DRUGS WHICH ACT ON THE BLOOD. 447 



for the therapeutist to remember that the red corpuscles consist 

 chiefly of hemoglobin, with a small quantity of salts, which 

 have potassium as their principal base united with phosphoric acid. 

 Iron is an essential component of haemoglobin (C 600 H 9GO N 15 4 

 FeS 3 Oi7 9 ). 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 pro- 

 ducts forming the colouring matters of the various secretions. 



H. PHARMACODYNAMICS. 



1. Our power over the blood plasma in health is easily appre- 

 ciated. 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 regu- 

 lating 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 medicse enter the 

 system through the plasma, where they exist in every possible 

 form, whether unchanged, or as albuminates, chlorides, sulphates, 



