MODES OF BACTERIAL INFECTION 145 



or receptacles for substances taken into the body. They are the natural 

 points of invasion and, as might be expected, they are furnished with means 

 of defence against it. The epithelium of the stomach and intestine, the 

 mucous membrane of the mouth, and the epidermis of the outer surface of 

 our bodies in short, every intact epithelial coat seems to be impenetrable 

 to bacteria. Even when highly virulent germs in large quantities are 

 brought into contact with uninjured epithelium, no infection takes place 

 unless a certain unhealthy condition of the tissues, a ' predisposition,' pre- 

 vails, and for the present this is unexplained. But where there is the 

 smallest break in the continuity of the cells, there the bacteria can penetrate 

 into the tissues and do their deadly work. We see this illustrated every 

 day in wounds on the outer surface of the body, and there can be no doubt 

 that it takes place in the same manner on the inner surfaces, the mucous 

 membranes of the bronchi and intestines. And here it is instructive to 

 recall what happens in plants. These close their wounds from below by 

 impenetrable layers of cork and thus cut off from bacteria the fluids by 

 which they are nourished. In animals, on the contrary, the blood and lymph 

 that exude from a wound form a richly nutritive medium for the growth 

 of the bacteria, which can then penetrate into the tissues and set up ' infec- 

 tion,' localized or general as the case may be. In experiments on animals 

 we imitate the natural course of events by injecting the bacteria beneath 

 the skin or into a blood-vessel. Insect stings represent a natural form of 

 this injection and not infrequently inoculate the body with disease germs. 



Investigations have yet to be made regarding the number of germs 

 necessary to cause infection, particularly in the case of natural infections. 

 In our experiments on animals the smallest doses must contain thousands 

 of bacterial cells, but it is stated that of the anthrax bacilli ten are enough, 

 injected hypodermically, to cause death in guinea-pigs (125). 



When bacteria have penetrated below the skin there is always the 

 possibility that they may be taken up by the blood-vessels or the lymphatics 

 and conveyed to other parts of the body. If they are, they may either 

 give rise to local lesions elsewhere or else endanger life by setting up 

 general infection. Anthrax bacilli, for instance, may produce merely a local 

 abscess (malignant pustule), or may spread and multiply in the blood- 

 vessels all over the body. Pus cocci may give rise to little pimples or 

 to the severer form of carbuncle, or scattered abroad by the blood may 

 induce fatal pyaemia or septicaemia. Some bacteria develop most rapidly 

 in the blood (anthrax, spirilla of recurrent fever), others principally or 

 entirely in the tissues (tubercle). 



The fundamental phenomena underlying all infections are the same 

 although the clinical and pathological features of a disease may be very 

 different in different cases. For these aspects of zymotic complaints medical 

 literature must be consulted. 



