Blood is red, opaque, and is, in the fowl, quite viscid. The exact tint of the blood depends on whether it is drawn from an artery or from a vein. Blood from a vein has a purplish tinge while that from an artery is a bright scarlet. The color of blood is largely due to pigment in the erythrocyte, called hemoglobin. The _reaction of the blood_ is alkaline, due to the phosphate and the bicarbonate of soda. The alkalinity of the blood is reduced by work. This is due to the formation of sarcolactic acid in the muscle. The odor of blood is due to volatile fatty acids. Each kind of fowl has its own peculiar odor. The taste of the blood is saltish, due to a small amount of sodium chlorid it contains. The blood consists of the following substances: First, the unorganized part, or fluid, the liquor _sanguinis_ or _plasma_. It contains in solution proteids, extractives, mineral matter, and gases. The gases are held in loose chemical union. The liquor sanguinis constitutes fully 66 per cent. of the volume of the blood. It is albuminous in nature and contains a small amount of coloring matter of a fatty nature. It holds in solution three proteids—fibrinogen, serum globulin, and serum albumin. Second, the organized parts, or the cellular structure (Fig. 74, Nos. 1 to 21). The cells float in the plasma and consist of three groups: the erythrocytes, or red blood cells, the leucocytes, or white blood cells, and the thrombocytes. =Erythrocytes.=—The average number of red cells (Fig. 74, No. 21) of the domestic fowl range between 3,000,000 and 4,000,000 per cubic millimeter. The red blood cells are flattened and elliptical in shape, and possess an oval elliptical nucleus. The average length is ¹⁄₂₁₀₀ inch and the diameter ¹⁄₃₈₀₀ inch, or 7 to 8 micra in diameter and 12 to 13 micra in length. However the diameters vary in different kinds of birds. The cytoplasm is yellow and glassy, and the nucleus takes basic stains and appears somewhat picnotic. =Thrombocytes.=—The thrombocyte (Fig. 74, No. 19) is of about the same length as the erythrocyte but somewhat narrower. The nucleus is round, stains purple with the Wright’s stain, and the chromatin material is somewhat diffused. The diameter of the nucleus is nearly equal to that of the cell. The cytoplasm is pale and may show vacuoles near the nucleus. They may contain small circumscribed red structures. They vary somewhat in size and shape. There are in the domestic fowl between 45,000 and 55,000 per cubic millimeter. =Leucocytes.=—There are, in the blood of the hen, 28,000 to 35,000 leucocytes per cubic millimeter. The leucocytes may be divided into five distinct types. These are as follows: