LUNGS. 



339 



lobular cellular tissue. The parenchyma is divided into many 

 polyhedral lobules of various sizes, united by connective tissue ; 

 these are again made up of smaller ones, which are composed of 

 a small bronchial tube and its terminal air-cells, in the walls of 

 which lie the capillary vessels uniting the pulmonary arteries with 

 the pulmonary veins. The air-cells are vesicular cavities, arranged 

 as it were in bunches at the end of the tube ; they consist of a 

 thin membrane of connective and elastic tissue, with a layer of 

 pavemental epithelium. Each lobule is provided with its own 

 bronchial tube and air-cells, functional and nutritive vessels, 

 lymphatics, nerves, and interstitial cellular tissue, and is enve- 

 loped by the interlobular tissue. The interlobular cellular tissue 

 is very fine, and continuous with the subserous layer. The 

 lobular division is constant in. all mammiferous animals; in the 

 horse it is less distinct than 

 in ruminants, where there 

 is a much larger amount of 

 interlobular cellular tissue. 

 The pneumonic functional 

 vessels are the pulmonary 

 artery and veins. The artery 

 conveys venous blood from 

 the heart to the lungs. At 

 the lung-root it divides into 

 right and left branches, 

 which are distributed to 

 their respective lungs, in 

 company with the ramify- 

 ing bronchial tubes, terminating in a capillary network around 

 the terminal air-cells, whence arise the radicles of the pulmonary 

 veins, which return the purified blood from the lungs to the 

 heart. 



The nutrient vessels are the bronchial arteries and veins. 

 These arteries proceed from the posterior aorta; their branches 

 are smaller than the pulmonary, and also accompany the air- 

 tubes. The pulmonary lymphatics are numerous, and divided 

 into superficial and deep. The former are situated in the 

 subserous layer, forming a network, the latter in the interlobular 

 tissue. Both communicate at the lung-roots, and pass through 

 the bronchial glands. The nerves are from the pulmonary 

 plexus, formed by the vagus and sympathetic. 



Arrangement of capillaries round the air-ceUs of 

 the lung. 



