18 TOPOGRAPHICAL ANATOMY OF THE 



true form when exposed for examination. The unhardened organ 

 collapses immediately the cavity of the chest is opened, and assumes 

 a size not much more than one-third of that it possessed during life. 



With the exception of attachments at its root (radix pulmonis) 

 and the pulmonary ligament, each lung lies free in its own side of 

 the chest. The colour of the lungs depends largely upon whether 

 they contain much blood or not. In the healthy living animal they 

 are pale red or pink ; after death one is generally darker than the 

 other owing to hypostasis of blood. 



From a clinical point of view it is noteworthy that the thickest 

 part — that is, the greatest transverse diameter — of the lungs is 

 about the middle of the length and the middle of the external 

 dorso- ventral diameter of the chest. 



In all domestic animals, except those belonging to the Equidse, 

 each lung is divided into lobes (lobi pulmonis). These, from their 

 position, are known as apical, cardiac and diaphragmatic. The 

 right lung, in addition, possesses an intermediate lobe. In the 

 horse there are occasionally indications of a division into lobes, 

 but generally no real fissures are present. Consequently, the lungs 

 of the horse present one continuous mass, except that an inter- 

 mediate lobe is always present in association with the right organ. 



The intermediate lobe (lobus intermedius) has the form of an 

 irregular three -sided pyramid, and lies between the two lungs 

 caudal to the heart and medial to the double fold of pleura (plica 

 venae cavse) that encloses the caudal vena cava. Though the lobe 

 is structurally connected with and forms a part of the right lung, 

 it has also a connection with the left lung by connective tissue 

 and pleura. 



The lateral or costal surface (facies costalis) of each lung is smooth 

 and glistening from the presence upon it of visceral pleura. Being 

 applied closely to the wall of the chest, the surface follows the curve 

 of the ribs, and is, therefore, convex in its dorso ventral diameter. 

 When the lungs have been satisfactorily hardened with preservative 

 before the thorax is opened, the surface is often lightly marked by 

 grooves which indicate the position of the ribs. 



The medial or mediastinal surface (facies mediastinalis) is of much 

 smaller extent owing to the fact that the diaphragm encroaches more 

 upon the thoracic cavity medially than laterally. The presence of the 

 heart produces a deep depression (impressio cardiaca) on this surface. 

 Immediately dorsal to the cardiac impression is the hilus ^ imhnonis, 



^ Hilum [L.], (whence nihilum and nihil, nothing), a small part, a trifle. The 

 hilus of an organ is small relative to the total area of its exterior. 



