_Blood plasma_, when obtained free from corpuscles, is perfectly colorless, in thin layers, for example, in microscopical preparation; when seen in large quantities it shows a slightly yellowish tint. The red color of the blood is not due, therefore, to coloration of the blood plasma, but is caused by the mass of red corpuscles held in suspension in the liquid. The proportion by bulk of plasma to corpuscles is usually given roughly as two to one. The blood plasma is composed of two substances, blood serum and blood fibrin. You have noticed that blood, after it has escaped from the vessels, usually clots or coagulates. The clot, as it forms, gradually shrinks and squeezes out a clear liquid, to which the name blood serum has been given. Serum resembles the plasma of normal blood in general appearance, but differs from it in composition. Here it is sufficient to say that blood serum is the liquid part of the blood after coagulation has taken place. You can prepare this experiment for yourself: If shed blood is whipped vigorously with a rod or some similar object while it is clotting, the essential part of the clot, namely the fibrin, forms differently from what it does when the blood is allowed to coagulate quietly. It is deposited in shreds on the whipper. Blood that has been treated in this way is known as defibrinated blood. It consists of blood serum plus the red and white corpuscles, and as far as appearances go it resembles exactly the normal blood; it has lost, however, its power of clotting. _Red blood corpuscles_ are bi-concave, circular disks, without nuclei; their average diameter is 7.7 microns (1 micron equals 1-25,000 of an inch); their number, which is usually reckoned as so many to a cu. millimeter, varies greatly under different conditions of health and disease. The average number is given as 5,600,000 per cubic millimeter for males and 4,500,000 per cubic millimeter for females. The number of red corpuscles also varies in individuals with the constitution, nutrition and manner of life. It varies with age, being greatest in the fetus and in the new-born child. It varies with the time of the day, showing a distinct diminution after meals. In the female it varies somewhat with menstruation and pregnancy, being slightly increased in the former and diminished in the latter condition.