60 



going into details here we may say that if rats of approximately the 

 same size are kept together in a cage there will be practically no 

 mortality from fighting. Of course, there should be no overcrowding. 

 Rats should be fed meat or cheese and plenty of green food such as 

 carrots or cabbage. In our experience in San Francisco it has been 

 found practicable to keep for a year one series of ten inoculated wild 

 rats without any loss. Judging from my experience I have no hesi- 

 tancy in saying that the natural mortality in the laboratory is higher 

 among both guinea pigs and white rats than it is among wild rats. 



It is almost certain that some of the lesions described below are 

 due to animal parasites, or to bacteria, but no such causative agent 

 has been identified. 



CIRCULATORY APPARATUS. 



We have seen no lesion of the circulatory system with the exception 

 of a few cases of pericardial effusion. The most extreme example 

 was one in which the pericardial sac was dilated to such an extent 

 that it filled almost the entire cavity of the thorax. The fluid in the 

 sac was blood stained and there were a number of recent adhesions 

 between the visceral and the parietal surfaces of the pericardium. 



PULMONARY APPARATUS. 



Pleural effusion, as is stated in another place, is an important sign of 

 plague infection. A clear, watery effusion has been found in a few 

 cases in rats that were not plague infected. 



One example has come under observation of a large Mus nor- 

 vegicus that had both pleural cavities almost entirely filled with a 

 milky fluid. The lungs were compressed and congested. Micro- 

 scopical examination for animal parasites and for bacteria was 

 negative. 



A condition of consolidation of the lungs which closely resembles 

 the stage of gray hepatization in lobar pneumonia in man is seen 

 occasionally. The area may involve half of a lung. Upon micro- 

 scopical examination one finds the air spaces and the small bronchi 

 filled with leucocytes. There was no cavity formation in any of the 

 cases that have come under observation. 



Two relatively common purulent conditions of the lungs are 

 encountered. In the first of these, large and more or less distinctly 

 loculated sacs are found, which are filled with yellow semifluid caseous 

 matter; in the second, the lesion is of much the same nature, but the 

 material in the sac has the consistency of tough, ropy mucus. Aside 

 from the main focus of this sort, numerous smaller areas of the same 

 nature are seen scattered through the otherwise normal parts of the 

 lungs. The extent of some of these purulent processes is remarkable. 

 We have seen cases in which the chest cavity was almost filled by 

 the lesions described. 



