THORAX AND ABDOMEN OF THE HORSE 21 



thickness of its wall and by its position cranial and somewhat dorsal to 

 the veins. (3) Bronchus, a large air-tube produced by the division of 

 the trachea. In position it is the most dorsal of the larger constituents 

 of the root. (4) Bronchial artery, of small size, running along the 

 dorsal surface of the bronchus. (5) Nerves in the form of the 

 pulmonary plexus, produced by the intercommunication of branches 

 from the vagus and sympathetic. (6) Lymphatic vessels and lymph 

 glands. 



Each root has certain structures closely related to it. The vena 

 azygos curves over the root of the right lung ; and the aorta is similarly 

 disposed on the left side. The oesophagus crosses the dorsal aspect of 

 the left root. Both roots are crossed by a vagus nerve ; and a phrenic 

 nerve, though not in actual contact with each root, is ventral thereto. 



If the bronchus be followed from the hilus into the interior it will 

 be found to pass backwards parallel to the dorsal border of the lung, its 

 size being gradually reduced by the production of branches. A branch 

 of notable size, and larger in the right lung than in the left, arises as 

 the bronchus is entering the lung and passes to the equivalent of the 

 apical lobe. Another large branch leaves the ventral border of the 

 bronchus and ramifies in that part of the lung that is the homologue 

 of the cardiac lobe. The right bronchus furnishes a special branch to 

 the intermediate lobe. All bronchial branches divide repeatedly until 

 small tubes, the lobular bronchioles, enter the lobules of the lung and 

 are there connected with the ultimate microscopic air-vesicles of the 

 respiratory tract. In the lung of the adult horse the lobules are not 

 very distinct because the amount of connective tissue between them is 

 scanty. They may, however, be recognised, both on the surface of the 

 lung and in sections, as polygonal areas varying considerably in 

 size. 



Naked-eye examination suffices to show that the bronchus and its 

 branches are provided with a skeleton in the form of curved plates of 

 cartilage, and that they are lined by a longitudinally folded mucous 

 membrane. As the branches become smaller their walls are reduced in 

 thickness, and it may be determined that the cartilaginous skeleton has 

 disappeared by the time the diameter of the tubes has been reduced to 

 about 1 mm. Finer details of structure can only be determined by the 

 aid of the microscope. 



Branches of the pulmonary artery follow the bronchial tubes, sub- 

 divide with them, and finally end in the lobules where they form rich 

 capillary plexuses in the walls of the air-vesicles. Branches of the very 

 much smaller bronchial artery also accompany the bronchial tubes to 

 the lobules, but do not extend as far as the air-vesicles. The blood they 



