l62 



FIFTEENTH LECTURE. 



as in the bronchial system. Lymphatic lacuni also occur in 

 the pulmonary vesicles, and their efferent vessels subsequently 

 invest the blood-vessels (Wywodzoff). 



We have, finally, to mention the epithelial lining of the 

 alveoli. This has occasioned much discussion. In the mam- 

 malial and human embryo there is a continuous covering of 

 flat, protoplasmatic, nucleated cells. A change occurs after 

 birth, however, with the commencement of aerial respiration. 



FlG. 149. — The epithelium from the basis portion of an infundibulum. situated just 

 beneath the pleura of the developed cat ; treated with nitrate of silver. 



Only a small contingent of our cells now retain their old 

 characteristics (Fig. 149). The epithelial element, over the 

 incurvations of the pulmonary vessels, and over all the other 

 prominences, has become a much more considerable proto- 

 plasmless and non-nucleated scale. 



