the body; and, so long as it remains alive, the blood contained in it will not coagulate, though, if a portion of the same blood be removed from the heart, it will coagulate in a few minutes.

Blood taken from the body of the turtle, and kept from coagulating by cold for some time, may be poured into the separated, but still living, heart. and then will not coagulate.

Freshly deposited fibrin acts somewhat like living matter, coagulable blood remaining fluid for a long time in tubes coated with such fibrin.

14. The coagulation of the blood is an altogether physico-chemical process, dependent upon the properties of certain of the constituents of the plasma, apart from the vitality of that fluid. This is proved by the fact that if blood-plasma be prevented from coagulating by cold, and greatly diluted, a current of carbonic acid passed through it will throw down a white powdery substance. If this white substance be dissolved in a weak solution of common salt, or in an extremely weak solution of potash or soda, it, after a while, coagulates, and yields a clot of true pure fibrin. It would be absurd to suppose that a substance which has been precipitated from its solution, and redissolved, still remains alive.

There are reasons for believing that this white substance consists of two constituents of very similar composition, which exist separately in living blood, and the union of which is the cause of the act of coagulation. These reasons may be briefly stated thus:-The pericardium and other serous cavities in the body contain a clear fluid, which has exuded from the blood-vessels, and contains the elements of the blood without the bloodcorpuscles. This fluid sometimes coagulates spontaneously, as the blood plasma would do, but very often shows no disposition to spontaneous coagulation. When this is the case, it may nevertheless be made to coagulate, and yield a true fibrinous clot, by adding to it a little serum of blood.

Now, if serum of blood be largely diluted with water and a current of carbonic acid be gas passed through it, a white powdery substance will be thrown down; this, redissolved in a dilute saline, or extremely dilute alkaline,

solution will, when added to the pericardial fluid, produce even as good a clot as that obtained with the original

serum.

This white substance has been called globulin. It exists not only in serum, but also, though in smaller quantities, in connective tissue, in the cornea, in the humours of the eye, and in some other fluids of the body.

It possesses the same general chemical properties as the albuminous substance which enters so largely into the composition of the red corpuscles (§ 4), and hence, at present, bears the same name. But when treated with chemical reagents, even with such as do not produce any appreciable effect on its chemical composition, it very speedily loses its peculiar power of causing serous fluids to coagulate. For instance, this power is destroyed by an excess of alkali, or by the presence of acids.

Hence, though there is great reason to believe that the fibrino-plastic globulin (as it has been called) which exists in serum does really come from the red corpuscles, the globulin which is obtained in large quantities from these bodies, by the use of powerful reagents, has no coagulating effect at all on pericardial or other serous fluids.

Though globulin is so susceptible of change when in solution, it may be dried at a low temperature and kept in the form of powder for many months, without losing its coagulating power.

Thus globulin, added, under proper conditions, to serous effusion, is a coagulator of that effusion, giving rise to the development of fibrin in it.

It does so by its interaction with a substance contained in the serous effusion, which can be extracted by itself, and then plays just the same part towards a solution of globulin, as globulin does towards its solution. This substance has been called fibrinogen. It is exceedingly like globulin, and may be thrown down from serous exudation by carbonic acid, just as globulin may be precipitated from the serum of the blood. When redissolved in an alkaline solution, and added to any fluid containing globulin, it acts as a coagulator of that fluid, and gives rise to the development of a clot of fibrin in it. In accordance with what has just been stated, serum of blood which has completely coagulated may be kept in one

vessel, and pericardial fluid in another, for an indefinite period, if spontaneous decomposition be prevented, without the coagulation of either. But let them be mixed, and coagulation sets in.

Thus it seems to be clear, that the coagulation of the blood, and the formation of fibrin, are caused primarily by the interaction of two substances (or two modifications of the same substance), globulin or fibrinoplastin and fibrinogen, the former of which may be obtained from the serum of the blood, and from some tissues of the body; while the latter is known, at present, only in the plasma of the blood, of the lymph, and of the chyle, and in fluids derived from them.

15. The proverb that "blood is thicker than water" is literally true, as the blood is not only "thickened" by the corpuscles, of which it has been calculated that no fewer than 70,000,000,000 (eighty times the number of the human population of the globe) are contained in a cubic inch, but is rendered slightly viscid by the solid matters dissolved in the plasma. The blood is thus rendered heavier than water, its specific gravity being about 1055. In other words, twenty cubic inches of blood have about the same weight as twenty-one cubic inches of water.

The corpuscles are heavier than the plasma, and their volume is usually somewhat less than that of the plasma. Of colourless corpuscles there are usually not more than three or four for every thousand of red corpuscles; but the number varies very much, increasing shortly after food is taken, and diminishing in the intervals between meals.

The blood is hot, its temperature being about 100 Fahrenheit.

16. Considered chemically, the blood is an alkaline fluid, consisting of water, of solid and of gaseous matters. The proportions of these several constitutents vary according to age, sex, and condition, but the following statement holds good on the average :

In every 100 parts of the blood there are 79 parts of water and 21 parts of dry solids; in other words, the water and the solids of the blood stand to one another in about the same proportion as the nitrogen and the oxygen of the air. Roughly speaking, one quarter of the blood

=

is dry, solid matter; three quarters water. Of the 21 parts of dry solids, 12 (= #ths) belong to the corpuscles. The remaining 9 are about two-thirds (67 parts ths) albumin (a substance like white of egg, coagulating by heat), and one-third (= 4th of the whole solid matter) a mixture of saline, fatty, and saccharine matters, sundry products of the waste of the body, and fibrin. The quantity of the latter constituent is remarkably small in relation to the conspicuous part it plays in the act of coagulation. Healthy blood, in fact, yields in coagulating not more than from two to four parts in a thousand of its weight of fibrin.

The total quantity of gaseous matter contained in the blood is equal to rather less than half the volume of the blood; that is to say, 100 cubic inches of blood will contain rather less than 50 cubic inches of gases. These gaseous matters are carbonic acid, oxygen, and nitrogen; or, in other words, the same gases as those which exist in the atmosphere, but in totally different proportions; for whereas air contains nearly three-fourths nitrogen, onefourth oxygen, and a mere trace of carbonic acid, the average composition of the blood gases is nearly twothirds carbonic acid, rather less than one-third oxygen, and not one-tenth nitrogen.

This

It is important to observe that blood contains much more oxygen gas than could be held in solution by pure water at the same temperature and pressure. power of holding oxygen appears in some way to depend upon the corpuscles, firstly, because mere serum has no greater power of absorbing oxygen than pure water has; and secondly, because red corpuscles suspended in water instead of serum absorb oxygen very readily. The oxygen thus held by the red corpuscles is readily given up by them for purposes of oxidation, and indeed

can be removed from them by means of a mercurial gas pump. It would appear that the connection between the oxygen and the red corpuscles is of a peculiar nature, being a sort of loose chemical combination with one of their constituents, that constituent being the hæmoglobin; for solutions of hæmoglobin behave towards oxygen exactly as blood does.

The corpuscles differ chemically from the plasma, in

containing a large proportion of the fats and phosphates, all the iron, and almost all the potash, of the blood; while the plasma, on the other hand, contains by far the greater part of the chlorine and the soda.

17. The blood of adults contains a larger proportion of solid constituents than that of children, and that of men more than that of women; but the difference of sex is hardly at all exhibited by persons of flabby, or what is called lymphatic, constitution.

Animal diet tends to increase the quantity of the red corpuscles; a vegetable diet and abstinence to diminish them. Bleeding exercises the same influence in a still more marked degree, the quantity of red corpuscles being diminished thereby in a much greater proportion than that of the other solid constituents of the blood.

18. The total quantity of blood contained in the body varies at different times, and the precise ascertainment of its amount is very difficult. It may probably be estimated, on the average, at not less than one-thirteenth of the weight of the body.

19. The function of the blood is to supply nourishment to, and take away waste matters from, all parts of the body. It is absolutely essential to the life of every part of the body that it should be in such relation with a current of blood, that matter can pass freely from the blood to it, and from it to the blood, by transudation through the walls of the vessels in which the blood is contained. And this vivifying influence depends upon the corpuscles of the blood. The proof of these statements lies in the following experiments :-If the vessels of a limb of a living animal be tied in such a manner as to cut off the supply of blood from the limb, without affecting it in any other way, all the symptoms of death will set in. The limb will grow pale and cold, it will lose its sensibility, and volition will no longer have power over it; it will stiffen, and eventually mortify and decompose.

But, even when the death stiffening has begun to set in, if the ligatures be removed, and the blood be allowed to flow into the limb, the stiffening speedily ceases, the temperature of the part rises, the sensibility of the skin returns, the will regains power over the muscles, and, in short, the part returns to its normal condition.