354 BACTERIA PATHOGENIC TO MAN 



animal is by passage through the bodies of highly susceptible animals 

 of the same species. Growth in fresh blood also increases it for the 

 homologous animal. 



Toxin Production. We have little exact knowledge upon the nature 

 of the substances produced by or through the growth of the pneumo- 

 cocci in animal tissues or artificial media. 



Occurrence in Man during Health. It is probable that in crowded 

 communities the pneumococcus is present on the mucous membranes 

 of most persons. We have found it generally present not only in the 

 throats of persons living in New York City, but also in those of persons 

 living on farms and in the Adirondack Mountains. It is commonly 

 present only on the mucous membranes of the bronchi, trachea, pharynx, 

 and nostrils. The healthy lung seems to be generally free from it. 

 Judging from animal tests it is very possible that the virulence for man 

 of the organisms present in health is much less than the virulence of 

 those in a pneumonic lung. 



Presence of Pneumococci in Lobar and Bronchopneumonia. Fully 95 

 per cent, of characteristic cases of lobar pneumonia are due primarily 

 to characteristic or atypical pneumococci. Usually no other bacteria 

 are obtained from the lungs. Atypical cases are frequently due to 

 pneumococci, but they may be due to streptococci, influenza bacilli, 

 etc.. The more recent the infection the greater is the number of bac- 

 teria found in the diseased lung area. As the disease progresses these 

 decrease in number until finally at the crisis they disappear from the 

 tissues, though at this time and long after convalescence they may be 

 present in the sputum. In atypical forms of pneumonia they may 

 remain longer in the tissues, and in walking pneumonia they may be 

 absent in the original centres of infection or present only as attenuated 

 varieties, while the surrounding, newly formed foci may contain fully 

 virulent cocci. It has been shown by Netter that more than one-half 

 of the cases of bronchopneumonia, whether primary, or secondary to 

 some other disease, as measles and diphtheria, both in children and 

 adults, are due to the diplococcus of pneumonia. Others, such as 

 Pearce, have found other micro-organisms, especially the streptococci, 

 in the majority of cases. These findings will be considered at the end 

 of the chapter. 



The pneumococci are found partly in^ the alveoli and bronchioles 

 of the inflamed lung and partly in the lymph channels and blood capil- 

 laries. Most of the organisms are found free, but a few are found in 

 the leukocytes. Through the lymph channels they find their way to 

 the pleura and to adjacent lymph glands. From the capillaries they 

 find their way to the general blood current, and thus to distant parts 

 of the body. In about 20 per cent, of cases the pneumococci are so 

 abundant that they can be found in cultures made from 5 to 10 c.c. of 

 blood. In a number of instances the foetus has been found infected. 



Occurrence in Inflammations Complicating Pneumonia. In every case 

 of lobar pneumonia and in most cases of bronchopneumonia pleurisy 

 is developed, which is excited by the same micro-organism that was 



