Blood is a tissue of which the red corpuscles are the essential and active elements, while the plasma is the liquid matrix. There are two kinds of corpuscles, the white and the red. The protoplasm of the white corpuscles is native indifferentiated protoplasm, in no respect fitted for any special duty, as far as we know at present. The white corpuscles are in reality embryonic structures, concerned chiefly in the production of other forms, such as red corpuscles, and it may be under certain conditions various elements of the other tissues. The red corpuscles have a definite respiratory function. But these form a part only of the blood. The largest portion of the blood, the whole mass of the plasma, is an unorganized fluid with no proper physiological (vital) properties of its own. Its function is to serve as the great medium of exchange between all the tissues of the body. Just as the whole organism lives on the things around it, its air and its food, so the several tissues live on the complex fluid by which they are all bathed and which is to them their immediate air and food. Blood within the living vessel is a fluid; but when shed, or after the death of the vessels, becomes solid by the process known as coagulation. The average specific gravity of human blood is 1056, varying from 1045 to 1075 within the limits of health. It has an alkaline reaction, which in shed blood rapidly diminishes up to the onset of coagulation. Blood may, in general terms, be considered as consisting by weight of more than one-third and less than one-half of corpuscles, the rest being plasma, the corpuscles being supposed to retain the amount of water proper to them. Human blood: corpuscles 513, plasma 487. The average quantity of fibrine in the human blood is said to be two per cent. Composition of serum: In 100 parts there are in round numbers: Water, 90 parts. Proteid substance, 8 to 9   ,,   Fat extractives and saline matter, 2 to 1   ,,   Of the proteid substances the great mass consists of the so-called serum-albumen. Composition of red corpuscles: The red corpuscles contain less water than the serum. In 100 parts of red corpuscle there are: Water, 56.5 Solid, 43.5 The solids are almost entirely organic matter, the inorganic salts in the corpuscles amounting to less than 4 per cent. In 100 parts of dried organic matter of the corpuscles of human blood there are: Hæmoglobin, 90.54 Proteid substance, 8.67 Lecithin, .54 Cholestrin, .25 The blood is distributed as follows in round numbers: