PRINCIPLES OF VETERINARY SURGERY 163 



very marked protective or bactericidal (fixative) property 

 and therefore the part it plays in immunity is not suffi- 

 ciently constant to base any theory upon its presence. 



A number of facts appear paradoxical. If cavies are 

 immunized against the vibrio Metchnikovi, the blood of 

 the cavy is very bactericidal as regards the vibrio. If some 

 of these vibrios are placed into a test-tube containing blood 

 serum from one of these pigs, they will be destroyed within 

 less than an hour. Nevertheless, if a small dose of this cul- 

 ture is injected subcutaneously into these hyper-vaccinated 

 cavies, the vibrio will remain alive for several days up until 

 the time that they are ingested and destroyed by the leu- 

 cocytes. This apparent contradiction Metchnikofi^ explains 

 by the fact that in the test-tube the vibrios encounter 

 the microcytase which has escaped from the microphages 

 at the time of the formation of the clot and the separation 

 of the serum ; while in the subcutaneous tissue of the im- 

 munized pig the vibrios live on until the leucocytes collect 

 in sufficient numbers to destroy them by the cytase which 

 they set free. Then again, the blood of animals which are 

 immune to a certain disease, is not always bactericidal 

 against the germ causing the disease. This Metchnikoff 

 explains from the fact that cytase escapes from the leu- 

 cocytes with great difficulty and hence an animal can be 

 immune whose serum is not bactericidal because it does not 

 contain the free cytase. 



There must exist, then, something other than the powers 

 of the fluids of the body, that is to say some other factor 

 which plays the predominant part in immunity, and accord- 

 ing to Metchnikofif, phagocytosis is the most constant and 

 general phenomenon. He finds it in cases where the 

 humoral properties are the most marked, as well as in those 

 in which they are only slightly developed or entirely absent. 



With these factors at hand — the phagocytes, and their 



