THE THORAX 



95.^ 



fibro-elastic membrane and circularly-disposed muscular fibres, with a thin 

 mucous coat, destitute of mucous glands, and covered with simple cihated 

 columnar epitheUum, there being here and there patches of squamous, non- 

 ciliated ceUs. The walls of the vestibule, atria, and air-ceUs are very thm 

 and consist of areolar, elastic, and muscular tissues, the elastic element being 

 specially developed at the margins of the orifices of the cells. This elastic 

 tissue enables the ceUs to recoil after distension. The interior of the vestibule^ 

 atria and air-cells is Uned with a single layer of squamous. non-cUiated 

 epitheUum, which is of extreme delicacy in the air-cells. Upon the outer walls 

 of the ceUs there are dense networks of capillary- bloodvessels, which also per- 

 vade the septa between the cells, these 

 septa being formed by infoldings of the 

 contiguous cell-walls. Each septum 

 contains only one capillary layer. The 

 venous blood is thus brought into the 

 most intimate relation \vath the air, all 

 that separates the two being the very 

 thin walls of the cells and the very 

 delicate walls of the capillary blood- 

 vessels. Moreover, there being only 

 one capillary layer in each intercellular 

 septum, the blood in the septal 

 capillaries is exposed to the air on 

 each side. 



Summary of the Structure of the 

 Lung. — The lung consists of an im- 

 niense number of lobules, irregularly 

 polygonal in outline, and each of .these 

 is provided with its own bronchial 

 tube. A lobule is composed of groups 

 of infundibula or ultimate lobules, and 

 the lobular bronchial tube, on enter- 

 ing the lobule, divides into as many 

 bronchioles as there are groups of in- 

 fundibula. Each bronchiole, on approaching a cluster of infundibula, presents 

 a dilatation, called the vestibule, from which atria lead to the infundibula. 

 The walls of the infundibula are freely beset with air-cells, which are also 

 present, though more sparsely, on the walls of the atria. Upon the walls of 

 the air-cells are dense networks of capillary bloodvessels, and each inter- 

 cellulcLT septum contains a single capillciry layer. 



Bloodvessels of the Lungs. — ^Two sets of arteries are associated 



with each lung, namely, pulmonary and bronchial, the former 

 having to do with the respiratory function of the organ, and the 

 latter with the nutrition of its component tissues. The pulmonary 

 arteries are two in number, right and left. They result from 

 the bifurcation of the pulmonary trunk, and convey venous 

 blood to the lungs. Each artery ramifies freely within the lung, 

 its branches accompanying the bronchial tubes, but they never 

 anastomose with one another. Ultimately they terminate in 

 dense capillary networks which lie upon the walls of the air-cells, 

 and also in the septa between adjacent cells. 



The pulmonary veins commence as radicles in the capillary net- 

 works already referred to, and they pass to the root of each lung, 

 where they give rise to two pulmonary veins, which proceed to the 

 left auricle of the heart and convey to it arterial or oxygenated 

 blood. The pulmonary veins and their tributaries are destitute of 



Fig. 391. — Section of Lung 

 (injected). 



