THE RESPIRATORY SYSTEM. 



129 



OUTLINE OF THE LUNGS. 



r Bronchial tubes. 



Ducts { Bronchioles. 



Alveolar ducts. 



Pleura 



L Blood vessels, lymphatics, and nerves. 



(" Endothelium. 



<( Connective tissue. 



[ Subpleural tissue. 



The hings are invested with a connective tissue sheath, the pleura, 

 which is made up of endothelium, a connective tissue matrix, con- 

 sisting of fibrous tissue bundles and elastic fibres, and the sub- 

 pleural tissue, composed of areolar tissue and elastic fibres. Each 

 lung is comprised of lobes ; each lobe, of lobules ; and each lobule con- 

 sists of ducts, air spaces, and pulmonary parenchyma. 



The smaller bronchial tubes, or bronchioles, become the terminal 

 bronchioles when their diameter does not exceed 1 mm. They end 

 in the alveolar ducts, which are lined with alveoli, or air sacs; and 

 extending from these ducts are irregular cavities, the infundibula, 

 which are also studded with alveoli. 



The pulmonary parenchyma comprises the walls of blood-vessels, 

 lymphatics, bronchioles, and alveolar ducts, the alveolar walls, and 

 the infundibular septa. A bronchiole may be distinguished from 

 a blood-vessel under the microscope by the crenated appearance of 

 its inner surface. The terminal bronchioles are lined with a single 

 layer of ciliated epithelial cells, and their walls consist of elastic 

 fibres and smooth muscle. Each alveolar duct is lined with cu- 

 boidal cells, and its wall otherwise resembles that of a bronchiole, 

 but is much thinner. The wall of an alveolus is lined with simple 

 squamons epithelium and comprises also a connective tissue frame- 

 work and a dense capillary network. The connective tissue frame- 



