56 A MANUAL OF PHYSIOLOGY 



be typical of the struggle which the phagocytes of higher 

 animals and of man seem to engage in when the germs of 

 disease are introduced into the organism. He supposes 

 that the immunity to certain diseases possessed naturally 

 by some animals, and which may be conferred on others by 

 vaccination with various protective substances,* is, to a 

 large extent, due to the early and complete success of the 

 phagocytes in the fight with the bacteria ; and that in 

 rapidly-fatal diseases such as chicken-cholera in birds and 

 rabbits, and anthrax in mice the absence of any effective 

 phagocytosis is the factor which determines the result. 

 Others have laid stress on the action of protective sub- 

 stances supposed to exist in the living plasma itself, although 

 only as yet demonstrated in the serum. It is possible that 

 such substances are manufactured by the leucocytes, and 

 either given off by them to the plasma by a process of 

 'excretion,' or liberated by their complete solution. And it 

 may be that it is only when the bacteria have been crippled 

 by contact with these defensive ' alexins ' that the leucocytes 

 are able to ingest them and complete their destruction. It 

 has been actually observed that the oxyphile cells of frog's 

 lymph retard the growth of certain bacteria by coating them 

 with a kind of slime derived from the oxyphile granules. 



Diapedesis. The fact that leucocytes can pass out of the 

 bloodvessels into the tissues (Waller, Cohnheim) has a very 

 important bearing on the subject of phagocytosis. The 

 phenomenon is called diapedesis, and is best seen when a 

 transparent part, such as the mesentery of the frog, is 



* The most recent investigations go to show that MetschnikofTs 

 phagocytic theory of immunity requires modification in the case of the 

 higher animals and man, although the brilliant biological observations on 

 which it was originally built retain all their value. He supposed that 

 in the immunizing process the leucocytes underwent certain changes, 

 acquired, so to speak, a sort of * education ' that enabled them to cope 

 with bacteria against which they were previously powerless. It seems 

 more probable that in the presence of the substances that confer immunity, 

 not only the leucocytes, but other cells, are stimulated to produce bodies 

 which cut short the life, or at least inhibit the growth, of the bacteria. It 

 may be, however that the leucocytes take the lead in this reaction. And 

 the voracious and at first sight almost undiscriminating appetite displayed 

 by cells which appear to englobe with equal avidity a granule of carmine 

 or a particle of proteid, a globule of fat or a fragment of carbon, renders 

 it difficult to believe that they do not also act, to some extent, directly 

 as phagocytes in the presence of pathogenic organisms. 



