This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
Action on Special Organs. - Opium has little action on muscular contractility. The action of opium on the motor nerves is doubtful. According to some observers, it first increases and then diminishes their excitability, the action commencing at the centres and proceeding towards the periphery. Others, however, have found that opium has little or no action on them, except towards the end of poisoning, when all the functions are paralysed.
The sensory nerves are first excited and then paralysed. Opium applied locally is said by some observers to have no action on the sensory nerves, but it has been found to have a paralysing action by Baxt. The results of his experiments are confirmed by clinical experience, for when the drug is injected subcutaneously it lessens sensibility, diminishes the power of distinguishing tactile impressions, and relieves pain when present.
When applied externally to the eye, or to the skin denuded of its epidermis, opium also relieves pain.
Opium lessens first the conducting power of the spinal cord, then the reflex functions, producing first inco-ordination of the movements of the hind limbs and then paralysis of reflex action.
Opium acts on the centres of the brain in the order of their importance; thus in the frog, there is, first, loss of voluntary motion, such as may be produced by extirpation of the cerebral hemispheres; next, loss of co-ordination, such as is produced by extirpation of the optic lobes; and lastly, effects like those produced by destruction of the medulla (p. 183).
The pupil is markedly contracted by opium, the action of the drug being probably central and not peripheral; but the exact mode of action has not been definitely made out. Stimulation of sensory nerves causes reflex dilatation of the pupil, and it is not improbable that the contraction which opium produces is due to its paralysing this reflex action more or less completely (p. 219).
The pupil sometimes dilates just before death. This dilatation is probably due to the excessively venous condition of the blood, as in the later stages of chloroform-poisoning.
The first effect which opium has on the circulation is to cause a dilatation of the vessels of the skin, sometimes giving rise to a cutaneous eruption of a roseolous character accompanied by itching, and coming on either before or after sleep.
The vaso-motor centre in the medulla is slightly, if at all, affected by small doses of opium. Large doses paralyse it.
The drug appears to have a peculiar action on the peripheral vaso-motor apparatus. It is well known that stimulation of the chorda tyrnpani causes dilatation of the vessels of the submaxillary gland; but I found that after the exhibition of opium the vessels of the gland no longer dilated, but on the contrary contracted, on stimulation of the chorda tyrnpani, so that the blood which issued from the gland was not of a bright arterial hue, but was very dark, and flowed drop by drop. This observation requires confirmation; but if it be confirmed, this result might serve to explain the effect of opium in cutting short inflammations, e.g. of the peritoneum.
In peritonitis as in other inflammations the blood-vessels are greatly dilated. Opium by its action on the vaso-motor centre, and (if these experiments be correct) by its action also on the peripheral terminations of vaso-motor nerves, will prevent or diminish the reflex dilatation of the vessels which the local irritation would otherwise produce. Congestion will thus be diminished and inflammation be relieved. The action of opium in peritonitis is therefore probably twofold. First, it lessens peristaltic movements of the intestines, and thus diminishes local irritation. Secondly, it lessens the reflex activity of the centres through which local irritation causes dilatation of the vessels, and thus diminishes peritoneal congestion. The late Sir Robert Christison used to say that not only coryza, but probably all inflammations, could be nipped in the bud by opium if it were only given sufficiently early and sufficiently freely.
The blood-pressure appears to be but little affected by opium. It varies after the injection of the drug, but this variation is probably due to an alteration in the general functions of the body, for example, great quiet, etc.
On Secretion. - The secretions of the body, except those of urine and of sweat, are lessened by opium.
If the lingual nerve of an animal be stimulated, a reflex flow of saliva takes place; but when opium has been given in sufficient quantity this reflex action is paralysed, and stimulation of the nerve no longer excites a flow. Very large doses, however, may cause salivation (p. 355). Opium also diminishes the other secretions of the alimentary canal, causing impairment of digestion, and this action serves partially to explain the constipation produced by opium.
The quantity of sweat secreted is increased by opium, and especially so when it is combined with ipecacuanha. Just before death by opium the secretion of sweat is greatly increased, so that the surface is bathed in it; but this is due to stimulation of the sweat-centres in the spinal cord by the increasing venosity of the blood (p. 438).
The quantity of urine is frequently lessened. Sometimes it may be really increased, but appear diminished in consequence of paralysis of the bladder preventing its being evacuated. Sometimes there is a constant desire to pass water.
On the Intestines. - The action of opium on the intestines varies with the dose. On isolating a piece of intestine and supplying it artificially with blood, the action of opium may be observed by mixing it with the blood (p. 383). When used in large quantity all peristaltic action ceases, and the intestine becomes tetanically contracted. Hence in large doses, injected directly into the jugular vein, it acts as a most powerful purgative, causing one very copious evacuation. It acts chiefly on the small intestines. In moderate doses it lessens peristaltic action and causes constipation. In very small doses it generally increases peristalsis and acts as a purgative, but not invariably so. This property is made use of in cases of constipation due to reflex irritation starting from the ovary. The mode of action has already been discussed (p. 385).