The White Or Colourless Corpuscles, also termed leucocytes, are spherical in form, larger than the red, being, in man, about 1/3000 inch in diameter, or more than that. They have a turbid or mottled appearance, which, on addition of water, disappears and discovers a nucleus. When coagulation is retarded, and the red corpuscles sink, the white corpuscles rise gradually to the top, showing that they are lighter than the fluid part of the blood, while the red corpuscles are heavier. Watching them as they circulate in the capillary vessels of the web of a frog's foot, one may see that the white corpuscles often show a tendency to adhere to the wall of the vessel, while the red corpuscles keep in a stream in the centre of it; and it has been proved by repeated observation that white corpuscles are capable of making their way through the capillary wall, which comes together again behind them without apparent breach of continuity, while they pass on into the tissue. It would appear that red corpuscles sometimes pass out in the same way; but the white have a much greater tendency to escape, and after they have done so, no line can be drawn between them and the other amoeboid bodies, that is to say, the connective-tissue-corpuscles, for the white corpuscles have amoeboid properties. Moreover, pus, the matter thrown out in suppuration, consists of a fluid rich in corpuscles, which cannot be separated by any line of distinction from white corpuscles; and it is not yet settled to what extent these consist of transuded white corpuscles, or how far they are derived from processes of multiplication among the connective-tissue-corpuscles. But whatever may be the occasional functions of the white corpuscles exercised by escaping into the tissues, they seem to have a much more important purpose within the circulation, for it is probable that the red corpuscles are formed from them by disappearance of the nucleus and alteration of their contents.

We shall find that the white corpuscles take origin in the spleen and in the lymphatic glands; they appear in great numbers immediately after eating, and quickly disappear again. Thus, a German observer (Hirt) computed the proportion of white to red corpuscles in his own blood, and found that before breakfast it was 1 in 1800, an hour afterwards 1 in 700, and between eleven and twelve o'clock 1 in 1500. He took dinner at one o'clock, after which the proportion was 1 in 400, while two hours afterwards it sunk to 1 in 1475. After an eight o'clock supper it was 1 in 550, and at eleven o'clock 1 in 1200.

78. Turning now to the chemical composition of the blood, we find that the liquor sanguinis is essentially an albuminoid solution. It is slightly alkaline, and contains about 97 parts of solid matter in every thousand. Of these only four parts consist of fibrin; while the albumen, which is the principal constituent of the serum, forms nearly 79 parts; the mineral matters constitute more than 8 parts; urea, kreatin, and other matters soluble in water, usually grouped together under the name of extractive, make up about 4, and the fats less than 2 parts in the thousand.

The blood is, in health, very uniform in its composition, and it will naturally occur to ask how the uniformity is maintained, seeing that the additions made to it must vary much with the character of the diet. It may also be asked how it happens that the blood, which nourishes the whole body, has so little resemblance to the total composition of the body. Both these questions admit of one answer, namely, that the amount of any substance in the blood at one time depends not only on the quantity which enters the circulation, but on the length of time that it remains there. Thus, there is very little fatty matter in the blood, although quantities enter with the supplies of nourishment from the food, and at least one substance, roughly classed under this head, choles-terine, is returned from the brain; and there is only a very minute quantity of urea in the blood, although there is reason to believe that a considerable amount of what is eliminated by the kidneys pre-exists in it: but the explanation is simply that none of these substances are allowed to accumulate in the blood, that they are removed from it as speedily as they enter it.