A cough is a more or less violent expiratory effort, indicating irritation of some part of the breathing organs. The act is commenced by a deep inspiration, during which the lungs are distended with air. The larynx or entrance to the windpipe is then momentarily closed, after which it is forcibly opened by the contraction of the muscles of the chest and abdomen, when the contained air is expelled, carrying with it any offending matter to which the cough may be due.

The irritation by which coughing is induced may be direct or sympathetic, i.e. it may result from something brought into direct contact with some part of the respiratory tube, as cold air, dust, irritating gases, or it may be a sense of irritation transmitted by nervous influence from some other organ, as in the case of cough arising out of stomach derangement, worms in the bowels, teething, etc, etc. The sensibility of the respiratory passages is greatly enhanced when any part of them suffers from active disease, hence coughing is always present in such ailments as laryngitis, bronchitis, and pneumonia. Cough, moreover, is frequently associated with heart disease, especially with those forms that interfere with the pulmonary circulation and keep up a certain degree of congestion of the lungs.

As a means of diagnosis, much importance is to be attached to the character of the cough: (l) as to whether it is short and hacking, as in stomach and intestinal derangements, or long drawn out, as in chronic lung disease; (2) whether it is occasional, as in pleurisy, or frequent, as in bronchitis; (3) whether it is moist, as in catarrh of the air passages, or dry, as in broken wind; (4) whether it is free, as in common cold, or painful and suppressed, as in laryngitis. Cough, it will be seen, is a mere symptom of disease, and not a disease in itself; and although usually associated with disorders of the respiratory apparatus, it is sometimes induced by nervous impulses excited in remote organs. The remedy for coughing is the remedy for the particular ailment out of which it arises, and every case must therefore be dealt with on its merits. The treatment necessary to allay the cough of laryngitis would have no beneficial effect in that resulting from intestinal irritation, for, while in the former case sedatives would be indicated, in the latter aperients and perhaps a vermifuge would be most promising of success. In the matter of treatment, therefore, the particular medicines and applications to be employed will be governed by the nature of the cause to which the cough is due.