MODE OF SPREAD OF INFECTIVE DISEASES. 745 



This is a mode of infection which we frequently produce 

 artificially in our experiments on animals, but which 

 seldom occurs under natural conditions. The experiments Even then in- 



. , . . f f ection does 



on animals, however, teach us further that in many mtec- not a i ways 

 tive diseases the penetration of the infective agents into occur - 

 the hlood is not sufficient for the production of infection; 

 we see that the bacilli of pneumonia, of malignant oedema, 

 of cholera, the micrococci of erysipelas, &c., do not cause 

 disease when injected into the hlood, with suitable pre- 

 cautions against the entrance of other bacteria, such as 

 disinfection of the wound in the skin, while they are 

 virulent in the same animals when introduced into 

 other parts, such as into a wound of the lung, into a 

 wound of the skin, into an altered intestine, &c. This 

 is explained in part by the fact demonstrated by Wysso- 

 kowitsch that bacteria circulating in the blood are not 

 excreted through certain surfaces of the body, such as 

 the wall of the intestine, and cannot pass into the 

 lumen of the intestine, for example, except when haemor- 

 rhages occur in the mucous membrane ; and that there- 

 fore cholera and typhoid bacilli, even when they have 

 penetrated into the blood, do not always reach their 

 proper seat of action. Wyssokowitsch's experiments 

 also give a partial explanation, these experiments show- 

 ing that the endothelial cells of the capillaries have the 

 power of fixing and destroying bacteria circulating in 

 the blood ; hence only those bacteria which are able to 

 overcome this protective arrangement can set up disease 

 from the blood. In the case of those resistant infective 

 agents which can penetrate with the blood into various 

 organs, and multiply there or in the blood itself, injection 

 into the blood is a very active mode of infection, e.g., 

 in the case of the exciting agents of septicaemia, anthrax, 

 and tuberculosis. 



Besides this infection through the blood, which occurs Or the infcc- 

 comparatively seldom, infection evidently occurs in many e irterat n 

 dieases by the primary establishment and development ^^af 1 ^ 1 ^ 

 of the infective agents at those parts of the body in wards become 



T -I ,i - c i ., the seat of the 



which the specific process occurs or begins ; so that the future disease. 

 seat of the alterations characteristic of the infective 



