Venous blood has a darker hue, and the pigment has lost a part of the loosely combined oxygen present in oxyhemoglobin and has been converted into hemoglobin. This also has a characteristic spectrum.

The blood consists, roughly, of two parts, the corpuscles, or organized elements of the blood, and the plasma, in which the corpuscles float.

An average of human blood places the blood-corpuscles at 48 per cent. by weight, and the serum therefore at 52 per cent.

The blood-corpuscles contain a large amount of iron-containing protein substance. This is oxyhemoglobin, and forms about 40.5 per cent. by weight of the corpuscles.

The blood-plasma is essentially a solution of protein substances, in which also are dissolved some salts. It contains 8.9 per cent. of solid matter, of which 6.9 per cent. is protein in character; 0.84 per cent. of this latter is made up of inorganic salts. The inorganic portion of the serum is chiefly sodium chlorid, although salts of potassium, calcium, and magnesium are also present.

The reaction of the blood is alkaline, due to the carbonates of the alkaline metals. Phosphoric acid is also present, but is combined with the proteins into a complex molecule. The protein substances in the blood-plasına are metaglobulin (fibrinogen), paraglobulin (serum-globulin), and serum-albumin. Metaglobulin, the first mentioned of the proteids, is the most important, as it is the precursor of the fibrin formed in the clotting of the blood.

The corpuscles are of two kinds, red and white. The red blood-corpuscles consist of the hemoglobin and stroma. The stroma contains the same chemical substances as are found in other protoplasmglobulins, lecithin, protagon, and cholesterin. The amount of water contained in the red bloodcorpuscles is about 57.7 per cent. The chemical constitution of the white corpuscles or leukocytes is not known, owing to the difficulty of their isolation. It has been assumed that they do not differ materially in composition from the corpuscles found in the lymph-glands. These contain, besides globulins, lecithin, protagon, and a nucleo-albumin not precipitated by saturated magnesium sulfate solution, called nucleohiston. This substance is acid in character, and contains a relatively large amount of phosphoric acid-4.99 per cent. Glycogen is also found to a small amount in the leukocytes.

The hemoglobins are immensely important, and their relation to one another should be carefully followed out.

The blood-pigments are: 1. Oxyhemoglobin; 2. Hemoglobin; 3. Reduced hemoglobin; 4. Methemoglobin.

Those formed by the combination of chemical reagents with hemoglobin are: 1. Carbon-monoxid hemoglobin; 2. Hydrogen sulfid hemoglobin; 3. Cyanhemoglobin (hydrocyanic acid).

Hemoglobin altered by chemical reagents.
Action of acids-acid hematin.

On reduction

gives a reduced hematin, oxygen-hematoporphyrin,

carbon-hexahydrohematoporphyrin.

Action of alkalies-alkaline hematin.

Action of acids with a chlorid-hemin.

The relation of the different pigments to one another will be seen in the following diagram.

The blood as existing in the blood-vessels is a

fluid of a specific gravity of about 1060.

On flow

ing from the blood-vessels and being allowed to stand for some minutes it suddenly becomes solid. The alkaline reaction diminishes and a slight evolution of heat takes place. This phenomenon is known as coagulation. Among the many theories which have been advanced to account for this sudden change, part of two may be used. In the first, the process is supposed to be caused by a change taking place in a constituent of the bloodplasma. This substance is fibrinogen. In the

second the change is said to be due to a breaking down of the leukocytes. A third factor in coagulation is the presence of a certain amount of calcium salts. All of these may be taken into consideration when seeking for a cause of coagulation.

If fibrinogen be prepared in a pure condition, and dissolved in a slightly alkaline solution of sodium chlorid, on the addition of a small amount of a solution of a calcium salt a typical coagulation takes place. If, on the other hand, a fresh blood be received in a solution of potassium oxalate sufficient to convert all the calcium salts into insoluble calcium oxalate, the spontaneous coagulation.

fluid loses its power of The clotting in blood is in part due to a ferment, called the fibrin-ferment or prothrombin, and which belongs to the class of enzymes. It may be prepared from a solution containing it by precipitation with alcohol, in which it is insoluble. If a solution of this substance be injected intravenously, death speedily takes place from multiple thromboses.

That the disintegration of the leukocytes is also a factor in the coagulation of blood may be shown by allowing blood to flow from an artery into a vessel whose walls are smeared with vaselin. Blood received in this way, and which is protected from the air by being carefully covered, will not coagulate for a much longer time than blood which has been allowed to collect in an ordinary open vessel. In the former case the white. corpuscles retain their form, as will be seen on microscopic examination.

The leukocytes themselves contain a ferment— nucleohiston-which has the property of causing coagulation. This ferment is peculiar in that when used in larger quantities it has the property of not only not causing coagulation, but of actually inhibiting it.

Chemical Reactions of Blood.-Take a sample of blood, and dilute with 10 times its volume of water, and use for the following tests.

I. Reaction to litmus. Saturate a plate made of plaster of paris, by making a cream of the calcium sulfate with water and pouring on a clean glass plate, and allowing to set. Pour on the plate some litmus solution, which should be neutral or very slightly acid. Touch the plate with a drop of fresh blood, wash off the excess of blood, and examine the spot. This test may be performed with litmus-paper, but the reaction is not so well marked.

2. The catalytic action of blood. To 5 c.c. of hydrogen peroxid add 1 c.c. of blood. Note the