534 RESPIRATORY SYSTEM OF THE HORSE 



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

 the anterior part of the pericardium. 



The lungs of the horse are not divided into lobes by deep fissures, as is the case 

 in most mammals. The left lung may be regarded as consisting of a chief part, 

 the body of the lung (Corpus pulmonis), and the apex (Apex pulmonis). But 

 the right lung has in addition an intermediate lobe (Lobus intermedius),^ which is 

 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, enclosed 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 puhnonary nerves; (6) the lymph vessels, which go to 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 below and behind 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 bronchus at its entrance into the lung gives off a 

 branch to the apex of the lung (Bronchus apicalis) . It is then continued backward 

 as the main stem bronchus, parallel with the dorsal border of the lung, giving off 

 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 

 intermediate lobe. The apical bronchus of the right lung is larger than that of 

 the left lung. A large ventral bronchial branch is given off from each stem 

 bronchus to the part of the lung which is homologous with the cardiac lobe of other 

 animals. 



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 a continuous layer of unstriped muscle, composed of circularly arranged bundles. 

 The mucous membrane presents numerous longitudinal folds; it contains many 

 elastic fibers, mucous glands, and lymph nodules, and is lined by ciliated columnar 

 epithelium. As the tubes diminish in size the coats become thinner and the car- 

 tilages smaller; in tubes about 1 mm. in diameter the cartilages and mucous glands 

 are absent. 



By repeated branching the interlobular bronchi are formed, and from these 

 arise the lobular bronchioles. The latter enter a lobule and branch within it, 

 forming the respiratory bronchioles (Bronchioli respiratorii) ; these give off the 

 alveolar ducts (Ductuli alveolares), the walls of which are pouched out to form 

 hemispherical diverticula, the alveoli or air-cells. 



A pulmonary lobule (Lobulus pulmonis) , the unit of lung structure, is made up 

 of a lobular bronchiole with its branches, and their air-cells, blood- and lymph- 

 vessels, and nerves. Between the lobules is the interlobular tissue, which forms 

 the supporting framework of the lung. 



Vessels and Nerves. — The branches of the pulmonary artery carry venous 

 blood to the lungs. They accompany the bronchi, and form rich capillary plexuses 



1 This is also commonly called the mediastinal lobe. 



^ Some authors consider each Imig 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. Not 

 rarely a fissure or notch in the dorsal border partially marks off the apex from the body of the lung, 

 and in some cases there is a corresponding fissure in the ventral border. 



' In foetal 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. 



