332 The Outline of Science 



corpuscles succeed in devouring the microbes before they multiply 

 to a dangerous extent, we are saved. But bacteria multiply at 

 a terrible speed, and sometimes they beat the corpuscles and we 

 pass into a perhaps dangerous illness. 



Biologists had hardly ceased to wonder at this new romance 

 of the blood when others were discovered. Bacteria produce a 

 poison (or toxin) with which they taint the blood. But it was 

 found that the blood produces an "anti-toxin," a chemical for 

 neutralising the toxin; and after years of experiment the anti- 

 toxin was prepared in the laboratory and injected into the blood. 

 It also became possible to help the white corpuscles in the fray, 

 or spur them on to it, so to speak, by preparing a sort of sauce 

 an opsonin, the man of science calls it from dead bacteria 

 and injecting it into the blood. The opsonin makes the living 

 bacteria more attractive or palatable to the corpuscles, and our 

 "brave defenders" go to work more vigorously. Sir Arthur 

 Keith, in The Engines of the Human Body, refers to "the im- 

 mense and movable armies of microscopic corpuscles which can 

 be mobilised for police or sanitary duties. They swarm in the 

 blood stream as it circulates round the body ... it is extremely 

 probable that one variety of them, if not more, are really errand- 

 boys on their way to deliver messages or parcels, and that the 

 gland masses which are built up in and around lymph channels 

 serve both as nurseries for the upbringing of such messengers, 

 and also as offices from which they are dispatched on their 

 errands." 



6 



The Heart 



It is clear that the many-sided value of the blood depends 

 upon its regular coursing through the whole body, and we have 

 now to see how this is accomplished. 



Until three centuries ago there was not a man in any civilisa- 

 tion who knew anything about this "circulation of the blood.' 



