13 



Many observers have shown that the intracellular digestion of the 

 lower organisms is brought about by the formation of vacuoles around 

 the food particles, these vacuoles being idled with a digestive fluid 

 whereby the food is broken up and rendered soluble and assimilable. 

 In the phagocytes, as Metchnikoff pointed out years ago, a similar pi-ocess 

 obtains in relation to the incepted bacteria when these are destroyed. 

 So that clearly the phagocytes, under favourable conditions, elaborate 

 substances capable of killing and breaking up the microbes. 



This view that the leucocytes contain a bactericidal substance or sub- 

 stances is strengthened, if it be not fully established, by Hankin's* dis- 

 covery of a bacteria-killing globulin present in the lymphatic glands of 

 the dog and cat, and also obtainable from the spleen. The former organs 

 and, to a large extent, the latter are composed of what may be termed 

 potential phagocytes. 



We thus arrive at the conclusion that the leucocytes and probably 

 other mesodermal cells function as phagocytes under appropriate condi- 

 tions within the organisms, and kill, or it may be, only modify the mi- 

 crobe by means of their digestive fluids ; that outside the organism, where 

 there has been destruction of the leucocytes, the soluble constituents of 

 these cells are liberated into the blood serum, lymph, etc., which now 

 gain the power of killing or modifying the microbes present. 



It is to the fact that Metchnikoff is now prepared to receive this doc- 

 trine and to extend his theory in this direction that I referred in the 

 earlier part of this paper, for, in his last communication,! he shows that 

 the same attenuation and alteration in the mode of growth of the vibrio 

 Metchnikovi is observable when the vibrio has been acted upon by phago- 

 cytes within the body of " vaccinated " guinea-pigs, and when it has been 

 acted upon outside the body by the blood serum of like animals. He 

 points out that in the latter case the change may be due to substances 

 derived from the broken down leucocytes. 



Having arrived thus far, the next problem to be dealt with is whether 

 the dissolution of leucocytes within the organism is capable of aiding the 

 action of the intact leucocytes and other phagocytes in preventing the 

 growth and extension of pathogenic microbes. Such dissolution is, we 

 know, of constant occurrence under normal conditions. During the pro- 

 cess of digestion, for example, not only is there a very definite increase 

 in the number of the white corpuscles of the blood, but as Heidenhaiu 

 has shown, there is a most active breaking down of the same, and in the 

 course of infectious disease it would seem that the production and destruc- 

 tion is largely increased wherever there is a well-marked reaction on the 

 part of the system. That there is increased formation is evidenced by the 

 proverbial "buSy coat" of febrile blood in certain states ; and having 



• U&nUa.— Proceedings of the lio'ial Societ;/, Vol. XLVIII, 1S90, p. 93. 

 t Metchnikoff.— .<nna?«« * V lastitut Pasteur, 1S91, No. 8. 



