DIFFERENTIAL DIAGNOSIS 299 



tized condition so characteristic of this disease. It will be 

 easily understood how the diflferent shades of color from dark 

 red to grayish or yellowish red are produced if we bear in 

 mind that the veins in different parts of the lung tissue are 

 plugged at different times and that, the affected regions are 

 in different stages of the disease. 



One of the most conspicuous features in a microscopic 

 section from a lung affected with acute contagious pleuro- 

 pneumonia is the presence of intensely stained foci and lines. 

 These lines, to which Prof. Welch seems to have been the first 

 to call attention are visible to the naked eye and when viewed 

 with a hand lens suggest by their peculiar curves the contour 

 lines on a map. They are situated at the margin of and 

 within the inflamed connective tissue which surrounds the 

 large vessels and separates the lobules from one another. 

 A closer examination of these lines indicates that they coincide 

 with the boundaries of the lobules and of the individual lymph 

 spaces of the interlobular tissue. Under a high power of the 

 microscope they are resolved into dense masses of leucocytes 

 in various stages of degeneration. These dense bands are 

 presumably attracted to the connective tissue boundary of the 

 lobules and to the walls of lymph spaces within the connective 

 tissue by the unknown cause of the inflammation, presumably 

 the cause of the disease itself. The space between the lines 

 is filled with fibrin, in which ver^^ few leucocytes are found. 



§224. Differential diagnosis. It is extremely difficult 

 to form an exact diagnosis of the disease during its develop- 

 ment, because the symptoms which are present are few in 

 number and by no means characteristic. The slight fever 

 and cough are the only symptoms of diagnostic importance in 

 the prodromal stage. In the second or acute stage a positive 

 diagnosis intt-a vitam can be made only when cases of pleuro- 

 pneumonia have previously occurred or when several occur 

 simultaneously. As a -rule, a correct diagnosis can only be 

 made by a post mortem examination. The following diseases 

 are to be differentiated from it : — 



I. Non-infectious iiijiammation of the lungs. This may be 

 distinguished clinically, as a rule, by its more acute and more 



