We have seen in the last chapter (XIX, 3) the law by which the nerves become stimulated, more especially by the yellow color, and to some extent by the orange and even the red, these principles being included in the substance of the nerves themselves. We have seen that the more violent nerve stimuli include something of the red or orange as well as the yellow, that drugs taken internally, when sufficiently active and exciting and working, no doubt, to some extent upon the vascular as well as the nervous tissues of the stomach, cause that quick repulsive action which is termed EMETIC; that those drugs whose yellow principle works somewhat more slowly, do not exert their expansive and repulsive action until they reach the bowels and thus constitute LAXATIVES, or when sufficiently active, PURGATIVES; that certain drugs which have an affinity for the liver and bile, causing them to act, are called CHOLAGOGUES; that those which stimulate the kidneys are called DIURETICS; those which stimulate the uterus, from some special affinity they may have for that organ, are called EMMENAGOGUES; those which stimulate the nerves of the skin and to some extent the vascular glands in a way to cause perspiration are called DIAPHORETICS; those which stimulate the nerves of the skin and call the blood outward until the surface becomes reddened are called RUBEFACIENTS. In all cases yellow is the central principle of nerve stimulus as well as the exciting principle of the brain which is the fountain head of the nerves, although, as we have seen, the more violent elements of stimulus approach the red, especially where vascular action is called forth. Those elements which act more directly to excite the brain, are called CEREBRAL STIMULANTS. I will give a few examples of the different drugs and foods which belong to the various departments of nerve action.
