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.
These are remedies which lessen the irritability of the respiratory centre or of the nerves connected with it. The chief drugs which diminish the excitability of the respiratory centre are opium and its principal alkaloid, morphine. Morphine and opium have a double action in lessening cough: they not only lessen the excitability of the respiratory centre, but they diminish the secretion of mucus in the bronchial tubes, and probably thus also lessen the irritation. Hydrocyanic acid has also a sedative action on it, but it is by no means so powerful as the others.
Belladonna and stramonium have a rather peculiar action, stimulating the respiratory centre, and at the same time appearing to lessen the excitability of the ends of the vagi in the lungs. Atropine has but a very slight and uncertain action on the respiratory centre in preventing cough, if indeed it has any at all. It has, however, a powerful effect - much more powerful than that of opium, - in completely arresting the secretion from the bronchial tubes. The cases in which it is useful are therefore those where the cough depends upon excessive secretion. . In cases where the mucous membrane is already too dry, it would be injurious rather than beneficial.
When apomorphine and morphine are given together they do not destroy each other's action, so that from the combination we get increased secretion from the mucous membrane, with diminished irritability of the respiratory centre, and consequently lessened cough. The cases in which this combination, then, is useful, are those where there is difficulty of breathing, continual cough, and thick tenacious mucus. When morphine and atropine are given together, also, they do not destroy each other's action; and thus dryness of the mucous membrane is produced, along with diminished irritability of the centre for coughing. This combination is therefore useful in cases of catarrh, emphysema, and phthisis, where there is copious secretion of mucus. In phthisis it is especially indicated on account of the beneficial action of atropine in also lessening sweating. Where the copious expectoration depends upon the presence of a cavity, and not on excessive secretion from the bronchi, it will not be much affected by the use of these remedies.