fibrinogens) obtained from most of the cellular organs of the body produce intravascular clotting when injected into the circulation of a living animal.

THE PLASMA AND SERUM

The liquid in which the corpuscles float may be obtained by employing one or other of the methods already described for preventing the blood from coagulating. The corpuscles being heavy sink, and the supernatant plasma can then be removed by a pipette or siphon, or more thoroughly by the use of a centrifugal machine (see fig. 60).

On counteracting the influence which has prevented the blood from coagulating, the plasma then itself coagulates. Thus plasma obtained by the use of cold clots on warming gently; plasma which has been decalcified by the action of a soluble oxalate clots on the addition of a calcium salt; plasma obtained by the use of a strong solution of salt coagulates when this is diluted by the addition of water, the addition of fibrin ferment being necessary in most cases; where coagulation occurs without the addition of fibrin ferment, no doubt some is already present from the partial disintegration of the corpuscles which has already occurred. Pericardial and hydrocele fluids resemble pure plasma very closely in composition. As a rule, however, they contain few or no white corpuscles, and so do not clot spontaneously, but after the addition of fibrin ferment or liquids like serum that contain fibrin ferment they always yield fibrin.

Pure plasma may be obtained from horse's veins by what is known as the living test tube' experiment. If the jugular vein is ligatured in two places, so as to include a quantity of blood within it, then removed from the animal and hung in a cool place, the blood will not coagulate for many hours. The corpuscles settle, and the supernatant plasma can be removed with a pipette.

The plasma is alkaline, yellowish in tint, and its specific gravity is about 1026 to 1029.

Its chief constituents may be enumerated as follows:1000 parts of plasma contain

In round numbers plasma contains 10 per cent. of solids, of which 8 per cent. are proteid in nature.

Serum contains the same three classes of constituents-proteids, extractives, and salts. The extractives and salts are the same in the two liquids. The proteids are different, as is shown in the following table:

Proteids of Plasma

Fibrinogen
Serum globulin

Serum albumin

Proteids of Serum

Serum globulin
Serum albumin

Fibrin ferment

The gases of the plasma and serum are small quantities of oxygen, nitrogen, and carbonic acid. The greater part of the oxygen of the blood is combined in the red corpuscles with hæmoglobin; the carbonic acid is chiefly combined as carbonates.

We may now consider one by one the various constituents of the plasma and serum.

A. Proteids.-Fibrinogen.-This is the substance acted on by fibrin ferment. It yields, under this action, an insoluble product called fibrin, and a soluble proteid of the globulin class.

Fibrinogen itself is a globulin. It differs from serum globulin, and may be separated from it by the fact that half-saturation with sodium chloride precipitates it. It coagulates by heat at the low temperature of 56° C.

Serum globulin and serum albumin.-The properties of these substances have been already considered (see Lesson II.).

Fibrin ferment.-Schmidt's method of preparing it is to take serum and add excess of alcohol. This precipitates all the proteids, fibrin ferment included. After some weeks the alcohol is poured off; the serum-globulin and serum-albumin have been by this means rendered insoluble in water; an aqueous extract is, however, found to contain fibrin ferment, which is not so easily coagulated by alcohol as the other proteids.

B. Extractives.-These are non-nitrogenous and nitrogenous. The non-nitrogenous are fats, soaps, cholesterin, and sugar; the nitrogenous are urea (0·02 to 0·04 per cent.), and still smaller quantities of uric acid, creatine, xanthine, and hypoxanthine.

C. Salts. The most abundant salt is sodium chloride: it constitutes between 60 and 90 per cent. of the total mineral matter. Potassium chloride is present in much smaller amount. It constitutes about 4 per cent. of the total ash. The other salts are phosphates and sulphates.

THE WHITE BLOOD CORPUSCLES

Our chemical knowledge of the white corpuscles is small. Their nucleus consists of nuclein, their cell-protoplasm yields proteids belonging to the globulin and nucleo-albumin groups. The nucleo-albumin obtained from them is probably the same thing as fibrin-ferment, or may be the zymogen of that ferment, the addition of a calcium salt converting it into the ferment. The protoplasm of these cells often contains small quantities of fat and glycogen.

it

THE RED BLOOD CORPUSCLES

The red blood corpuscles are much more numerous than the white, averaging in man 5,000,000 per cubic millimetre, or 400 to 500 red to each white corpuscle. The method of enumeration of the corpuscles is described in the Appendix.

They vary in size and structure in different groups of vertebrates. In mammals they are biconcave (except in the camel tribe, where they are biconvex) non-nucleated discs, in man averaging 3200 inch in diameter; during fœtal life nucleated red corpuscles are, however, found. In birds, reptiles, amphibians, and .fishes they are biconvex oval discs with a nucleus: they are largest in the amphibia.

a b

d

e

Water causes the corpuscles to swell up, and dissolves out the red pigment (oxyhæmoglobin), leaving a globular colourless stroma. Salt solution causes the corpuscles to shrink: they become crenated or wrinkled. The action of water and salt solution suggests the existence of a membrane on the surface of the corpuscles through which osmosis. takes place, but the existence of such a membrane is still a matter of discussion. If there is no actual membrane, the outer denser portion of the stroma plays the rôle of one during osmotic phenomena. cent. potash) dissolve the corpuscles. Dilute acids (1 per cent. acetic

FIG. 29.-a-e, successive effects of water on a red blood corpuscle ; f, a red corpuscle crenated by salt solution; g, action of tannin on a red corpuscle.

Dilute alkalis (0-2 per

acid) act like water, and in nucleated corpuscles render the nucleus distinct. Tannic acid causes a discharge of hæmoglobin from the stroma, but this is immediately altered and precipitated. It remains adherent to the stroma as a brown globule, consisting probably of hæmatin. Boracic acid acts similarly, but in nucleated red corpuscles the pigment collects chiefly round the nucleus, which may then be extruded from the corpuscles.

Composition.-1000 parts of red corpuscles contain—

The proteid present appears to be identical with the cell-globulin (nucleo-albumin ?) of white corpuscles. The mineral matter consists chiefly of chlorides of potassium and sodium, and phosphates of calcium and magnesium. In man potassium chloride is more abundant than sodium chloride; this, however, does not hold good for all animals.

Oxygen is contained in combination with the hæmoglobin to form oxyhæmoglobin. The corpuscles also contain a certain amount of carbonic acid (see RESPIRATION, at the end of this lesson).

Hæmoglobin and Oxyhemoglobin. The pigment is by far the most abundant and important of the constituents of the red corpuscles. It is a substance which gives the reactions of a proteid, but differs from other proteids in containing the element iron, and in being crystallisable.1

1 Hæmoglobin and oxyhæmoglobin are both crystallisable; like other proteids they have enormously large molecules, and so are indiffusible. Though crystalline they are therefore not crystalloid in Graham's sense of that term (see p. 20). Blood pigment is, however, not the only crystallisable proteid. Long ago crystals of proteid (globulin or vitellin) were observed in the aleurone grains of many seeds, and in the similar proteid occurring in the egg-yolk of some fishes and amphibians. By appropriate methods these have been separated and recrystallised. The crystals do not appear to be pure proteid, but compounds with some inorganic substance like lime or magnesia. Further egg-albumin itself has been crystallised. If a solution of white of egg is diluted with half its volume of saturated solution of ammonium sulphate, the globulin present is precipitated, and is removed by filtration. The filtrate is now allowed to remain some days at the temperature of the air, and as it becomes more concentrated from evaporation, minute spheroidal globules of varying size, and finally minute needles, either aggregated or separate, make their appearance. Whether these are pure egg-albumin or compounds of egg-albumin with the ammonium sulphate is at present a matter of dispute.

It exists in the blood in two conditions: in arterial blood it is combined loosely with oxygen, is of a bright red colour, and is called oxyhæmoglobin; the other condition is the deoxygenated or reduced hæmoglobin (better called simply hæmoglobin). This is found in the blood after asphyxia. It also occurs in all venous blood-—that is, blood which is returning to the heart after it has supplied the tissues with oxygen. Venous blood, however, always contains a considerable quantity of oxyhæmoglobin also. Hæmoglobin is the oxygen-carrier of the body, and it may be called a respiratory pigment.

Crystals of oxyhæmoglobin may be obtained with readiness from the blood of such animals as the rat, guinea-pig, or dog; with difficulty from other animals such as man, ape, and most of the common mammals. The following methods are the best :

1. Mix a drop of defibrinated blood of the rat on a slide with a drop of water; put on a cover glass; in a few minutes the corpuscles are rendered colourless, and then the oxy

hæmoglobin crystallises out from the solution so formed.

2. Microscopical preparations may also be made by Stein's method, which consists in using Canada balsam instead of water in the above experiment.

3. On a larger scale the crystals may be obtained by mixing the blood with one-sixteenth of its volume of ether; the corpuscles dissolve and the blood assumes a laky appearance. After a period, varying from a few minutes to days, abundant crystals are deposited.

The accompanying figures represent the form of the crystals so obtained.

2

I

FIG. 30.-Oxyhæmoglobin crystals magnified: 1, from human blood; 2, from the guinea-pig; 3, squirrel; 4, hamster.

In nearly all animals the crystals are rhombic prisms; in the guinea-pig they are rhombic tetrahedra (four-sided pyramids); in the squirrel, hexagonal plates; in the hamster, rhombohedra and hexagonal plates.

The crystals also contain a varying amount of water of crystallisation this may in part explain their different crystalline form and solubilities.

Different observers have analysed hæmoglobin. They find carbon, hydrogen, nitrogen, oxygen, sulphur, and iron. The percentage of