Under the microscope the white corpuscles look like little round disks, about one-third larger than the red corpuscles, and with a large kernel, or _nucleus_, in their centre.

They have the same power of changing their shape, of surrounding and swallowing scraps of food, as has the ameba, and are a combination of scavengers and sanitary police.

When disease germs get into the blood, they attack and endeavor to eat and digest them; and whenever inflammation, or trouble of any sort, begins in any part of the body, they hurry to the scene in thousands, clog the blood-tubes and squeeze their way out through the walls of the smallest blood-tubes to attack the invaders or repair the damage.

This causes the well-known swelling and reddening which accompanies inflammation.
Blood, then, is a sticky red fluid, two-thirds of which is food-soup, and the other third, corpuscles.

How tiny the blood-corpuscles are, may be guessed from the fact that there are about 5,000,000 red cells and 10,000 white cells in every _cubic centimetre_ (fifteen drops) of our blood.
How the Blood Circulates through the Body.