Here another step is made in the explanation of the action of medicines. They are said to have particular tendencies towards certain parts of the body, over which parts they exert a peculiar and special influence. It is thought that we shall make an advance in our knowledge of the subject, if we can discover what these tendencies are. An arrangement of medicines may be made accordingly. It is certainly more scientific than a merely empirical arrangement; and it will be so far of use that it will enable us, when we wish to make an impression on a certain organ or set of organs, to select those medicines which especially influence it or them. There is no doubt whatever of the existence of these local tendencies. There is no doubt that some medicines, such as Iodine, Bromine, Mercury, and Iron, tend especially to affect the blood and the blood-making organs, as the liver and spleen, rather than to act on the nervous or glandular systems. That some tend particularly towards the nerves, and prefer individually different parts of the nervous system, as Opium acts on the brain, Aconite on the superficial sensory nerves, Digitalis on the organic nerves of the heart, and Stramonium on those of the lungs. Again, it is evident that some medicines tend to act on the organs of secretion; and of these, particular sets select particular glands, as Diuretics the kidneys, Diaphoretics the glands of the skin, and Purgatives those of the bowels. There may be disagreements on minor points, but there can be no dispute as to the fact of the existence of these local partialities.[7]