This section is from the book "Materia Medica And Therapeutics: An Introduction to the National Treatment of Disease", by John Mitchell Bruce. Also available from Amazon: The pharmacology and therapeutics of the materia medica.
c. All these measures act upon the peripheral structures. The trunks of the afferent nerves may also be affected so as to interfere with the convection of the impressions. Opium, and possibly other drugs, heat and cold, electricity, properly regulated pressure, and section or stretching of the nerves are different means of removing sensibility or at least pain.
d. The sensitive and perceptive centres in the cerebrum may he the seat of action of anaesthetics. Amongst the substances possessing this effect are Opium, Chloral, Chloroform, Ether, and Cannabis Indica, consciousness as a whole being affected by these measures, which are called general anaesthetics, general anodynes, or narcotics-a series of titles which will be presently noticed. Lastly, it will be observed that certain substances, such as Opium, arrest the afferent impressions at every point-at their formation, in the course of their conduction, and where they impinge upon the sensorium, that is, they act upon the sensory tract from one extremity to the other.
e. The special senses also can be directly influenced by various measures, including drugs. Local anaesthetics reduce the keenness of the sense of touch. Deafness and subjective noises are produced by Quinia, Salicylic Acid, and Alcohol. Santonin causes green vision. Taste is excited by a variety of influences which we have already studied; depressed and peculiarly disturbed by Aconite and other alkaloids.
Motion. Our command of the motor side of the nervous system is greater than our influence over sensation, for the reason that motor parts can be acted on not only directly, but also reflexly through sensory parts, as we have just seen-local irritants exciting muscular movements, and local depressants arresting them.
a. Motor stimulants are specially interesting, as different drugs act on different parts of the motor apparatus from the cerebrum to the muscles. Alcohol, in moderate doses, increases the activity of the "motor" convolutions, and so probably do Chloroform and Ether for a very short time. The medulla, as the centre of the respiratory movements, is excited by Strychnia, Ammonia, Belladonna, and by small doses of Alcohol, Ether, and Chloroform. The anterior cornua of the cord (probably in association with the posterior cornua) are powerfully stimulated by Strychnia-convulsions being readily induced. Stimulation of the motor nerve-trunks can be used to excite the muscles by means of faradaic electricity.
Our most valuable motor stimulants, however, are applied to the terminations of the nerves, the terminal apparatus, and the muscles themselves, in the form of local motor stimulants. Strychnia acts also in this way. Electricity is in constant use for this purpose-as the faradaic, occasionally as the galvanic, current. Passive movements of the limbs, rubbing, shampooing, and douching, by rousing the local circulation and metabolism, are also means of preserving or increasing muscular nutrition and activity.
b. Motor depressants are a parallel series of agents. The motor convolutions are disturbed, depressed, and finally completely "paralysed" by large doses of Alcohol, Chloroform, and Ether, which completely arrest all voluntary movements. The motor functions of the medulla are so powerfully depressed by Opium, Chloral, Aconite, Conium, Physostigma and large doses of Alcohol and Chloroform, that death from poisoning by these substances occurs in this way. The anterior cornua of the cord are depressed by Physostigma and many less powerful drugs, which cause paralysis of the limbs through this channel. The same effect is produced by Conium and other substances, through depression of the motor nerves, not of the cord. The motor nerve-endings are remarkably under the influence of Belladonna; more, however, those of the involuntary muscles, with which we are not at present concerned. Galvanism is the most powerful local depressant of muscular activity, and is our ordinary means of producing this effect directly.
c. The co-ordination of movement is peculiarly interfered with by certain drugs, at any rate by Alcohol, which in considerable doses produces staggering gait, disturbance of the ocular muscles with double vision, thickness of speech, and awkwardness of the manual movements.
Consciousness. From the very exalted position which it occupies in the system, consciousness is peculiarly amenable to a variety of influences at our command.
a. It can be roused by powerful, especially by painful impressions: for instance, the cold bath or douche; heat, or hot applications such as mustard to the surface; loud sounds, or powerful odours. Besides these, many drugs directly excite the brain, the cerebral stimulants and deliriants, such as Caffein, Camphor, Alcohol and Chloroform in the first stage; Opium, Chloral, and Cannabis Indica, in some individuals; Belladonna and its allies; Camphor, Salicylic Acid; laughing gas, etc.
The mental faculties are readily disordered by many of the same measures which increase consciousness, leading to laughing, crying, brilliancy of the imagination, increase of the appetites, confusion of the intellect, loss of control of the will, delirium in its many forms, and even convulsions. Alcohol, Opium, Cannabis Indica, Chloral, Chloroform, Camphor, and Belladonna, are specially active in producing these effects, which are seldom or never desired by the therapeutist for their own sake.
b. Equally valuable are our means of reducing consciousness, or removing it, and thus producing general anaesthesia, which, in appearance at least, closely resembles sleep, and is associated with suspension of all the other mental faculties. This effect may he secured by temporarily arresting the functions of the convolutions by means of drugs which directly depress the nervous tissue of the convolutions, such as Chloroform, Ether, Bichloride of Methylene, Alcohol in large doses, Chloral, and Opium. The Bromides, Caffein and Zinc, are valuable cerebral depressants, as they diminish reflex excitability, and thus promote rest of the nervous centres. Beyond these, a number of powerful substances, such as Aconite, and other vegetable and mineral poisons, produce a condition of coma with unconsciousness. The question arises, Which of the many active substances which possess this power are convenient and suitable for use ? Careful observation has taught us that the order of involvement of the various parts of the nervous system by these substances-the line of march of their phenomena- differs widely with the different drugs. With some of them, such as Ether and Chloroform, the very first phenomenon is disturbance of the convolutions; and it is not until consciousness has been completely removed, that any serious depression of the medulla and its vital functions occurs. With others, for example, Opium and Chloral, the cerebrum and medulla appear to be simultaneously and equally involved; and before consciousness has been completely removed, the centres of respiration and circulation in the medulla may be dangerously depressed. A third set of nervous depressants have hopelessly paralysed the medulla before consciousness is much disturbed; such are Aconite and the irritant poisons. In selecting for use a drug which will remove consciousness, we entirely reject the third set. The first set, with Ether and Chloroform as their types, we retain as our general anaesthetics; the second set, including chiefly Opium and Chloral, are used under special circumstances, and are generally called narcotics ( a deep sleep), or, as we have already seen, anodynes, pain destroyers.