The epithelium lining the alveolar walls is a continuous epithelium.

In both inflammatory and mechanical congestion of the pulmonary blood vessels, it is the accumulation of serum behind the epithelial cells which causes them to be shed.

The epithelium must be shed from diametrically opposite sides of an alveolar wall before a "pore" can be formed.

In both lobar pneumonia and mechanical edema pores are present, but they are not normal structures.

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