Whether any precise connexion may exist between these various actions of Opium, is a difficult thing to determine. It is certain that there are other Narcotics and Sedatives which are able to relieve pain, but which neither cause constipation, nor produce cerebral congestion. Purgative medicines, which open the bowels and diminish cerebral congestion, are so far opposed to Opium. Those effete matters which it is the function of the bowels to excrete from the blood, and whose excretion is promoted by purgative medicines, have an action on the brain which in some degree resembles that of Opium. Constipation produces drowsiness, and torpor of the mental faculties. And it is worthy of remark that Opium does not seem to produce quite its proper effect when combined with a medicine which causes purging. Thus it is barely possible that the action of Opium in causing congestion of the brain may be in part referrible to the influence of those excrementitious matters which it hinders from passing out through the intestinal glands (p. 291.) But nothing conclusive is known on the subject.
How, we may ask in the next place, does Opium produce constipation? It has none of the characters of a general Astringent, and yet there is no Astringent like it in its action on the bowels. An attempt has been made to explain it on mechanical grounds. Poisseuille some time ago asserted that a solution of Morphia prevented the process of endosmosis through a dead animal membrane. Matteucci and others account for the operation of Opium on this principle, supposing that by exerting this action on the living mucous membrane, it is able to prevent the transudation of fluids from the blood into the cavity of the bowel. But this view seems even at first sight unsatisfactory. If such a force is really exerted, and the passage of fluids through a membrane prevented by the action of Opium, must it not apply to the stomach as well as to the bowel? Why is Morphia absorbed at all? It should stop the digestive process altogether. And why is the secretion of the skin increased by Opium, while that of the bowels is suppressed? (Vide Matteucci's Lectures, p. 74.)
But there is reason to suppose that a serious error of observation has been made by those who first asserted that a solution of Morphia tended to arrest the process of endosmosis. Dr. Cogswell has lately made some careful experiments on the subject, and he finds that a solution of Muriate of Morphia passes readily through the bladder of an endosmometer to serum or a heavy saline solution. And when experiments are made on two other liquids of different density, the addition of Morphia to one or other of them is found to effect no alteration in the direction or rapidity of the current.
Opium is a general paralyzer to muscular fibre, both of the voluntary and involuntary kind, but particularly of the latter. And the only reasonable attempt that can be made to explain the action of Opium in producing constipation, is by a reference to this its paralyzing influence on the coat of the bowel, taken in conjunction with the torpid condition of the general system, and suspension of the animal functions, produced by the secondary action of this Narcotic on the nervous forces.
A considerable dose of Opium produces contraction of the pupil of the eye. A larger dose causes coma. The Deliriant Narcotics, i. e. Hyoscyamus, Belladonna, and Stramonium, dilate the pupil, and in large doses cause delirium. Their action is thus readily distinguished in cases of poisoning.
Opium is the best internal remedy in all cases of pain and nervous irritation, except where contraindicated by great febrile excitement, by stomach irritation, or by cerebral congestion. It acts powerfully on children, and should be administered to them very cautiously, or not at all. It is given to lessen the secretion in some cases of diarrhœa. It is generally unadvisable to give it when there is constipation or hepatic congestion. In wakefulness, mania, and delirium, when unattended with congestion of the brain or fever, it is highly serviceable. In spasmodic disorders, convulsions, and cough, it is more or less applicable.
That the active principles of Opium are absorbed, and pass into the blood, has been proved by their detection in the urine. (P. 223.) It does not act on the system before time has been allowed for this absorption. If taken by a mother during suckling, it may act upon the infant by passing into the secretion of milk. It acts upon the nervous system, and probably on the muscles also (see note, p. 224,) through the medium of the blood.
As a Diaphoretic in fevers and inflammations, Opium is to be preferred in cases where the skin is already moist and cool, where the pulse is soft, and the tongue not coated. But where there is febrile excitement, Antimony, or Ipecacuanha, is rather indicated. Opium may act on the glands of the skin as a true Eliminative; or it may possibly operate in the same indirect way as a nauseant dose of Tartar Emetic, relaxing the ducts of the sudorific glands by its paralyzing influence on their muscular fibre.
The chemical construction of Opium is a matter of considerable interest. Morphia, the chief active principle, is an alkaloid which is almost identical with Opium in its narcotic actions but slightly more sedative. Unencumbered by inert matters, it presents us with an anodyne in a more concentrated form. But the combination of the Morphia with other principles in the crude Opium is worthy of remark. Both Morphia and Opium are liable occasionally to produce nausea, headache, loss of appetite, and depression of the spirits. There are various ways in which we may try to obviate these unpleasant symptoms. To prevent nausea, we may combine the dose with an aromatic. Dr. Copland has successfully combined Morphia with an aromatic spirit, in cases where alone it was apt to bring on hypochondriasis. And to prevent the loss of appetite in Delirium tremens, Dr. Todd recommends to combine Morphia or Opium with Quinine or some other Tonic. But it is very curious that in the construction of Opium an attempt is made by nature at the very things that we so much desire in practice. For the drug contains, in addition to Morphia, an aromatic volatile oil, and a neutral principle, wrongly called Narcotine, which has been proved by some experiments made in India to be a simple Tonic, like Quinine. So that if we could only separate from the other parts of the drug the Morphia, volatile oil, and Tonic principle, we should obtain an admirable combination made ready to our hand. But Opium contains also other matters which are mere incumbrances, but which are more or less taken up by the various liquid menstrua that we employ; as, an irritant resin; other neutral principles, as narceine, Meconine, and Papaverine,—and Meconic acid,—all of which are probably inert; and, what is worst of all, a small quantity of an alkaloid called Thebaia, said by Magendie to have an action identical with that of Strychnia.
Though Morphia constitutes only about twelve per cent. of the best Opium, the dose of it required is at least one fourth of that of the latter. It seems therefore that Opium must contain some other narcotic principle. Codeia is a Narcotic, of less power than Morphia, and though it is contained in Opium, it is scarcely in sufficient quantity to explain the strength of the drug. It is likely that Opium contains more than one volatile oil, and that the odorous principle to which its peculiar smell is owing is itself narcotic in its action. A parallel case to this is verified in the instance of bitter almonds, which not only yield Hydrocyanic acid, the most powerful Sedative known, but contain a large quantity of a volatile oil, which remarkably resembles the acid in its poisonous action. (Vide pp. 238, 268.)