This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
The therapeutics of this class of medicines will be most conveniently treated of under the several individual articles; as there is so much specialty in their uses, that few general observations would be applicable to the whole, or even the greater number. There are, however, a few considerations which it may be proper to present in this place, to spare inconvenient explanations and useless repetition hereafter.
General Stimulation. Among the cerebral stimulants are medicines which are most powerful and most relied on under circumstances calling for the most vigorous support to the depressed or exhausted system. In consequence of the universality of their stimulant power, and especially of their influence over the functions of the brain, the due maintenance of which is essential to life, they are better adapted than any other class of medicines to diseases of debility, in which these functions may participate in the general prostration. The only circumstance which contra-indicatcs their use, and which might preferably direct the attention to other stimulants, is the existence at the moment, or the probable occurrence at the period of reaction, of active congestion or inflammation of the brain.
Relief of Pain, Spasm, etc. Besides general stimulation, they are much employed for the relief of pain, the relaxation of spasm, and the composure of nervous irritation generally. All these effects they produce by rendering the nervous centres insensible to impressions of an irritant character, and incapable of radiating influence from themselves. It must be remembered, however, that, in thus operating, they stimulate the centres instead of depressing them. Their influence comes under the general law already referred to in these preliminary remarks, namely, that irritation of an organ, in its highest grade, diminishes or depresses the function of that organ, probably by overloading it with blood. When the sensorial centres are thus stimulated, the impression sent to them from the suffering part is no longer felt, and pain of course ceases. Or, if the pain originated in disordered function of the centre itself, the feeling is equally abolished; because the function itself is temporarily suppressed. The same explanation applies to spasm and other forms of nervous derangement. If they originate in external irritation transmitted to the sensorium, they are relieved when the sensorium can no longer feel the irritation; if in disorder of the sensorium itself, they cease temporarily with the cessation of its function. But it frequently happens that pain, spasm, and other nervous disorder, originate in a depressed condition of the nervous centre; perhaps in a deficiency of its supply of good blood. The centre, under these circumstances, becomes a negative point towards which nervous force may be supposed to flow from the periphery corresponding with it, which suffers from the loss, and exhibits that suffering in one of the modes alluded to. The cerebral stimulants meet the indication here precisely. They sustain the depressed centre by a direct excitation, and by supplying it with good blood, when this is to be had, and sometimes even when it may happen to be wanting; for they often serve, by stimulating the blood-producing functions, to improve its character, and increase its quantity. But of course the cerebral stimulants are altogether unsuited for the relief of disordered nervous phenomena, when dependent on active congestion or inflammation of the brain itself. No one but a hornoeopathist would think proper to prescribe alcohol or opium in acute cerebritis or recent apoplexy; and the hornoeopathist himself, in doing so, escapes the crime of manslaughter only by the absolute nothingness of the dose.
The term anaesthesia has been applied to the loss of sensation under the influence of narcotic medicines, and anaesthetics to the substances capable of producing it. By some writers the anaesthetics have been raised to the dignity of a class; and if we regard symptoms or obvious effects simply as the basis of classification, such an association would be altogether proper; but I have elsewhere given my reasons for considering such a plan as very defective, of which the strongest is that similar symptoms are often produced by medicines wholly different in their essential mode of action, and a class founded on this basis must often include medicines which are, in other respects, wholly different, and consequently endanger serious errors in practice. Thus, anaesthesia may result from cerebral stimulants or cerebral sedatives, from excess of cold, from mechanical compression, and various other agencies, directly or indirectly, which could not, I think, be thrown together on any just basis of arrangement. After considering, however, the different agents used for their anaesthetic effects in the several classes to which they properly belong, it may not be amiss to enumerate them together, and thus recall them to the mind of the student in relation to their very important practical purpose; and I shall probably pursue this course before dismissing their consideration altogether.
Production of Sleep. This is another purpose which some of the cerebral stimulants are, beyond all other medicines, calculated to fulfil. They produce the effect, in all probability, simply by so congesting the sensorial centres as temporarily to suspend their sensibility to impressions, and their power of action. Sleep necessarily ensues, and is more or less profound, according as the centres are more or less deeply affected. It will be remembered that the congestion is, in these cases, merely the result of a stimulation of the centres inviting the blood into them, and may occupy, therefore, only the minute portions of cerebral matter of which the proper centres may possibly consist; differing altogether from the universal congestion caused by forces driving the blood into the brain, or by obstruction preventing its return. Sleep may equally be produced by influences on these same centres, depressing them below the point of impressibility or of action, as is probably the case with chloroform. This, too, is probably the source of sleep in health; the centres ceasing for a time to act, under the loss of excitability, temporarily exhausted by their activity in the waking state.
But several of the cerebral stimulants produce the therapeutic effects referred to in the last two paragraphs, in the manner of the nervous stimulants or antispasmodics, and in doses too small to give rise to their characteristic effects upon the brain. Pain, spasm, etc. are thus relieved by them, not by rendering the nervous centres insensible to impression, but probably by equalizing the excitement, the disturbance in the balance of which has occasioned the disorder. In producing sleep, their effect, thus given, is wholly different from their ordinary and characteristic operation. It is only when rest is prevented by nervous disturbance, that they are capable of acting in these small doses. The patient sleeps because the disorder which interrupted his rest has been relieved, not from the direct impression of the medicine on his nervous centres. A little paregoric, two or three grains of camphor in solution, or half a teaspoonful of Hoffmann's anodyne; doses which would have scarcely an observable effect in health, will often have this composing influence in disease. Different names have been conferred upon articles of this class, expressive of certain effects produced by them; as narcotics because they stupefy, anodynes because they relieve pain, anaesthetics from producing insensibility in general, and soporifics or hypnotics from causing sleep.