We have now concluded the subject of Hæmatic medicines. I have said that this class has been very generally neglected and overlooked by writers on the subject. For this reason, and because there are some ideas respecting their action which I have thought it worth while to work out and to investigate at some length, I have devoted more space to its elaboration than I shall be able to spare for either of the remaining classes, in the consideration of which we are not likely to encounter so many interesting and debateable points. PROP. VIII.--_That a second class of medicines, called_ NEUROTICS, _act by passing from the blood to the nerves or nerve-centres, which they influence._ 1. _That of these, some, called_ STIMULANTS, _act so as to exalt nervous force in general or in particular._ 2. _That others, called_ NARCOTICS, _act so as first to exalt nervous force, and then to depress it; and have also a special influence on the intellectual part of the brain._ 3. _That others again, called_ SEDATIVES, _act so as to depress nervous force, in general or in particular._ Having investigated at some length the action of Blood-medicines, we now commence the consideration of another class of remedies, which differ very widely from the last in their mode of operation. The action of Hæmatics is slow, but more or less durable, because it is evidenced in the blood; and a change in the blood produced by this action continues for a longer or a shorter time. The action of Neurotics, or Nerve-medicines, is rapid; but it is transient, and is soon over. A Neurotic medicine does not cause any change in the blood; and it cannot remain in it, but soon passes out. It acts by contact with nerve, apparently producing no lasting change even in nerve-fibre; and as the cause of the action cannot remain, the effect also soon passes away. Whereas Hæmatics, durable agents in the blood, are used to counteract the causes of inveterate and chronic disorders; these Neurotics, which produce a transitory, but more or less forcible impression on the nervous system, are employed to rouse it when torpid, or to depress it when over-excited. Rarely of use in chronic blood-disorders, they are given mostly in the temporary emergencies of acute diseases. But it is obvious that even a temporary agent may prove of permanent efficacy by remedying a temporary emergency. It was feasible to attempt some explanation of the actions of medicines in the blood, occasionally more or less analogous to known chemical influences. But the agency of nerve-medicines is of a far more incomprehensible kind. When we consider that little or nothing is known, or can be known, about the ultimate causes of sensation, or motion, or nervous excitement, there is no need for wonder that we find ourselves at a loss to explain the operation of medicines that influence these conditions.