150 THE RESPIRATORY SYSTEM. 



tional divisions of the lungs, where the admission of oxygen 

 into and the expulsion of carbon dioxide from the blood take 

 place. They are lined with a single layer of large polygonal 

 squamous epithelium-cells, at the junction of which appear 

 occasional small intervals or stomata opening into lymph- 

 passages. Among the squamous cells lining the vesicles occur 

 here and there small spheroidal cells ; in the foetal lung, before 

 it is expanded with air, the cells lining the air-vesicles are all 

 spheroidal. 



Underneath the epithelial lining is a thin network of elastic 

 tissue (the interalveolar connective tissue), which forms the 

 framework and support for the vesicles. The walls of the 

 alveoli also contain a close meshw r ork of blood-capillaries 

 derived from the pulmonary artery, in close relation with the 

 squamous lining of the vesicles, thus affording favorable con- 

 ditions for the exchange of gases between the venous blood 

 and the air in the alveoli. Adjacent air- vesicles are separated 

 from each other only by thin septa, each of which contains 

 but a single set of capillaries, the loops of which lie in relation 

 with the epithelial lining on both sides. 



The pleura is a serous sac, the parietal portion of which 

 lines the thoracic wall, and whose visceral portion forms a 

 covering for the lung. It facilitates the movements of the 

 lungs over the opposing surfaces. In structure it is a serous 

 membrane, consisting of a fibrous stratum covered with poly- 

 gonal endothelium-cells on the surface. Beneath it is the sub- 

 serous or subpleural areolar tissue, which in the visceral 

 portion of the sac is continuous with the superficial connec- 

 tive tissue of the lung. 



