Much can be learned about these blood cells by simple means. On the addition of water to the drop of blood, the red corpuscles swell up, lose their biconcave shape, and become round. They also become paler, while the fluid in which they float becomes yellowish. The meaning of this change is that the water has entered into the corpuscle, swelling it up, and has dissolved some of its colouring matter, which passes out and stains the plasma. The addition of a strong solution of sugar or salt causes them to shrink and become shrivelled looking, because the fluid parts of the cell have passed out to dilute the plasma, rendered more dense by the addition of the salt solution. The action of acetic acid causes the red blood cor puscle to disappear. It becomes paler and paler, and finally becomes invisible, or at least but the faintest shadowy indication of it is left, if the action of the acetic acid is not pushed. No trace of it may be left. On white blood cor puscles the action is similar, the cell becomes more and more transparent, till the bulk of the granular protoplasm of which it is com posed disappears. Something else is, however, brought into view, namely, small bodies nuclei—contained in the cell, but not easily seen, because obscured by the protoplasm, till the clearing up of the cell reveals them. A small white blood corpuscle may contain only one nucleus, the larger ones contain several. Thus, besides the differences already noticed, the red and white corpuscles of human blood differ in this, that the latter are nucleated while the former contain no nucleus.
Almost all vertebrate animals (animals hav ing a backbone) have the kinds of blood cells described, but they are not all of the same appearance as in human blood. In mammals (animals that suckle their young) the red cells are disc.shaped and without a nucleus, except in the camel, where they are oval though with out a nucleus. In all other vertebrate animals they are oval and have a nucleus. In man and in all mammals, with the exception of the camel tribe, the red corpuscles are biconcave as already described; but in birds, reptiles, and fishes they are biconvex, thicker in the middle than at the edges. They differ in size also in various animals.
When carefully measured, human red blood corpuscles are found to be about the of an inch across. [The white corpuscles are larger, being about the of an inch in diameter.] In the elephant they are ,21`4a of an inch ; in the musk-deer they are very In the proteus, a remarkable amphibian (an am phibian is au animal which may breathe both by gills and by lungs)—in the proteus there are the largest red corpuscles found in any vertebrate of an inch. In Fig. 132 are shown corpuscles of various animals mag nified by the same amount.
A remarkable thing about the colourless blood corpuscles is their power of altering their shape. The red corpuscle can have its shape altered, but only by pressure from without. The pressure of one on the other changes the shape, and on removal of the pressure the old shape returns. If a red corpuscle is passing
through a narrow channel it lengthens and becomes narrow till it has passed through, when its shape is restored. But the white cor puscle is active iu its change. It is constantly changing, but so slowly as to be with difficulty noticed in an ordinary preparation of a drop of blood. The change of shape is effected by what are called anmeboid movements, which have been sufficiently described on p. 53. The corpuscle can push out and draw in portions of its body, which is now globular and now elongated. By such changes of form the cell may wander from place to place. It has been shown, for instance, how the white corpuscles may wander out of the blood-vessels in the living body, pushing their way through the liv ing walls and insinuating themselves amongst the tissues outside the vessels. The cells have been called, in consequence, wander cells.
Moreover, the cells that form the matter of an abscess are not distinguishable from certain kinds of the white cells of the blood, and it is not certain whether they are not cells which have passed out of the blood-vessels in the process of inflammation. (See p. 328.) Again, the white cells found in lymph after it has passed through a lymphatic gland, found also in such numbers in lymphatic glands and in the spleen, are identical with those of the blood.
The number of white cells increases in the blood after meals and quickly diminishes again. Thus a German observer estimated the propor tion of white cells to red before breakfast as 1 to 1800, an hour after breakfast as 1 to 700, and some hours later as 1 to 1500.
When a more minute examination of the blood is desired the method described in the Introduction (p. 18) is followed, by which varie ties of red cells are detected and also of white. That portion of the Introduction should be read here, and the plates there introduced studied. By methods, which need not further be referred to, the number of red cells may be counted, as well as the number of each variety of white, and the amount of colouring matter each red cell contains may be estimated.
The Quantity of Blood in the body has been estimated in various ways, though, of course, the quantity cannot be stated with absolute accuracy. It is supposed in an adult man to be about one-thirteenth of the total weight of the body, that is about twelve pounds by weight in a person of eleven stones. When the multitude of corpuscles in a single drop of blood is considered, it will be evident that the total number of corpuscles in the blood of the body is scarcely countable, and certainly quite inconceivable. They have been counted, how ever, and the estimate is that in the of a cubic inch of blood there are a little over five millions of corpuscles. It has also been esti. mated that if all the red blood corpuscles in the blood of an adult man could be laid down side by side, they would cover an area of 3000 square yards.