The actions of individual drugs upon the circulatory, respiratory, and excretory systems differ so considerably that a small section has been devoted to this subject in each of the chapters devoted to nitrous oxide, ether, and chloroform respectively. One feature is, however, dependent upon the _state of anæsthesia_ rather than the action of the particular drug, and that is a certain slight fall of blood pressure. This phenomenon is seen even in natural sleep, and is presumably due simply to lack of normal stimuli such as tactile, visual, and auditory impressions which in the ordinary circumstances of life, help to maintain the tone of the vasomotor system. That such a fall is due to the _state_ of anæsthesia admits of little doubt, but the fact is not always easy to demonstrate since each of the drugs themselves have a marked influence upon the B.P., which masks the pure effect of the anæsthetic sleep. Action upon the Nervous System. It is in this system, of course, that we look for the characteristic action of anæsthetics, since if we had a choice, it is the brain only which we should desire to influence by our drug. It used to be said that anæsthetics paralyse the brain from above downwards, but that is only approximately true. More correctly we may say that the more highly developed parts of the brain are earliest affected, and that those portions, such as the vital medullary centres, which man shares in common with his humbler zoological relatives, maintain their activity until the last. Moreover, it must be remembered that before any brain centre succumbs, it passes through a preliminary stage of _excitement_, varying in intensity with varying drugs and also with different types of patients. Those who are accustomed to administer to their nervous centres repeated large doses of such nerve poisons as alcohol and tobacco, may show very evident signs of this preliminary cerebral irritation during the process of induction of anæsthesia; so do also the unhappy possessors of nervous systems deranged from other causes such as epilepsy.