PROBLEMS OF MODERN SCIENCE 



heart is the great pumping-station, and the tubes 

 that lead from the heart to the other portions of 

 the body are what we call the arteries, while the 

 tubes that lead back to the heart and carry the 

 blood home again are what we call the veins. 

 But what is the object of the blood being pumped 

 along the arteries ? It is not for the mere fun of 

 the thing. It is because the arteries terminate in a 

 network of extremely minute vessels which require 

 a microscope to see them, called the capillaries, and 

 it is in these that the blood does its work. And 

 * efficiency ' is the watchword here. This work 

 has to be very expeditiously performed, because 

 the blood only remains in these tiny tubes for 

 a second at most during each journey it makes. 

 They were originally called capillaries, I suppose, 

 because a hair was about the smallest thing that 

 people of those days could think of, but they are 

 very much finer than the finest possible hairs. 

 A hair is a solid thing, a capillary is a hollow 

 thing ; and capillaries together form a network of 

 minute vessels, through which the blood moves 

 comparatively slowly in order to enable it to give 

 its oxygen and nutriment to the tissues through 

 which it passes, and to remove from the tissues 

 the waste material that they do not want and 

 carry this to the organs of excretion, by means 

 of which it is got rid of. 



I think you will at once see the reason why 

 200 



