PATHOLOGY. 213 



the character of the exudate also renders assistance in arriving 

 at a diagnosis. Also the absence or scarcity of fibrin in the 

 alveolar exudate in pneumonic plague is in striking contrast to 

 the condition observed in croupous pneumonia. The alveoli are 

 frequently filled v^'ith plague bacilli. The gross lesions of the 

 human lung are illustrated in Plates VIII, IX, and X. 



Histological examination. — No cases in the series were found 

 in which the lungs exhibited no alteration and no part of any 

 lung examined was free from at least some pathological changes. 

 In the earlier cases (1) the presence of bacteria, (2) the changes 

 in the blood content of the vessels, and (3) the changes in the 

 bronchi and bronchioles constitute the prominent features. 



(1) The bacteria occur in enormous numbers and frequently 

 appear as dense blue clouds in thin sections stained by hjemo- 

 toxylin and eosin and examined even with a low magnification. 

 In general, this method of examination gives the most satisfac- 

 tory evidence of the distribution of the bacteria, and the higher 

 magnification with the oil-immersion objective is only necessary 

 when the bacteria must be searched for. 



In the earliest cases the bacteria are especially numerous 

 about the bronchioles, in the peribronchial lymph spaces, and 

 adjoining alveoli. Here they frequently form masses com- 

 pletely encircling the bronchioles, and are also present in large 

 numbers in the interlobular septa and beneath the pleura. In 

 the lungs which are the seat of anthracotic deposits, wherever 

 anthracotic pigmentation is found, there are enormous masses 

 of the bacteria, and the distribution of the bacilli about the 

 bronchioles, in the interlobular septa, and beneath the pleura 

 is recognized as the usual distribution of anthracotic pigments. 

 In these earlier cases there are but few bacilli in the blood 

 contained within the vessels and also few in the neighborhood 

 of the vessels. The alveoli in this stage also contain bacilli 

 when there is no recognizable change in their lining epithelium. 



(2) The blood vessels in the interalveolar and interlobular 

 septa are widely distended with blood and occasionally small 

 haemorrhages have taken place from these vessels into the al- 

 veoli. Very few, if any, bacteria can be found in the blood 

 vessels in the earlier cases. 



(3) The smaller bronchi and bronchioles are in the condition 

 of catarrhal inflammation. The lining cells are swollen and 

 frequently desquamated, and there are some few red blood-cells 

 and leucocytes among the lining cells and in the lumina of the 

 bronchioles. Mucus in the form of granular flakes is also con- 



