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 

 enveloped 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 cell- 

 ular 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, fio. 119. 



which are distributed to Arrangement of capillariea round the air-ceUs ot 



their respective lungs, in 



company with the ramifying 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 s^onpathetic. 



