Drugs sometimes seem to affect only one part of the body and to leave the other organs unaffected; although the drugs may be carried equally by the blood to every part of the body, they appear to combine with some and not with others. Manv dve-stuffs will not attach themselves to cotton fabrics, but will do so readily to wool or silk; and we find that different tissues, and even different parts of the same tissue, have very unequal attractions for stains: thus some anilin colours will deeply stain a nucleus, while leaving the cell in which it is contained entirely uncoloured. Although the different organs of the body contain many substances in common, yet their chemical composition varies within wide limits, and the products of the tissue-waste are also different. Even in the same organs the cells may have different properties, and even individual parts of the same cell may differ. Some have a reducing, and others an oxidising action; some an alkaline, and others - as may be ascertained from their action on anilin colours1 - an acid, reaction (p. 70). We would therefore expect that, just as the tissues exert a selective action upon dye-stuffs which we are able to see, they will also have a selective action on many organic substances, although this action may not be visible to our senses.

1 Hermann, Arch. f. Anat. u. Physiol, 1867, 64, 650.