_m. p. 3._--That it is necessary that they should pass from the blood to that part of the nervous system which is influenced by them.
_m. p. 4._--That they are of use in an over-excited or depressed state of the nervous system.
_m. p. 5._--That they are transitory in action, and cannot remain in the blood.
Some space and labour was required in the proof of the minor propositions relating to Hæmatics, for among them were included some things that are not universally acknowledged; but the above account of the action of Neurotics concerns matters that are very generally admitted, and will not occupy us nearly so long in its discussion.
In the first place it is affirmed that Neurotic medicines pass into the blood. In the consideration of Prop. II., it was shown that all of them,--whether vegetable alkaloids, volatile oils, resins, or mineral substances,--were more or less capable of being absorbed. That they do pass into the blood is proved by the fact that many of them have been detected there, as well as found in the secretions into which they must have entered from the blood. Ammonia, Hydrocyanic acid, Antimony, Assaf[oe]tida, Turpentine, Alcohol, and Camphor,--all of which are Neurotic agents,--have been chemically detected in the blood by Tiedemann and Gmelin. A still larger number have been discovered in the urine by Wöhler, particularly of volatile oils and odorous principles. The active principles of Opium, Belladonna, Stramonium, and Henbane, have been detected in the same secretion by others.
Thus Neurotics pass into the blood. That they act after this absorption, and not by contact with the mucous surface, was proved in the consideration of Prop. I.
The second minor proposition is borne out even by the names by which the recognised action of these medicines is distinguished. Considered as a class, they have no action on the blood, but their influence is quickly and obviously exerted on the nervous system, or on the vital functions which are universally attributed to that part of the animal frame. Stimulants are so called because they are found by experience to exalt nervous force; Sedatives, because they depress it. Thus Ammonia is given to prevent Syncope, caused by a weakness or failure in power of the nerves of the heart; and Aconite is prescribed to relieve pain caused by an over-excitement of the sensory nerves. Though in these examples Ammonia acts on the centre, and Aconite on the peripheral nerves, yet they are both capable of exalting or of depressing nervous force in general. But neither Stimulants nor Sedatives, as defined by me, exert any marked influence on the intellectual part of the brain. They do not affect the phenomena of mind, or of the four special senses which are immediately associated with it. This influence is confined to the intermediate division of Nerve-medicines, called Narcotics. Their general action is evidenced by a short or long primary stimulation,--and a subsequent depression of nervous force, which is also of variable intensity. At the same time they affect the mind in various ways; exciting it, confusing it or lulling it. Opium and Alcohol are examples of Narcotics. These distinctions are in most cases obvious and well defined.[42]