i THE LUNGS. 933 
The anterior mediastinum is that part of the interpleural space which lies 
between the pericardium behind and the sternum in front. In its upper part this 
space can hardly be said to exist, seeing that here the two pleural sacs come into 
contact with each other on the anterior aspect of the pericardium; but below the 
level of the sternal ends of the fourth costal cartilages the left pleura falls short 
of the right pleura, and an interval is apparent. The only contents to be 
noticed in the anterior mediastinum are a few lymphatie glands and some areolar 
tissue, in which ramify some lymphatic vessels, and some “Tninute twigs from the 
internal mammary artery. 
The posterior mediastinum is that part of the interpleural space which is 
situated behind the pericardium. It may be regarded as the continuation down- 
wards of the posterior part of the superior mediastinum, and many of the structures 
in the one are prolonged downwards into the other. The arbitrary upper limit of 
the posterior mediastinum is the lower border of the fourth dorsal vertebra. Zn 
front it is bounded by the pericardium, except in its lowest portion, where the 
anterior wall is formed by the back of the diaphragm. Lehind it is hmited by the 
bodies of the eight lower dorsal vertebrae, and on each side by the mediastinal pleura. 
It contains the descending thoracic aorta, the azygos veins, the thoracic duct and 
the cesophagus, with the two pneumogastric nerves. 
Structure of the Pleura.—The pleura on each side is a closed sac, and, like other 
serous membranes, it is attached by its outer surface to the wall of the cavity which it 
lines and to the surface of the viscus which it covers. It is composed of a thin connec- 
tive-tissue stratum in which bundles of fibres cross each other in various directions, and 
intermixed with which there is a considerable quantity of elastic tissue. On the inner 
surface of this there is a continuous coating of thin epithelial cells placed edge to edge. 
The pleura so formed is attached to the parts it lines and invests by a small amount of 
areolar tissue which forms a subserous layer. In the case of the visceral pleura the sub- 
serous tissue is continuous with the areolar tissue in the substance of the lung, and this 
accounts for the tight manner in which it is bound down. 
The pleura is plentifully supplied with blood. This is conveyed to it by minute 
twigs from the intercostal arteries, the internal mammary artery, and the bronchial 
arteries. Lymphatic vessels also are particularly abundant in the pleura and in the 
subserous layer, and it is by these that excess of fluid is conveyed from its cavity. Many 
lymphatic vessels communicate directly with the cavity by means of excessively minute 
orifices termed stomata. Dybkowsky has shown that the lymphatics and stomata of the 
pleura are not equally distributed throughout the membrane. Over the ribs and on the 
mediastinal pleura they are absent. 
THE LUNGS. 
When healthy and sound each lung les free within the corresponding pleural 
chamber, and is only attached by its root and the ligamentum latum pulmonis. 
It is not common, however, to meet with a perfectly healthy lung. Adhesions 
between the visceral and parietal layers of pleura, due to pleurisy, are generally 
present. 
Like the cavities in which they are placed, the two lungs are not precisely 
alike. The right lung is slightly larger than the left, in the proportion of about 
11 to 10. The right lung is also shorter and wider than the left lung. This 
difference is due to the great bulk of the right lobe of the liver, which elevates the 
right cupola of the diaphragm to a higher level than the left cupola, and likewise 
to the heart and pericardium projecting more to the left than to the right, and thus 
diminishing the width of the left lung. 
The lung is light, soft, and spongy in texture; when pressed between the 
finger and thumb it crepitates, and when placed in water it floats. The elasticity 
of the pulmonary tissue is very remarkable. A striking demonstration of this is 
afforded when the thoracic cavity is opened, and the atmospheric pressure acting 
upon the interior and exterior of the lung is equalised. Immediately the organ 
collapses to about one-third of its original bulk, and it becomes impossible in such 
a specimen to study its proper form and dimensions. 
The surface of the adult lung presents a mottled appearance. The ground 
