338 SPLANCHNOLOGY. 



and left, the former being a little the larger ; they are separated 

 by the mediastinum, heart, pericardium, and large blood-vessels. 

 During life, they occupy the major part of the cavity, resting on 

 the inner surface of the walls, and adapting themselves to its 

 varying capacity. They are light, porous, and highly elastic, 

 possessing considerable strength. Healthy lungs float in water, 

 their buoyancy being due to the air they always contain ; when 

 air is admitted into the thorax, they collapse considerably. In 

 the adult, they are of a rosy flesh colour, marked by an irregular 

 marbling or mottling. 



The lung is attached midway by the root, which consists of 

 the bronchus, vessels, and nerves, enveloped by the pleura. Each 

 lung presents two surfaces, external and internal, a base, an apex, 

 and three borders. The external costal surface is convex, corre- 

 sponding to the thoracic wall. The internal, or mediastinal 

 surface, forms a vertical plane in contact with the mediastinum, 

 and presents an anterior division, which rests against the anterior 

 mediastinum, in front of an excavation in which the heart is 

 lodged ; above and behind this is the root of the lung. Two 

 fissures present themselves on the superior aspect, an anterior 

 and posterior. The former crosses towards the superior border, 

 and receives the posterior aorta ; the latter, lower down, shallower, 

 and more distinct in the left lung, is destined for the passage of 

 the oesophagus. On the inner aspect of the right lung there is a 

 small lobule, wanting -on the left lung, formed by a deep fissure, 

 through which the posterior vena cava passes ; it is lined by a 

 fold of pleui-a. The concave base is sloped downwards and 

 forwards, fitting the convex surface of the diaphragm, where the 

 fissure above noted originates. The summit or apex lies behind 

 the first rib, and presents a partially detached appendage, the 

 anterior lobe. The superior border is round, thick, and convex, 

 and is lodged in the channel between the ribs aud bodies of 

 the vertebrge. The inferior border is thinner and shorter, deeply 

 cleft near the heart, and more so on the left than on the right 

 side. The posterior border is elliptical, surrounding the base, 

 which it separates from the costal and mediastinal surfaces. 



Structurally, the lungs consist of an external serous coat, a 

 subserous layer, and the lung tissue proper, or parenchyma. The 

 serous coat is the pleura pulmonalis. The subserous layer is com- 

 posed of cellular tissue, intermixed with yellow elastic fibres ; it 

 clothes the entire lung surface, and is continuous with the inter- 



