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 pulmonary 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 
This is also commonly called the mediastinal lobe. 
2Some 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. 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. 
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