CHAPTER XXIII 



INTRODUCTION 



Had the animal body no effective means of resistance to bacterial 

 invasion, man could not have developed, and the whole animal world 

 would have perished long since, so abundant in numbers and so wide 

 in distribution are the various forms of bacterial life, both saprophytic 

 and pathogenic. In life our skin is covered with bacteria and our 

 alimentary tract filled with them. Even in the most perfect states of 

 health we are the hosts and they are our guests. Willingly or unwill- 

 ingly, and for the most part unconsciously, we supply homes and food 

 to these parasites. With the approaching death of the host, these 

 myriads begin to penetrate every part of our bodies, and by the time 

 the respiration and pulse cease, both saprophytes and parasites are 

 tearing to pieces our mortal frames. It follows from this that the 

 living animal must possess certain agencies which serve to protect it 

 against bacterial invasion, and it is our purpose to inquire into the 

 nature and action of these agencies. 



Different species of animals differ widely in their susceptibility to 

 a given pathogenic bacterium. As a rule at least, it is true that the 

 more widely separated the species the more marked is the difference 

 in susceptibility to the same infection. We know of no bacterial infec- 

 tion common to the cold and warm-blooded animals. It is true that 

 we acquire typhoid fever by eating oysters taken from infected water, 

 but the oysters do not have typhoid fever any more than the leaves 

 of lettuce, which may be soiled with typhoid excreta, do. Fish, turtles 

 and possibly other cold-blooded animals have tuberculosis, but this 

 is due to quite a different organism from that which causes this dis- 

 ease in man. Some cold-blooded animals may be infected experimen- 

 tally with anthrax and plague, but under natural conditions they are 

 not known to develop these diseases. In some instances at least, the 

 immunity of cold-blooded animals to bacteria pathogenic to the warm- 

 blooded seems to be solely a matter of temperature. When kept at 



