ORGANS OF RESPIRATION. 381 



epithelial cells, and labouring under the impression that they 

 were really distinct structures, I have sometimes perceived 

 cells of glandular epithelium, one much larger than the rest, 

 with a transparent border ; and this I fancied was a fresh cell 

 membrane in process of development. 



The presence of epithelial scales in the air cells was first 

 observed by Mr. Addison * ; but that gentleman was unable 

 to determine whether these were ciliated or not; subse- 

 quently Mr. Rainey f advanced the statement that healthy air 

 cells contain no epithelium. In sections of recent lungs, it 

 is a very easy matter not merely to determine the existence 

 of epithelium in the air cells, but also the fact of its cylinder 

 and ciliated form and character. Circular and oval epithelial 

 cells also occur : these are most probably ciliated cells in a less 

 advanced condition of development. (See Plate XL VIII. 

 fig. 3. and Plate XLIX. fig. 2.) 



It has been stated that the air cells vary in size. This is the 

 case with even contiguous cells, as may be seen by a re- 

 ference to the figures : the air cells in the lungs of an adult 

 are also much larger than those of a child. 



The fact of the epithelium extending from the bronchial 

 tubes into the air cells, would seem in itself to imply that 

 the mucous membrane also lined them, at least in a modi- 

 fied condition, which is contrary to the opinion expressed by 

 Mr. Rainey. 



VASCULAR APPARATUS. 



The vascular apparatus of the lungs consists of arteries 

 and veins : these vessels, after numerous ramifications in the 

 interlobular spaces, unite together on the surface of the air 



* Observations on the Anatomy of the Lungs, by Thomas Addison, 

 MD., 1841. Transactions, Medico-Chirurgic Society. 



f On the Minute Structure of the Lungs, and on the Formation of 

 Pulmonary Tubercle, by George Rainey. Transactions, Medico-Chirurgic 

 Society, 1845. 



H H 2 



