All the varieties in action of Sedative medicines may be accounted for by considering that they may either derange or destroy the nervous forces. In the case of each set of nerves in the body we may distinguish an action of derangement, and an action of destruction, both producible by Sedative medicines. Thus by an action on the motor nerves of the cerebro-spinal system, convulsions or paralysis may be produced. By an influence on the nerves of sensation; pain, or anæsthesia. By an affection of the organic nerves of the heart, or of the brain which controls them; palpitation, or syncope. By an action on the nerves of the lungs; cough, or apn[oe]a. By the exertion of a sedative power over those of the stomach, nausea may result, or vomiting. These various symptoms are all brought about by Sedative medicines, but in each case the first effect is referrible to a derangement, the second to a loss of nervous power. In every instance there is an impairment of natural nervous force. Narcotics act on the mind. They cause death, with stupor or delirium. Sedatives act on the organic function of the brain, which is necessary to life, but they do not affect the mind. In poisonous doses, they kill by producing syncope, which is a suspension of the action of the heart. Hydrocyanic acid causes at the same time convulsions. These convulsions appear to be caused by a derangement of the nervous polarity of the spinal cord; and not by a stimulation of this centre, as the tonic spasm of Strychnia. (_Vide_ page 232.) Sedatives are divided into two orders, according to the extent of their action; in the same way as Stimulants have been divided. SEDANTIA. Ord. 1. Sedantia Generalia. Ord. 2. Sedantia Specifica. General Sedatives have a direct action upon all the nerves and nerve-centres in the body, the result of which is a diminution of nervous force. Special Sedatives exert the same depressing action upon particular nerves only. Like the order of Special Stimulants, they are peculiar and exceptional agents. All those with which we are acquainted appear to direct their action to the branches of the Vagus nerve. It is among General Sedatives that the types or representatives of this division are to be found. The definition of this order does not admit of so great a variety in action as must be allowed to Narcotics. A Stimulant medicine simply exalts or increases nervous force; and a Sedative simply depresses the same. But a Narcotic first does one thing and then the other; and according to the degrees of these two actions, so do some Narcotics resemble Stimulants, and others approach very nearly to Sedatives. But we have seen that Narcotics have also a peculiar and mysterious action on the mind, which action is of three separate kinds. And the same three orders which are named according to their respective actions on the mind, are found to coincide with three stages of transition from the action of a Stimulant to that of a Sedative medicine.