In the disease called anemia the number of red corpuscles is greatly diminished.
Plethora is the term applied to the opposite condition, in which the number is greatly in excess of the usual standard.
Coagulation of the blood be drawn from an animal into a vessel surrounded by a freezing-mixture it remains liquid. The corpuscles, being heavier than the fluid in which they float, fall to the bottom, and thus the blood is separated into the corpuscles at the bottom of the vessel, and the plasma or liquor sanguinis above. If the blood were kept in the fluid state for a sufficient time the difference in the layers of plasma and corpuscles would be marked by the great difference in colour, the plasma being almost colourless, and the layer of cor puscles of a deep red. If, then, the cold were withdrawn the blood would speedily cease to be fluid, and would become a coagulated or clotted mass, the solidified plasma having the appearance of a clear jelly with a yellowish tinge. The cause of the clotting is easily seen if one takes a drop of blood on a slide, and waits for a minute or two before putting on a cover glass and examining it. In two or three minutes the drop of blood on the slide forms a clot. Ou covering it with a covering glass, and exa mining with the microscope, fine glassy fibres are seen forming an irregular net-work, in the meshes of which lie the corpuscles. It is the formation of these fibres that has caused the setting of the drop of blood. The substance thus formed from blood, after it is withdrawn from the vessels, is called fibrin. It consists of white structureless filaments or threads. It does not seem to exist in the blood as fibrin, as we shall see, but is formed after the blood is drawn, or after contact of the blood with foreign bodies. Fibrin is formed very quickly in blood removed from the blood-vessels, but its forma tion may be delayed by cold, as we have already seen, or by the addition to the blood of certain salts, common salt, for example. Fibrin may be separated from blood, before coagulation takes place, by whipping the blood with twigs. The fibrin forms on the twigs, from which it. can be washed off, when it appears as a white stringy substance. The blood left behind will no longer coagulate, because the fibrin has been removed.
Suppose now that blood be drawn into a tall glass vessel, no precautions as to the main tenance of a low temperature being taken, in from five to ten minutes coagulation takes place throughout, and a sort of firm red jelly is formed. The corpuscles, not having time to &ink to the bottom, are entangled in the meshes of the fibrin, and the whole mass is red in colour. The clot takes the shape of the vessel in which it is contained. If a little delay has taken place iu coagulation the corpuscles have sunk to some extent, and the clot will have a deeper colour towards the deeper parts, the bottom layer being deepest of all. The white blood corpuscles are,
however, lighter than the red. They do not sink so fast, and are, therefore, entangled to wards the surface of the clot, giving it a whitish or creamy look on the top. This used to be called the huffy coat. Where the coagulation has been very quick, the separation has no time to be effected, and the huffy coat is absent. If the clot be left alone in the glass vessel, by and by other changes take place. The fibrin, whose formation has caused the coagulation, begins to shrink. As the clot is attached to the sides of the vessel the shrinking is more pronounced towards the 'centre, and thus the surface of the clot gets hollowed or cupped, as it is called. The shrinking of the clot squeezes out a clear yellowish fluid, which soon separates the clot from the sides of the vessel, and thus we have the clot floating in a fluid of a straw. yellow colour. This straw-yellow fluid is serum, Now let us distinguish between serum and plasma. In blood as drawn from an animal we have corpuscles, and plasma or liquor san guinis. By coagulation the plasma is separated into fibrin and a fluid—the serum. Thus blood, less its corpuscles, is plasma or liquor sanguinis; and plasma, less its fibrin, is serum. This dif ference may be represented in the fallowing way :— It was formerly supposed that the fluid part of the blood held fibrin in solution, and that, when coagulation took place, the fibrin was pre cipitated or became solid, as in well-known chemical reactions, or that it became solid in consequence of something escaping from the blood, which held it in solution. The view DOW held is that fibrin does not exist as such in the blood, but that a substance called fibrinogen exists, from which fibrin is formed by the action of a ferment in the presence of lime salts. Why the changes do not readily occur in the blood in the vessels of a living animal has not yet been satisfactorily deter mined. In certain diseased conditions, how. ever, they do occur. Thus foreign bodies in troduced into the current of the circulation— a thread drawn through a blood-vessel and left there, for example—soon become covered with a layer of fibrin. It is well knowb, also, that a deposit of fibrin may readily occur on a part of the surface of a blood-vessel, or of the valves of the heart, that has been roughened by inflammation. When a clot is thus formed, either in the heart or vessels, it is called a thrombus. The risk of such an occurrence is great, and it is specially so when the deposit is on the edges of one of the valves of the heart, for a part of the clot may be detached and whirled away in the current of blood till a vessel is reached too small to give it passage. The clot blocks the vessel, and thus the area which it supplied with blood is either perma nmtly or temporarily deprived of its supply. In such a case the clot is called an embolus, and the occurrence embolism.