Catarrhal affections of the respiratory passages may be excited by irritants of various kinds, and it is probable that these irritants are frequently living organisms. The form of coryza usually called hay-fever is probably due to irritation of the nasal mucous membrane by pollen-grains commencing to grow on it and sending pollen-tubes into its substance.

Other forms of respiratory catarrh, e.g. measles and influenza, are probably associated with specific microbes.

When the respiratory mucous membrane is perfectly healthy it is probable that the invading organisms are quickly expelled or destroyed (p. 85) so that no injury results. But when the resisting power of the mucous membrane is weak, either on account of general constitutional tendencies, or from local and temporary condition of congestion due to a chill (p. 252), the microbes may begin to grow and cause great irritation.

Among the remedies useful in arresting colds we may recognise antiseptics, which destroy microbes, and also sedatives, which remove congestion.

Hay-fever has been treated by Binz with a watery solution of quinine in order to stop the growth of organisms in the nose. In some cases this treatment is successful. There is a form of cold sometimes known as influenza-cold. Like true influenza it is extremely infectious and is easily communicated, not only by one member of a family to another, but even by casual visitors. It sometimes begins as a cold in the head, passes down the throat to the trachea and bronchi, leading to severe bronchitis with much depression and occasionally also to gastro-intestinal catarrh. Sometimes it begins in the throat and spreads upwards into the nostrils and downwards into the air-passages. It may frequently be arrested or rendered less severe by the use of dilute carbolic acid applied to the nostrils in the form of spray or by a syringe or nasal douche when the cold begins in the head. When the cold begins in the throat it may be arrested by the use of a carbolic acid gargle, and such a gargle is also useful when the cold begins in the head and is spreading down the throat.

Inhalations of carbolic acid and ammonia appear to be frequently useful in arresting colds. It seems probable that their effect may be due partly to an antiseptic action and partly to their lessening congestion. Carbolic acid inhalations appear to be useful in whooping-cough, probably from an antiseptic action.

Camphor inhaled and also taken internally is useful in arresting colds, though it may be rather hard to give an explanation of its modus operandi.

The sedatives which remove congestion of the nasal mucous membrane may be either general or local. Amongst the local may be mentioned bismuth, bismuth and morphine, and cocaine; and amongst the general, preparations of opium, especially Dover's powder, and aconite.