and the portions before and behind this the 

 anterior mediastinum and the posterior mediastinum respectively. Adopt- 

 ing this arbitrary division of the mediastinum, the organs included 

 in it may be tabulated thus : — 



In the Anterior Mediastinum. — The trachea; the oesophagus; the 

 axillary and innominate arteries and their collateral branches; the 

 anterior vena cava and its tributaries ; the thoracic duct ; the pneumo- 

 gastric, recurrent, phrenic, and cardiac nerves ; the tracheal lymphatic 

 glands ; and, in the foetus and young animal, the thymus gland. 



In the Middle Mediastinum. — The pericardium and the heart; the 

 common aorta and its bifurcation into anterior and posterior aorta ; the 

 terminations of the anterior vena cava and vena azygos ; the pulmonary 

 vessels; the thoracic duct; the trachea and its bifurcation into the 

 bronchi ; the oesophagus ; the pneumogastric, phrenic, and left recurrent 

 nerves; and the bronchial lymphatic glands. 



In the Posterior Mediastinum. — The posterior aorta, the vena azygos r 

 the thoracic duct, the oesophagus, the oesophageal continuations of the 

 pneumogastric nerves, the left phrenic nerve, and the oesophageal lym- 

 phatic glands. 



The posterior vena cava and the right phrenic nerve in the latter 

 part of its course are not in the mediastinum, being included in a 

 special doubling belonging to the right pleural membrane. 



the lungs (plates 22 and 25). 

 The lungs are two in number, and they occupy the greater part of 

 the cavity of the thorax. As now seen, however, they are collapsed, 



