THE LUNGS 531 
third of its original size, and loses its proper form; this is due to its highly elastic 
character and the fact that the tension of the lung tissue caused by the air pressure 
in its cavities has been relieved by the external air pressure.! The color varies 
according to the amount of blood contained in the lung. During life the lung has a 
pink color, but in subjects which have been bled for dissection it is light gray or 
faintly tinged with red. In unbled subjects it is dark red; the depth of color varies, 
and is often locally accentuated by gravitation of blood to the most dependent parts 
(hypostasis). The foetal lung, since it contains no air and has a relatively small 
blood-supply, differs from that of an animal which has breathed in the following 
respects: (1) It is much smaller; (2) it is firmer and does not crepitate; (3) it sinks 
in water; (4) it is pale gray in color. In form the lungs are like casts of the cavities 
in which they are situated. When well hardened in situ, their surfaces present 
porsal border 
Cardiac 
notch 
Fic. 472.—Rieut Lune or Horse; Costat SURFACE. 
Specimen hardened in situ. 
impressions and elevations corresponding exactly to the structures with which they 
are in contact. Each lung presents two surfaces, two borders, a base, and an apex. 
The costal surface (Facies costalis) is convex, and lies against the lateral 
thoracic wall, to which it is accurately adapted. 
The mediastinal surface (Facies mediastinalis) is much less extensive than the 
costal surface. It is molded on the mediastinum and its contents. It presents a 
large cavity adapted to the pericardium and heart; this is termed the cardiac 
impression (Impressio cardiaca), and is larger and deeper on the left lung than on 
the right. Above and behind this is the hilus of the lung (Hilus pulmonis), at which 
the bronchus, vessels, and nerves enter the lung; the bronchial lymph glands are 
also found here. Behind this the two lungs are adherent to each other over a 
triangular area. Above this there is a groove for the cesophagus, which is deepest 
on the left lung. A groove for the aorta (Sulcus aortze) curves upward and back- 
ward over the hilus, and passes backward near the dorsal border of the lung; the 
1No idea of the natural form and size of the lung can be obtained from a specimen in this 
state, nor can an accurate conception be gained by examination of the soft inflated organ. The 
lungs should be hardened in situ for this purpose, and if the process has been successful they con- 
iorm in every detail to the structures wi ich they were in contact. 
