Preliminary Considerations.--The blood consists of a fluid of complicated and variable composition, the plasma, in which are suspended great numbers of microscopic structures: viz., red corpuscles, white corpuscles, blood-platelets, and blood-dust. _Red corpuscles_, or _erythrocytes_, are biconcave discs, red when viewed by reflected light or in thick layer, and straw-colored when viewed by transmitted light or in thin layer. They give the blood its red color. They are cells which have been highly differentiated for the purpose of carrying oxygen from the lungs to the tissues. This is accomplished by means of an iron-bearing proteid, hemoglobin, which they contain. In the lungs hemoglobin forms a loose combination with oxygen, which it readily gives up when it reaches the tissues. Normal erythrocytes do not contain nuclei. They are formed from preëxisting nucleated cells in the bone-marrow. _White corpuscles_, or _leukocytes_, are less highly differentiated cells. There are several varieties. They all contain nuclei, and most of them contain granules which vary in size and staining properties. They are formed in the bone-marrow and lymphoid tissues. _Blood-platelets_, or _blood-plaques_, are colorless or slightly bluish, spheric or ovoid bodies, about one-third or one-half the diameter of an erythrocyte. Their structure, nature, and origin have not been definitely determined. {140} The _blood-dust of Müller_ consists of fine granules which have vibratory motion. Little is known of them. It has been suggested that they are granules from disintegrated leukocytes. _Coagulation_ consists essentially in the transformation of fibrinogen, one of the proteins of the blood, into fibrin by means of a ferment derived from disintegration of the leukocytes. The resulting coagulum is made up of a meshwork of fibrin fibrils with entangled corpuscles and plaques. The clear, straw-colored fluid which is left after separation of the coagulum is called _blood-serum_. Normally, coagulation takes place in two to eight minutes after the blood leaves the vessels. It is frequently desirable to determine the coagulation time. The simplest method is to place a drop of blood upon a perfectly clean slide, and to draw a needle through it at half-minute intervals. When the clot is dragged along by the needle, coagulation has taken place. This method is probably sufficient for ordinary clinical work. For very accurate results the method of Russell and Brodie, for which the reader is referred to the larger text-books, is recommended. Coagulation is notably delayed in hemophilia and icterus and after administration of citric acid. It is hastened by administration of calcium chlorid.