454 IMMUNITY. 



or non- susceptibility of the fowl to the tetanus poison 

 evidently resides in the tissues, and it has not been shown 

 that in them antitoxic substances are present, though the 

 possibility of this has not been excluded in all cases. 

 It is possible then that an animal might be immune against 

 the anthrax bacillus, for example, if the toxines of the latter 

 were simply inert towards the animal tissues, or, in other 

 words, if its tissues enjoyed a natural insusceptibility 

 to the toxines. In such a case the anthrax bacillus would 

 be in the position of the bacillus subtilis, and would be 

 destroyed in the tissues by the same means. 



(b) Natural Bactericidal Powers, The second factor 

 may now be considered, namely, the power of killing the 

 organism, though it appears to us that natural immunity 

 has been too exclusively looked at from this side. Special 

 powers of destroying organisms in natural immunity have 

 been ascribed to (a) phagocytosis, and (b) the action of the 

 serum. 



(a) The chief factors with regard to phagocytosis have 

 been given above. The bacteria in a naturally immune animal, 

 for example, the anthrax bacillus in the tissues of the white 

 rat, are undoubtedly taken up in large numbers by the phago- 

 cytes, whereas in a susceptible animal this only occurs to a 

 small extent ; and Metchnikoff has shown that they are taken 

 up in a living condition, and are still virulent when tested 

 in a susceptible animal. But is this phagocytosis the cause 

 or the effect of immunity ? The fact of artificial immunity 

 would rather point to its being the latter. The following 

 experiment performed by Metchnikoff, though belonging to 

 the subject of artificial immunity, may be given here. He 

 injected into a guinea-pig a virulent culture of the bacillus 

 of hog-cholera, and at the same time injected the anti- 

 serum of the same organism into a vein. At the end of a 

 few hours a local swelling formed at the site of injection, 

 in which there were enormous numbers of bacilli but 

 no leucocytes. After another injection of the serum, 

 however, the leucocytes gathered around and attacked the 

 bacilli. From this experiment he infers that the serum 



