through the fine vessels of the lungs, and carries it to the tissues in every part of the body, which greedily take the oxygen from the red corpuscles. The red corpuscles of man's blood and that of the hairy suckling animals--the mammals--are not nucleated cells, but are regularly formed and renewed as they daily wear out, as fragments of larger mother-cells, which break up into these corpuscles, in the marrow of the bones, and some other situations where they are found. In all other vertebrates the red blood corpuscles have a kernel, or dense nucleus, and are complete "cells," usually oval, smooth and flattened in shape--a curious difference not easily accounted for. There are in a pint of the blood of an average man about two billions of these red corpuscles, and the amount of blood in the body is about one-twentieth of the total weight of the body--say, in a man weighing 160 lb., about 8 lb. or pints of blood. The clear, colourless lymph existing in all the lymph spaces of the body is probably about twelve pints. In many animals the red corpuscles are much less numerous than in man; for instance, a drop of human blood contains a thousand times as many red corpuscles as does an equal-sized drop of frog's blood. It is true that the frog's red corpuscles are a good deal bigger than those of man, but the result is that the human blood is some hundreds of times richer in hæmoglobin than the frog's, and has a proportionately greater power of carrying oxygen from the lungs to the tissues, and keeping up the slow, burning process, or oxidation, upon which the activity of the body, as well as its warmth, depend. The body depends upon its supply of oxygen as a steam-engine depends upon the oxygen of the air, which keeps its coal-fire burning. The pace of the blood-stream which is produced by the force-pump action of the contractions or beats of the heart is tremendous. It courses along at the rate of ten inches in a second in the big arteries and veins, and it has been carefully ascertained by experiment that a heartful of blood (which in a big man is about half a pint for each half or "side" of the heart)--or let us speak of a single corpuscle--is driven out of the heart through the great artery or aorta to the most remote parts of the body, and is back again at the heart, after running through endless branches of arteries, smallest capillaries, and thence into fine veins, bigger veins, and the biggest vein, in twenty to thirty seconds, the time 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.