AND THE HEALING ART 513 



ments of the white corpuscles outside the body were therefore not astonishing ; 

 but they long remained matters of mere curiosity. ]\Iuch interest was called to 

 them by the observation of the German jmthologist Cohnheim, that in some 

 inflammatory conditions tliey passed through the pores in the walls of the 

 flnest blood-vessels, and thus escaped into the interstices of the surrounding 

 tissues. Cohnheim attributed their transit to the pressure of the blood. But 

 why it was that, though larger than the red corpuscles, and containing a nucleus 

 which the red ones have not, they alone passed through the pores of the vessels, 

 or why it was tliat this emigration of the white corpuscles occurred abundantly 

 in some inflammations and was absent in others, was quite unexplained. 



These white corpuscles, however, have been invested with extraordinary new 

 interest by the researches of the Russian naturalist and pathologist, Metchnikoff. 

 He observed that, after passing through the walls of the vessels, thev not only crawl 

 about like amoebae, but, like them, receive nutritious materials into their soft 

 bodies and digest them. It is thus that the effete materials of a tadpole's tail are 

 got rid of ; so that they play a most important part in the function of absorption. 



But still more interesting observations followed. He found that a micro- 

 scopic crustacean, a kind of water-flea, was liable to be infested bv a fungus 

 which had exceedingl}' sharp-pointed spores. These were apt to j)enetrate the 

 coats of the creature's intestine, and project into its body-cavity. No sooner did 

 this occur with any spore than it became surrounded by a group of the cells which 

 are contained in the cavity of the body and correspond to the white corpuscles 

 of our blood. These proceeded to attempt to devour the spore ; and if they 

 succeeded, in every such case the animal was saved from the in^'asion of the 

 parasite. But if the spores were more than could be disposed of by the devouring 

 cells (phagocytes, as Metchnikoff termed them), the water-flea succumbed. 



Starting from this fundamental observation, he ascertained that the microbes 

 of infective diseases are subject to this same process of devouring and digestion, 

 carried on both by the white corpuscles and b\' cells that line the blood-vessels. 

 And by a long series of most beautiful researches he has, as it aj^pears to me, 

 firmly established the great truth that phagoc3'tosis is tlie main defensive means 

 possessed by the living body against the invasions of its microscopic foes. The 

 power of the system to ]:)roduce antitoxic substances to counteract the poisons 

 of microbes is undoubtedly in its own place of great importance. But in the 

 large class of cases in which animals are naturally refractory to jiarticular m- 

 fective diseases the blood is not found to yield any antitoxic element b\- which the 

 natural immunity can be accounted for. Here ]-)hagoc\-tosis seems to be the sole 

 defensive agency. And even in cases in w hich the serum does possess antitoxic, 

 or, as it would seem in some cases, germicidal jMOjierties. the bodies of the dead 



