The readiness with which leucocytes sacrifice themselves in their efforts to remove germs and decaying tissue is a matter of almost every-day experience. The fatty matter produced in the sebaceous glands of the skin normally overflows on to the surface. It serves to render the skin supple and impervious to water. Germs get into one of the sebaceous glands of the face or of the eyelid. The contents of the gland begin to decompose. Leucocytes enter it for the purpose of removing the putrescent substance. They lose their vitality and turn into pus corpuscles. The pimple or the stye bursts, and pus and fatty matter are discharged together.
That the conversion of leucocytes into pus cells is due to want of oxygen has been shown by the following experiment: A minute piece of phosphorus is placed beneath the skin. Leucocytes gather round the spot with a view to removing the tissue which the phosphorus has destroyed. But phosphorus has so strong an affinity for oxygen that it exhausts the supply in the area of tissue which surrounds it. The leucocytes die before reaching the tissue immediately adjacent to the piece of phosphorus. Their dead bodies form round it a raised ring of pus cells. We can explain this readiness of leucocytes to sacrifice themselves in their efforts to reach foreign matter which needs to be removed, only by saying that the attraction of the food is greater than the repulsion of lymph destitute of oxygen. An amœba placed in comparable circumstances gives up the quest of food, however strongly chemiotaxic, and retreats towards water which contains oxygen sufficient to provide for its respiratory needs.
=Blood.=—A portion of the body fluid is enclosed within vessels and kept in circulation by the heart. The heart pumps blood into the aorta. This trunk gives off large arteries, which in turn divide until the finest capillary vessels are reached. The capillary tubes reunite to form veins, which, with the exception of those which collect food from the digestive organs, convey the blood right back to the heart. The veins which drain the stomach and intestines (the organs in which food is prepared for absorption) and the spleen (the organ in which worn-out red blood-corpuscles are in a sort digested) break up in the liver into a second set of small vessels. The pseudo-capillary vessels of the liver reunite to form the hepatic veins, which add the blood that has passed through that organ to the rest of the blood which is passing up the inferior vena cava to the heart. A second capillary circulation is found in the kidney also.