454 RESPIRATORY SYSTEM OF THE HORSE 



tissue caused by the air pressure in its cavities has been reheved 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 l)led for dis- 

 section it is light gray or faintly tinged with red. In luibled 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 ami has a relatively small l^lood-supply, 

 differs from that of an animal which has l)reathed 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 



JDofScil hori^er 



m 



Fig. 361. — Left Lung of Horse, Costal Surface. 

 Specimen hardened in situ. 



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 (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 l^ehind 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 oesophagus (Sulcus a^sophageus), 

 which is deepest on the left lung. A groove for the aorta (Sulcus aortie) curves 

 upward and })ackward over the hilus, and passes Imckward 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 



^ A (!orrect idea of the natural form and size of the king cannot be obtained from a specimen 

 in this state. The lungs should be hardened in situ for this purpose. 



