Few of us realise what a wonderful and exceptional fluid the blood of a higher animal is. The Australian natives attach so little importance to it that they actually cut themselves and use their blood as a sort of paste for sticking decorative feathers on to a pole! The Papuans are more advanced, since they regard the flow of blood from a cut or graze as an evil portent. And some respect to the greatness and wonder of blood is shown by those persons among civilised peoples (more frequently men than women) who faint when they see blood, or even at the mention of its name! This stream of red fluid within us (of which an average man has about fifteen pints in his vessels) courses at a tremendous rate from the heart through all the endless branches and networks of arteries, capillaries and veins, and back to the heart. It feeds, cleanses, warms and takes "vital air" (the old name for oxygen gas) dissolved in it to every particle of our bodies, fresh and fresh at every pulse-beat as it rushes on. It not only absorbs crude digested food through the walls of the gut, but conveys it to where it is worked up and distributes the worked-up product. It removes the quickly used-up substances from every part, and the choke-damp or carbonic acid which would stop the whole machine, and kill us, were it not got rid of through the lungs as the blood hurries through the walls of these air-sacs, whilst other used-up materials are carried by it to the kidneys and passed out of the body through them. Every part of the body is brought into common life with every other part by this impetuous blood-stream--which is here, there, and everywhere, right round, and back again, in twenty-five seconds! It is obviously a very serious thing if a poison-producing microbe gets into this blood-stream and multiplies within it, or if poison-producing microbes lodge somewhere beneath the skin in a wound, and keep on discharging virulent poison into the blood! The mischief is spread all over the body at once.