DISSECTION OP THE BACK AND THOEAX. 103 



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 posterioi- mediastinum respectively. Adopt- 

 ing this arbitrary division of the mediastinum, the organs included in 

 it may be tabulated thus : — 



In the Anterior Jlediastinum. — 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 Jliddle Mediastinum. — The pericardium and the heart ; the 

 common aorta and its bifurcation into anterior and posterior aortse ; 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 pneumogastrio, phrenic, and left recurrent 

 nerves ; and the bronchial lymphatic glands. 



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

 the thoracic duct, the oesophagus, the (Esophageal continuations • of the 

 pneumogastrio nerves, the left phrenic nei-ve, and the oesophageal 

 lymphatic 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, 

 and occupy but a small moiety of the cavity, a condition which makes 

 their examination more easy. Each lung appears to lie somewhat loosely 

 in the chest ; but if it be grasped, and an attempt be made to remove 

 it bodily, it will be found to be attached at a point on its inner surface. 

 This, which is termed the root of the lung, is the point where the bronchi 

 and vessels enter it. Each lung presents for examination two surfaces, 

 three borders, a base, and an apex. 



The External (or costal) surface is much the larger of the two. It is 



