"We will now suppose that the products of ahsorption and hepatic metaholism have entered the blood. The peculiar relations which the blood hears to the solid organs gives a special character to its pathology and therapeutics. It possesses of itself no active functions, but is simply a great fluid medium which conveys nutrient material and oxygen to the tissues, and carries away the products of their activity. In the same way it is the medium by which the active principles of drugs reach the internal organs, without, as a rule, materially disturbing the functions of the blood itself. It is not surprising that the blood should have comparatively few primary disorders, whilst it is constantly liable to suffer in consequence of disease of the digestive organs from which we have traced its supply, and of the excreting organs by which its constituents finally leave the body.

I. Physiological Relations

The physiological relations of the liquor sanguinis are very obvious: it is the medium of nutrition. It carries between the different organs the materials which are the sources of energy, namely albumins, fats, sugar, water, and salts, as well as the products of the vital processes-carbonic acid, water, urea, salts, and other substances. It possasses a mean volume, an alkaline reaction depending on the presence chiefly of salts of soda, and a certain general uniformity of composition, which, however, varies considerably at different parts of the circulation-for instance, before and after exposure of the blood to the liver, lungs, muscles, or other active organs. The composition of the liquor sanguinis is indeed the balance of two opposed processes-a process of supply, income, or ingestion, which we have traced through the liver from the food; and a process of production, expenditure, or egestion, carried on by the active organs of the body, with their measurable products, energy and excretions. The white corpuscles are physiologically associated with the plasma, that is, are essentially nutritive, in function; but are probably also the source of the red corpuscles.

The function of the red corpuscles is perfectly distinct from the functions of the plasma. They are the great medium of respiration, carrying oxygen from the lungs to the tissues, and are thus the respiratory elements of the body. It is important for the therapeutist to remember that the red corpuscles consist chiefly of haemoglobin, with a small quantity of salts, which have potassium as their principal base united with phosphoric acid. Iron is an essential component of haemoglobin (C600H960N194 FeS3O179). Whatever may be the immediate source of the red corpuscles, there can be no doubt that the most important factors in their development are food, air, and free exposure of the blood to light. Ultimately they are broken up, their products forming the colouring matters of the various secretions.

II. Pharmacodynamics

1. Our power over the blood plasma in health is easily appreciated. The most obvious means of influencing it is through the income or supply. We can alter a man's diet, his digestion, and his hepatic functions, and by these indirect means we retain a hold on the vital fluid. We can also modify its several constituents during their ingestion-the albumen, sugar, water, phosphates, carbonates, chlorides, sulphates, etc.-by regulating the food or administering them in the form of drugs. A fact of great therapeutical importance is that we can increase, within certain limits, the alkalinity of the plasma by means of alkalies or alkaline earths, given as the Bicarbonates of Potash or Soda, as the various solutions of these or of Lithia, Lime, and Magnesia; or in a more moderate degree over a longer period, by means of the many natural alkaline waters, such as those of Vichy, Carlsbad, Baden Baden, Ems, and Bilin. Alkalinisers of the blood act upon the plasma not only directly, but indirectly by combining with uric acid, and carrying it with them out of the system by virtue of their diuretic influence. Potash is the most rapid and evanescent alkaliniser; Soda is slower and more permanent, as is fully described in Part I. The citrates and tartrates are also alkalinisers of the blood, being decomposed, as we shall presently see, in the presence of the red corpuscles, into alkaline carbonates. It is much more difficult to reduce the natural alkalinity of the blood. Mineral Acids have very little effect in this direction, as they enter the blood in the form of neutral salts of potash, soda, etc., which pass out undecomposed. Citric and Tartaric Acids remain partly unchanged in the plasma, and Benzoic, Cinnamic, and Salicylic Acids also pass through it, the two first being partly converted into hippuric acid. Free Iodine may be temporarily liberated in the plasma from the iodides.

Besides these, most of the materiae medicae enter the system through the plasma, where they exist in every possible form, whether unchanged, or as albuminates, chlorides, sulphates, etc., or as highly complex compounds. It is most important, however, for the student to observe that, beyond the alkalies and acids, hut few drugs act upon the plasma. The great majority of them simply exist in it, and are conveyed by it to the tissues and organs of elimination, where they exert their specific influence.

But we may go beyond this, and alter the total amount of blood or plasma in the body by actually adding to it from the blood of another person or animal. This is done by transfusion, a powerful means of restoring the blood, but one which is not always readily available.

2. We can affect the value of the plasma through the expenditure or egesta. We have seen that purgation is a ready means of influencing the water, salts, albumen, and other constituents of the plasma in the portal system, and thus in the blood generally. We shall find in subsequent chapters that in the same way we can stimulate excretion by the kidneys and by the skin. We shall also discover, under the head of metabolism, that we can so far either tax or spare the great organs which are the source of vital energy and therefore of waste, such as the muscles, and thus the metabolic and nutritive value of the whole blood. But we can go much farther than this: we can actually abstract a certain quantity of blood by venesection, cupping, or leeching, as we have already seen in the case of the portal vein; and such alteration in quantity will cause a decided alteration in quality, for, as we shall find in chapter x (The Circulatory System)., abstraction of blood increases the amount of water in the plasma.