31' president's adddess, 



wounded tissue. He therefore called these v/hite or colourless corpuscles 

 'phagocytes,' the eater-cells, and in his beautiful book on Inflammation, 

 published twenty years ago, proved the extreme importance of their 

 activity. At the same time he had shown that they eat up intrusive 

 bacteria and other germs ; and his work for the last twenty years has 

 mainly consisted in demonstrating that they are the chief, and probably 

 the only, agents at work in either ridding the human body of an attack 

 of disease-causing germs or in warding off even the commencement of an 

 attack, so that the man or animal in which they are fully efficient is 

 ' immune ' — that is to say, cannot be eiFectively attacked by disease-germs. 

 Disease-germs, bacteria, or protozoa produce poisons which sometimes 

 are too much for the phagocytes, poisoning them and so getting the upper 

 hand. But, as Metschnikoff showed, the training of the phagocytes by 

 weak doses of the poison of the disease-germ, or by weakened cultures of 

 the disease-germ itself, brings about a power of resistance in the phagocytes 

 to the germ's poison, and thus makes them capable of attacking the germs 

 and keeping them at bay. Hence the value of inoculations. 



The discussion and experiments arising from Metschnikoff's demon- 

 strations have led to the discovery of the production by the phagocytes 

 of certain exudations from their substance which have a most important 

 efifect in weakening the resistance of the intrusive bacteria and rendering 

 them easy prey for the phagocyte. These are called ' sensitisers,' and 

 have been largely studied. They may be introduced artificially into the 

 blood and tissues so as to facilitate the work of the phagocytes, and no 

 doubt it is a valuable remedial measure to make use of such sensitisers as 

 a treatment. Dr. Wright considers that such sensitisers are formed in 

 the blood and ti.ssues independently of the phagocytes, and has called 

 them ' opsonins,' under which name he has made most valuable application 

 of the method of injecting them into the body so as to facilitate the work 

 of the phagocytes in devouring the hostile bacteria of various diseases. 

 Each kind of disease-producing microbe has its own sensitiser or opsonin ; 

 hence there has been much careful research and experiment required in 

 order to bring the discovery to practical use, Metschnikoff himself holds 

 and quotes experiments to show that the ' opsonins ' are actually produced 

 by the phagocytes themselves. That this should be so is in accordance 

 with some striking zoological facts, as I pointed out nearly twenty years 

 ago. For the lowest multicellular animals provided with a digestive sac 

 or gut, such as the polyps, have that sac lined by digestive cells which 

 have the same arareboid character as ' phagocytes,' and actually digest to 

 a large extent by swallowing or taking into their individual protopla,sm 

 raw particles of food. Such particles are enclosed in a temporary cavity, 

 or vacuole, into which the cell-protoplasm secretes digestive ferment and 

 other chemical agents. Now there is no doubt that such digestive 

 vacuoles may burst and so pour out into the polyp's stomach a digestive 

 juice which v/ill act on food particles outside the substance of the cells, 

 and thus by the substitution of this process of outpouring of the secretion 



