Cough, as I have already said, is a reflex act which is performed by means of a reflex mechanism, and is adopted for the purpose of expelling foreign bodies from the air-passages. It is evident that, when the source of irritation may be removed by efforts at coughing, these efforts are useful, and require to be sustained rather than prevented; but if the irritant cannot be removed, the effort of coughing is injurious rather than beneficial, and the same is the case when the amount of effort is disproportionately great to the good that it effects. In these cases we must try to lessen the cough.

The source of irritation in the respiratory passages may either be free in the lumen of the bronchial tubes, or may be situated in the mucous membrane lining the bronchi, or in the substance of the lung itself. Thus we may have foreign substances, such as dust, which have been inhaled, or mucus secreted from the bronchi, resting on the surface of the mucous membrane, and leading to irritation. Such foreign matter may be expelled by coughing, and so may purulent matter lying in a cavity, and the cough may be useful by expelling them.

But if the irritation be simply due to a congested condition of the bronchial mucous membrane; to congestion or consolidation of the lung-tissue itself; to a caseous or calcareous nodule which is firmly embedded in the lung; or to inflammation of the pleura, it is evident that the efforts at coughing will not remove the irritant, but will rather tend to produce exhaustion; and consequently we must either try to remove the source of irritation by other means, or to lessen the irritability of the nervous mechanism by which coughing is produced. Where the cough is due to irritation caused by indigestion we may give alkalies to relieve acidity, but we sometimes find that a blue pill and a black draught are amongst the most efficient remedies for coughs of this character, by the permanently beneficial action they exert on the digestion. When there is irritation of the pharynx, as well as of the trachea, mucilaginous substances, such as jujubes or linseed tea, are exceedingly useful.

Where cough depends on congestion of the mucous membrane of the trachea or bronchi, we not unfrequently find that the inhalation of cold air, by causing contraction of the vessels, and lessening the congestion, will arrest the cough, so that patients are able to walk out on a cold frosty morning for a length of time without coughing. On coming into a warm room the vessels of the respiratory mucous membrane again dilate : the mucous membrane becomes congested, and the congestion leads to violent and prolonged efforts at coughing. In such cases counter-irritation over the neck, upper part of the chest, and between the shoulders is useful, probably by causing contraction of the vessels (p. 252), and thus lessening congestion. But congestion, not only of the trachea and bronchi, but also of the smaller bronchial tubes, may be relieved, not only by counter-irritation, but by inducing secretion. Congestion of the smaller bronchi indicated by loud whistling rales all over the chest, is often accompanied by great shortness of breath. The inhalation of hot aqueous vapour tends to relieve the congestion by inducing secretion, but more powerful agents still are antimony, ipecacuanha, and apo-morphine. In such a condition as the one just mentioned, where secretion is absent and congestion is great, one or other of these drugs should be given frequently until secretion occurs freely, as indicated by abundant moist rales in the chest.

Along with these depressant expectorants, some preparation of opium should be given, in order to lessen the cough, which at this stage is of no advantage. It is advisable not to stop the administration of these expectorants immediately on the occurrence of secretion, but to continue them for some time longer, and gradually to lessen their amount. When secretion has become copious, either from the administration of depressant expectorants or from the natural course of the disease, we have resort to such drugs as will tend to cause its expulsion, and also to lessen its formation. Amongst those which tend to lessen its formation are balsams and terebinthinates (p. 255), and those which tend to assist expulsion have already been mentioned (p. 254). Along with these we generally combine some preparation of opium if the cough is disproportionately severe, and in chronic bronchitis cod-liver oil (p. 254) is perhaps the most efficient of all remedies.