THE BLOOD AND ITS CIRCULATION 349 



occupied by twenty-five to thirty heart-beats. The walls 

 of the arteries are firm, though elastic, and it is no wonder, 

 with this tremendous pressure and pace on the liquid 

 within, that when an artery is cut the blood spurts out to 

 a distance of several feet. 



The colourless liquid of the blood contains, besides 

 the red corpuscles floating in it, others brought to it in 

 the lymph and derived from various connective-tissue 

 spaces and special nodules or " glands." They are out- 

 numbered by the red corpuscles in the proportion of five 

 hundred to one. They are colourless, and bigger than 

 the red corpuscles. Most of them continually change 

 their shape, and consist of active, moving protoplasm. 

 These are the " phagocytes," which, besides acting 

 chemically upon the constituents of the blood-liquid, 

 take into their substance (as does the amoeba or proteus- 

 animalcule) and digest and destroy all foreign or dead 

 particles, and the bacteria which may find their way into 

 it. They pass out, forcing their way through the ex- 

 cessively thin walls of the finest capillaries blood- 

 vessels not wide enough to admit two of them side by 

 side and enter, to the number of thousands, the tissues 

 which have been wounded or poisoned by bacteria, to 

 carry on their all-important protective " scavenger " or 

 " police-constable " work. 



Inflammation is the slowing of the blood-stream by 

 dilatation of the vessels at an injured spot, in order to 

 allow the phagocytes to make their way out of the blood- 

 stream into the tissues, and so get to close quarters with 

 the enemy. There are other excessively minute dust- 

 like particles called "platelets," which are sometimes very 

 abundant in the liquid of the blood. Besides the duties 

 of oxygen- carry ing and scavengering the blood has other 



