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SCIENCE. 



[N. S. Vol. IV. No. 91. 



the organ. If we gently scrape the surface 

 of the animal's tongue we can detach some 

 of these ciliated cells ; and on examining 

 them with the microscope in a drop of 

 water we find that they will continue for 

 an indefinite time their lashing movements, 

 which are just as much living or vital in 

 their character as the writhings of a worm. 

 And, as I observed many years ago, these 

 detached cells behave under the influence 

 of a stimulus just like parts connected with 

 the body, the movements of the cilia being 

 excited to greater activity by gentle stimu- 

 lation, and thrown into a state of temporary 

 inactivity when the irritation was more se- 

 vere. Thus each constituent element of our 

 bodies may be regarded as in one sense an 

 independent living being, though all work 

 together in marvelous harmony for the good 

 of the body politic. The independent 

 movements of the white corpuscles out- 

 side the body were therefore not astonish- 

 ing ; but they long remained matters 

 of mere curiosity. Much interest was 

 called to them by the observation of the 

 German pathologist Cohnheim that in 

 some inflammatory conditions they passed 

 through the pores in the walls of the finest 

 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 that 

 this emigration of the white corpuscles oc- 

 curred abundantly in some inflammations 

 and was absent in others, was quite unex- 

 plained. 



These white corpuscles, however, have 

 been invested with extraordinary new in- 

 terest by the researches of the Kussian 

 naturalist and pathologist Metchnikofi". 

 He observed that, after passing through the 

 walls of the vessels, they not only crawl 



about like amoebae, but, like them, receive 

 nutritious materials into their soft bodies 

 and digest them. It is thus that the efiete 

 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 microscopic 

 crutacean, a kind of water flea, was liable 

 to be infested by a fungus which had ex- 

 ceedingly sharp-pointed spores. These were 

 apt to penetrate the coats of the creature's 

 intestine, and project into its body cavity. 

 'Eo 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 invasion of the parasite. 

 But if the spores were more than could be 

 disposed of by the devouring cells (phago- 

 cytes, as Metchnikoff termed them) the 

 water flea succumbed. 



Starting from this fundamental observa- 

 tion, 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 by cells 

 that line the blood vessels. And by a long 

 series of most beautiful researches he has, 

 as it appears to me, firmly established the 

 great truth that phagocytosis is the main 

 defensive means possessed by the living 

 body against the invasion of its microscopic 

 foes. The power of the system to produce 

 anti-toxic 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 particular infective 

 diseases the blood is not found to yield any 

 antitoxic element by which the natural im- 

 munity can be accounted for. Here pha- 

 gocytosis seems to be the sole defensive 



