The action and uses of opium are due almost entirely to its morphine, and therefore they may be studied together. Codeine and other alkaloids will be considered on p. 366. Meconic acid appears to be nearly free from physiological properties.

External

Opium probably has no action when applied to the unbroken skin, but it has been said to be slightly anodyne. It can be absorbed from and relieve the pain of raw surfaces.

Internal

Alimentary canal. - Opium diminishes all the secretions of the body except the sweat. The mouth consequently becomes dry, and the patient feels thirsty, but after a small dose not markedly so. This effect is partly due to the direct action of the opium on the mouth, but to a less extent to its influence exerted after it has been absorbed. In the stomach and intestines, by the same double action, the secretion of the gastric and intestinal juices is diminished. The drug also paralyzes the peristaltic movements of the stomach and intestines. This is due to its action on the nervous or muscular structures in the wall of the intestine itself. The result of the diminution of secretion and peristalsis is that opium appeases hunger, often causes indigestion, almost always gives rise to constipation, and if vomiting or diarrhoea is present, it may prevent it. These actions are also in part due to its general sedative influence on the nervous system. If pain exists in the abdomen or elsewhere, opium is a powerful anodyne. Most of it is absorbed, but rather slowly. If injected subcutaneously it is excreted into the stomach. With some persons it causes vomiting. Whether the biliary and pancreatic secretions are diminished is not known.

Blood. -Morphine for the most part circulates in the blood as such, and is excreted by the kidneys, but a small part of it is destroyed in the liver. The fate of the other alkaloids is not known, nor are we aware of any direct action of any of the constituents of opium on the blood itself.

Circulation. - In an ordinary healthy man small doses of opium hardly affect the heart or vessels. Large doses diminish the force and frequency of the heart, which finally stops in diastole. These effects can be produced by applying the drug to the organ; it therefore directly affects either the cardiac muscle or the nerves in it. But this local action is said to be helped by the less important influence of opium on the vagal centre; this is at first stimulated, and about the time at which the heart itself is depressed, so that both these actions make the pulse slow. Patients rarely die from the effect of opium on the heart and its nervous apparatus, this being much less important than the influence on respiration, and some of the cardiac depression may be secondary to asphyxia.

The vaso-motor centres are slightly depressed by moderate doses, hence the vessels, particularly those of the skin, dilate; with large doses the depression is considerable.

Respiration. - Opium is a direct poison to the respiratory centre. Breathing therefore becomes slow, less air is taken in at each inspiration, and death takes place from asphyxia. The secretion of bronchial mucus is decreased.

Nervous system. - Brain. - The higher faculties are at first excited even by small doses. In a few persons there is no incoordination in this excitement. The intellectual power and mental vigor are increased, and therefore the drug is taken by some people to enable them to do their mental work. Usually, however, the excitation does not affect the mind evenly; generally the imagination is powerfully and pleasantly excited, much more so than the faculties of reason and judgment, which are a little dulled. The expression on the face is one of happiness and comfort, and this corresponds with the condition of the mind, which is in a state of peace, calm and happiness. This is soon succeeded by sleep, which is accompanied by pleasant dreams, generally of an impossible nature. With some persons, however, the sleep is quite dreamless. This, which is the beginning of the depression of the highest centres, is soon followed by depression of the others, the higher being influenced before the lower, so that soon the sleeper does not respond to any sound, light, or cutaneous stimulation, nor does he feel pain. It is this last fact that makes the drug so invaluable. The dose requisite to annul pain depends, of course, upon the severity of it. If a large amount is given, often there is no primary excitement, and then the first symptom that opium has been taken is drowsiness. On waking from sleep induced by opium some persons feel quite well, but usually there is a little languor, headache, and nausea. Opium-eaters take it for its stimulant effect. It is given medicinally as a hypnotic and anodyne. The pupil is contracted; this is due to the effect of the drug on the pupillary centre in the floor of the aqueduct of Sylvius. In man, just as the stimulation of the intellectual centres is brief, so is that of the cerebral motor centres - in fact, it is often difficult to detect any evidence of it. Their subsequent depression is never so marked as that of the intellectual faculties; for although there is languor and muscular weakness, and the patient always lies down, yet he can be walked about if he is supported. Vomiting is occasionally caused by transient irritation of the vomiting centre, but soon it is depressed, and therefore emetics do not act well in cases of opium poisoning.

The motor cells of the spinal cord are at first slightly stimulated, and consequently reflex excitability is exaggerated; but they are soon depressed, and it is difficult to obtain reflex movements.

The excitability of motor and sensory nerves is, perhaps, a little increased at first, but in the later stages of opium poisoning they are depressed, the sensory before the motor. The muscles remain irritable to the last.

Opium, in its action on the nervous system, illustrates the common fact that functions at first stimulated by a drug are usually subsequently paralyzed by it; and it affords an excellent example of the law of dissolution, for higher functions, such as the intellectual and imaginative, are first affected; motion is then disordered; next the pupillary centre, and then the medullary centres, for respiration and cardiac action are implicated. The spinal cord is influenced to a less degree, the nerves very slightly, and the muscles not at all.

In man the peculiarities of the action of morphine are its predominating influence on the higher mental functions, and the slight affection of the motor and the vaso-motor centres, the cord, the nerves, and the muscles. In frogs morphine produces violent convulsions, because its predominating action is to stimulate the spinal cord. Birds are peculiarly insusceptible to morphine. Mammals are for the most part affected in the same way as man, except that the first or excitement stage is more marked; hence with many mammals, especially cats, morphine is a violent convulsant; dogs and rabbits require large doses to produce symptoms.

Kidneys. - Sometimes opium slightly increases, sometimes it slightly decreases, the urinary flow. Perhaps morphine is decomposed in the body, for oxydimorphine has been found in the urine of those taking morphine.

Skin. - Opium is a mild diaphoretic. It may cause itching.

Metabolism. - If the person taking opium has glycosuria, the amount of sugar he passes in the urine is diminished. General metabolism appears to be decreased also, for it is stated that the amounts of uric acid and carbon dioxide excreted are lessened, but some experimenters contradict this statement.

Temperature. - Large doses depress this, probably from the effect of the drug upon the thermogenetic nerve centres.

Persistent use of large doses decreases the secretion of milk and the menstrual discharge. It is excreted by the milk, and so may affect the child.

Peculiarities. - There are few drugs which have such different effects upon different people. The above description states the manner in which most human beings are affected; but in some the stage of excitation is very evident, so that they become delirious and cannot sleep. In others, vomiting and indigestion are very marked. Some of these peculiarities are due, no doubt, to the varying composition of opium. Children are easily poisoned by it, and therefore only small doses should be administered to them; women are more readily affected than men. Persons who take it habitually soon tolerate enormous quantities. It may produce an erythematous eruption on the skin.

Differences in action between opium and morphine. - (1) Morphine being more readily absorbed, acts more quickly. It is especially suitable for subcutaneous injection; given in this way it acts very rapidly. (2) Opium is more liable to upset the digestion and to cause constipation; but this last fact often makes it the more valuable in many abdominal diseases. (3) Opium is the better diaphoretic. (4) Morphine is more certain in its action as an anodyne and soporific; possibly this is because of the other powerful alkaloids in opium. (5) It is less convulsant. (6) Opium is stated to act more powerfully in reducing the amount of sugar present in the urine in glycosuria. (7) Opium affects the bladder sphincter less. (8) Morphine causes more pruritus than opium.