456 RESPIRATORY SYSTEM OF THE HORSE 



opposite to the heart, the cardiac notch (Incisura earcUaca). On the left lung this 

 notch is opposite to the thirtl, fourth, and fifth ribs, so that a considerable area of the 

 pericardium here lies in direct contact with the chest-wall. On the right lung the 

 notch is less extensive, and is an intercostal space further forward. 



The base of the lung (Basis pulmonis) is oval in outline; its surface (Facies 

 diaphragmatica) is deeply concave in adaptation to the thoracic surface of the 

 diaphragm. Laterally and posteriorly it is limited by a thin basal border which 

 fits into the narrow recess (Sinus phrenico-costalis) between the diaphragm and 

 the chest-wall. The position of this border, of course, varies during respiration. 

 In the deepest inspiration it may reach the bottom of this recess; in ordinary 

 inspiration it lies about four or five inches (ca. 10 to 12 cm.) from the costal arch, 

 to which it is nearly parallel; in ordinary expiration it lies about twice as far from 

 the costal arch. 



The apex of the lung (Apex pulmonis) is prismatic, narrow, and flattened 

 transversely. It is partially marked off from the rest of the lung ])y the cardiac 

 notch. It curves downward, and is related internally to the anterior mediastinum 

 and behind to the anterior part of the pericardium. 



In the horse the lungs are not divided into distinct lobes by deep fissures, as 

 in most mammals. In the case of the left lung there is no lobation, but the right 

 lung presents a mediastinal lobe, separated from the body of the lung by a fissure 

 which forms in its dorsal part a canal for the posterior vena cava and the right 

 phrenic nerve, inclosed in a special fold of the right pleura.^ 



The root of the lung (Radix pulmonis) is composed of the structures which 

 enter or leave the lung at the hilus on the mediastinal surface. These are: (1) 

 The bronchus; (2) the pulmonary artery ; (3) the pulmonary veins ; (4) the bron- 

 chial artery; (5) the pulmonary nerves; (6) the lymph vessels, wdiich open into 

 the bronchial lymph glands. The bronchus is situated dorsally, with the bronchial 

 artery on its upper surface and the pulmonary artery immediately below it. The 

 pulmonary veins lie chiefly l^elow and ])ehind the artery. 



The lobulation of the lungs is not very evident on account of the small amount 

 of interlobular tissue. The lobules appear on the surface or on sections as irregular 

 polygonal areas of different sizes.- 



Bronchial Branches. — Each Ijronchus at its entrance into the lung gives off a 

 branch to the apex of the lung. It is then continued backward as the main stem 

 bronchus, parallel with the dorsal border of the lung, giving oft" branches (Rami 

 bronchiales) in monopodic fashion; these branch similarly and reach all parts of 

 the lung. The right bronchus gives off a special branch to the mediastinal lobe. 



The arrangement of the larger bronchi is not the same in the two hmgs. The right bronchus, 

 after detaching the apical branch, gives off a large ventral brancli wliich shortly bifurcates. One 

 of the divisions passes ventrally into the part of the lung behind the cardiac notch (homologue 

 of cardiac lobe), while the other passes downward and backwartl into the lower part of the base. 

 The stem bronchus then gives off the branch to the mediastinal lobe, and divides, about six inches 

 from the hilus of the lung, into two nearly equal branches; the dorsal branch is a direct con- 

 tinuation of the stem, the ventral one passes into the middle part of the base. The left bronchus, 

 after a short course, divides into two nearly equal branches. The dorsal branch may be regarded 

 as the continuation of the stem; it passes backward parallel with the dorsal border of the lung, 

 giving off a ventral branch much further back than in the right lung. The ventral division of 

 the stem bronchus divides, after a very short course, into two branches; the anterior one passes 

 ventrally into the homologue of the cardiac lobe, while the posterior one passes downward and 

 backward into the base of the lung. 



The structure of the larger bronchial tubes is, in general, similar to that of the 

 trachea. Their walls contain irregular plates of cartilage instead of rings. There is 



' Some authors consider each lung to be divided into anterior and posterior lobes by the 

 cardiac notch, so that the right lung would have three lobes and the left lung two lobes. 



- In fcetal lung the lobulation is much more distinct. Pigmentation of the lung is sometimes 

 seen in horses, and in such cases the pigment is deposited mainly in the interlobular tissue, thus 

 mapping out the lobules. 



