B. Red corpuscles much more highly magnified, seen in face; C. ditto, seen in profile; D. ditto, in rows, rather more highly magnified; E. a red corpuscle swollen into a sphere by imbibition of water.
F. A white corpuscle magnified same as B.; G. ditto, throwing out some blunt processes; K. ditto, treated with acetic acid, and showing nucleus, magnified same as D.
H. Red corpuscles puckered or crenate all over.
I. Ditto, at the edge only.
round bodies, the blood discs or blood corpuscles ([Fig. 4], A). If you look carefully you will notice that most of them are round, as B; but every now and then you see something like C. That is one of the round ones seen sideways; for they are not round or spherical like a ball, but circular and dimpled in the middle, something like certain kinds of biscuit. When you see one by itself it looks a little yellow in colour, that is all; but when you see them in a lump, the lump is clearly red. Remember how small they are: three thousand of them put flat in a line, edge to edge, like a row of draughts, would just about stretch across one inch. All the redness there is in blood belongs to them. When you see one of them, you see so little of the redness that it seems yellow. If you were to put a drop of blood into a tumbler of water, the water would not be stained red, but only just turned of a yellowish tint, so little redness would be given to it by the drop of blood. In the same way a very very thin slice of currant jelly would look yellowish, not red.
These red corpuscles are not hard solid things, but delicate and soft, very tender, very easily broken to pieces, more like the tiniest lumps of red jelly than anything else, and yet made so as to bear all the squeezing which they get as they are driven round and round the body.
Besides these red corpuscles, you may see if you look attentively other little bodies, just a little bigger than the red corpuscles, not coloured at all, and not circular and flat, but quite round like a ball ([Fig. 4], a, F, G). That is to say, these are very often quite round, only they have a curious trick of changing their form. Imagine you were looking at a suet dumpling so small that about two thousand five hundred of them could be placed side by side in the length of one inch—and suppose the round dumpling while you were looking at it gradually changed into the shape of a three-cornered tart, and then into a rounded square, and then into the shape of a pear, and then into a thing that had no shape at all, and then back again into a round ball, and kept doing this apparently all of its own accord while you were looking at it—wouldn’t you think it very curious? Well, one of these little bodies in the blood of which we are speaking, and which are called white corpuscles, may be seen, when a drop of blood is watched under the microscope, to go on in this way, continually changing its shape. But of these white corpuscles of the blood, and of their wonderful movements, you will learn more as you go on in your physiological studies.
23. Besides these red and white corpuscles there is nothing else very important in the blood that you can see with the microscope; but their being in the blood is one reason why blood is thicker than water.
Did you ever see a pig or sheep killed? If so, you would be sure to notice that the blood ran quite fluid from the blood-vessels in the neck, ran and was spilt like so much water—but that very soon the blood caught in the pail or spilt on the stones became quite solid, so that you could pick it up in lumps. Whenever blood is shed from the living body, within a short time it becomes solid. This becoming solid is called the clotting or coagulation of blood.
What makes it clot? Suppose while the blood was running from the pig’s neck into the butcher’s pail, and while it was still quite fluid, you were to take a bunch of twigs and keep slowly stirring the blood round and round in the pail. You would naturally expect that the blood would soon begin to clot, would get thicker and thicker and more and more difficult to stir. But it does not; and if you keep on stirring long enough you will find that it never clots at all. By continually stirring it you will prevent its clotting. Now take out your bundle of twigs: you will find it covered all over with a thick reddish mass of some soft sticky substance; and if you pump on the red mass you will be able to wash away all its red colour, and will have nothing left but a quantity of white, soft, sticky, stringy material, all entangled and matted together among the twigs of your bundle. This stringy material is in reality made up of a number of fine, delicate, soft, elastic threads or fibres, and is called fibrin.