35 



studded with an enormous number of yellowish white granules which 

 are about the size of a pin head. This lesion, which was very com- 

 mon in the San Francisco cases, is very readily recognized. 



The other lesion is a marking of the organ with grayish white 

 spots; "they are typically of the size of a pin's point, and give the 

 surface of the organ a stippled appearance as if dusted over with 

 gray pepper" (3, p. 331). This appearance, which is less frequently 

 encountered than is the preceding one, is more difficult to recognize; 

 indeed the most careful scrutiny is necessary to avoid overlooking it. 



Rats that have been fed with certain biological preparations used 

 to destroy rodents (Danysz's virus and similar preparations) often 

 present lesions in the liver resembling those due to plague infection. 

 The granules are, however, larger and more distinct. In these cases 

 the spleen is enlarged' and generally granular, but rarely dark and 

 friable as in plague infection. 



THE SPLEEN. 



The size of the spleen of healthy rats of the same weight varies so 

 greatly that often one can not be sure as to what constitutes an en- 

 largement of this organ. 



In plague rats this organ is markedly enlarged, firm, friable, 

 rather dark in color, and occasionally presents small granules under 

 the capsule. As Skschivan (1) pointed out, these granules are not 

 encountered as often as are granules in the liver. At times the organ 

 presents a very distinctly mottled appearance. This latter appear- 

 ance is much more frequently seen in artificially inoculated rats than 

 in those found infected in nature. We have seen the organ dis- 

 tinctly slate-colored on several occasions. 



PLEURAL EFFUSION. 



The last sign of rat plague is one of great importance when associ- 

 ated with other suspicious lesions. The effusion is bilateral, and is 

 serous in character, usually clear, though it is occasionally blood- 

 stained. Pleural effusion is rarely found in rats other than those 

 that are plague infected. The following table shows in percentage 

 the frequency of the various macroscopical lesions of acute natural 

 rat plague, as observed in Bombay and in San Francisco: 



