EXPERIMENTAL INOCULATION 207 



often enlarged and firm, and the blood contains capsulated 

 pneumococci in large numbers (Fig. 71). If the seat of inocula- 

 tion be in the lung, there generally results pleuritic effusion on 

 both sides, and in the lung there may be a process somewhat 

 resembling the early stage of acute croupous pneumonia in man. 

 There are often also pericarditis and enlargement of spleen. 

 We have already stated that cultures of the pneumococci on 

 artificial media in a few days begin to lose their virulence. 

 Now, if such a partly attenuated culture be injected sub- 

 cutaneously into a rabbit, there is greater local reaction ; 

 pneumonia, with exudation of lymph on the surface of the 

 pleura, and a similar condition in the peritoneum, may occur. 

 In sheep greater immunity is marked by the occurrence, after 

 subcutaneous inoculation, of an enormous local sero-fibrinous 

 exudation, and by the fact that few pneumococci are found in 

 the blood stream. Intra- pulmonary injection in sheep is 

 followed by a typical pneumonia, which is generally fatal. The 

 dog is still more immune ; in it also intra-pulmonary injection is 

 followed by a fibrinous pneumonia, which is only sometimes 

 fatal. Inoculation by inhalation appears only to have been 

 performed in the susceptible mouse and rabbit ; here also 

 septicaemia resulted. 



The general conclusion to be drawn from these experiments 

 thus is that in highly susceptible animals virulent pneumococci 

 produce a general septicaemia ; whereas in more immune species 

 there is an acute local reaction at the point of inoculation, and 

 if the latter be in the lung, then there may result pneumonia, 

 which, of course, is merely a local acute inflammation occurring 

 in a special tissue, but identical in essential pathology with an 

 inflammatory reaction in any other part of the body. When a 

 dose of pneumococci sufficient to kill a rabbit is injected sub- 

 cutaneously in the human subject, it gives rise to a local inflam- 

 matory swelling with redness and slight rise of temperature, all 

 of which pass off in a few days. It is therefore justifiable to 

 suppose that man occupies an intermediate place in the scale of 

 susceptibility, probably between the dog and the sheep, and 

 that when the pneumococcus gains an entrance to his lungs, the 

 local reaction in the form of pneumonia occurs. In this con- 

 nection the occurrence of manifestations of general infection 

 associated with pneumonia in man is of the highest import- 

 ance. We have seen that meningitis and other inflammations 

 are not very rare complications of the disease, and such cases 

 form a link connecting the local disease in the human subject 

 with the general septicsemic processes which may be produced 



