MEDICINAL INFLUENCE OVER METABOLISM. 457 



"blood, and allow them to participate in the vital processes. 

 Thus, if sujh foreign substances as Mercury or Arsenic be in- 

 troduced into the blood, the muscular and other tissues will 

 take them into their substance, just as they take up proteids, 

 salts of lime, and water, and incorporate them in a loose 

 chemical way, their own proper composition being essentially 

 unaltered. By whatever channel they may be introduced into 

 the blood, most of the active principles of the materia medica 

 are carried in the plasma to the tissues and organs, and are said 

 to " act upon" or to "have a specific action " upon them. Thus, 

 Iodine acts upon the glands, Bromine upon the brain, Potash on 

 the heart, and so on. By this expression we mean that the medi- 

 cines having reached an organ take part in the process of meta- 

 bolism; that they become loosely incorporated with the ana- 

 tomical elements of the part ; that they form, either in these, or 

 in the presence of these, certain chemical compounds with oxy- 

 gen, different from the ordinary ; that they are cast out again 

 in the metabolic products, either unchanged or in a new chemical 

 form ; and that, in thus passing through the organ and taking 

 part in its activity, they have modified the force which it displays. 

 Thus, Alcohol, in passing into muscle, becomes oxydised and 

 converted into carbonic acid and water, and in the process of 

 decomposition increases the force of muscular contraction. 

 Alcohol is accordingly said to act specifically upon muscles. 

 So with all tissues and organs: some incorporate from the 

 blood one substance, some another. Just as the life-processes 

 of the various tissues and organs differ from each other, so will 

 some select or be acted on by some principles, others by other 

 principles. Gland protoplasm is acted upon by Iodine, nervous 

 protoplasm by Bromine, muscle protoplasm by Potash, red 

 corpuscle protoplasm by Iron, and so on. 



Here it is necessary to offer a word of caution. The expres- 

 sion " action " of a medicine is generally used in a much wider 

 sense than that just indicated. When we say that a given 

 therapeutical substance acts upon " an organ," we do not always 

 mean that it acts upon the protoplasm of that organ. When we 

 say that alcohol acts upon the skin, flushing it and increasing 

 its heat and secretion, we do not imply that alcohol is de- 

 composed by the connective tissue-cells of the skin. An organ 

 possesses not only active protoplasmic cells but vessels and 

 nerves ; and a vast number of the effects of drugs upon organs 

 are due, as we shall see in subsequent chapters, to their action 

 upon the vessels and the nerves that supply these organs. 

 Ultimately, of course, all drugs do act upon protoplasm in some 

 form, on the protoplasm of muscular tissue, of nerve-ganglia, 

 of the walls of blood-vessels, or of the cells of the nerve-centres 



