This name is given to the clear fluid which oozes out of the clot of plasma. It only differs from the latter in its chemical composition in so far that fibrin is separated from it. Though chemically this is a slight difference, it signifies the change from a complex living body (blood plasma) into a solution of dead albumins, etc.

Serum is a clear, straw-colored, alkaline fluid of 1028-1030 sp. gr., holding in solution different organic substances and some inorganic salts. After a full meal the serum is said to be slightly milky, from the presence of finely divided fat.

It contains about 9 per cent, of solid matters, of which a large proportion, 7 per cent., are proteids. Of these the most abundant is (1) serum albumin (about 4 per cent, in man), a solution of which becomes opaque at 6o° C, and coagulates at a heat of 730 -750 C. The proteid next in importance is (2) serum globulin or paraglobulin (about 3 per cent, in man), which has already been mentioned. It may be precipitated imperfectly by C02, or completely by magnesium sulphate. (3) Serum casein has been obtained from serum by careful neutralization with acetic acid after the removal of the serum globulin by C02. This is said to be serum globulin which has failed to come down with the C02. (4) Neutral fats in a state of fine subdivision are present in a variable quantity: also (5) lecithin; (6) traces of sugar; (7) various products of tissue change - kreatin, urea, etc.; and (8) inorganic salts, viz., sodium chloride, about 5 per cent., and sodium carbonate, which probably existed in the blood as sodium hydric carbonate. There is also a small quantity of potassium chloride. But it should be remembered that about ten times more sodium than potassium salts exist in the serum, and probably in the blood plasma.