The =lungs= (Pulmones) occupy much the greater part of the thoracic
cavity. They are accurately adapted to the walls of the cavity and the
other organs contained therein. The two lungs are not alike in form or
size, the right one being considerably larger than the left; the
difference is chiefly in width, in conformity with the projection of the
heart to the left.
The lung is soft, spongy, and highly elastic. It crepitates when pressed
between the finger and thumb, and floats in water. When the thoracic
cavity of the unpreserved subject is opened, the lung collapses
immediately to about one-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.[128]
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 =fœtal 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 pleural cavities in which they
are situated. When well hardened _in situ_, their surfaces present
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.
[Illustration:
FIG. 361.—LEFT LUNG OF HORSE, COSTAL SURFACE. Specimen hardened _in
situ_.
]
The =costal= (or external) =surface= (Facies costalis) is convex, and
lies against the lateral thoracic wall, to which it is accurately
adapted.
The =mediastinal= (or internal) =surface= (Facies mediastinalis) is 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=, at which the bronchus, vessels, and nerves
enter the lungs. The bronchial lymph glands are also found here. Behind
this the two lungs are adherent to each other over a triangular area.
Above this is a groove for the œsophagus (Sulcus œsophageus), which is
deepest on the left lung. A groove for the aorta (Sulcus aortæ) curves
upward and backward over the hilus, and passes backward near the dorsal
border of the lung; the curved part of the groove for the aortic arch is
absent on the right lung, on which there is a groove for the vena
azygos. Anteriorly there are grooves for the trachea, the anterior
aorta, the anterior vena cava, and other vessels.
[Illustration: