Thus I must chiefly limit my remarks on Neurotics to defining _what_ their action is, finding it impossible to state with certainty _how_ they act. And the field of inquiry being so limited, it follows that there is much less to be said about them than had to be said of Hæmatics. Nearly all the powerful poisons that act after passage into the blood, belong to this class. Their action in most points of view is such as completely to exceed our means of comprehension. Sudden death may be produced by it. But there is no apparent cause for this. We find no erosion or perforation of the coats of the stomach or intestines; no mechanical disorganization of the tissues, or chemical change in them; no hæmorrhage, or vascular disease; no rupture of nerve-fibres. Whence, then, could death have arisen? How could the mere presence of a few atoms in the blood,--half a grain, one-twentieth, or even one-fiftieth of a grain--how could this apparently contemptible influence have produced so essential a derangement of the vital functions as to stop them altogether? It is impossible to answer. The action of such remedies in the sudden causation or alleviation of nervous symptoms, when applied in the cure of disease, is equally wonderful. How are we to account for their different actions on different nerves? How is it that Opium contracts the pupil, and Belladonna dilates it?--that Digitalis affects the heart, and Stramonium the respiration?--that Prussic acid will cause convulsions, and Hyoscyamus delirium? In what way are these various operations brought to pass? It is impossible to answer.