Acute inflammation of the nares occurs most frequently as a catarrh, constituting the ordinary cold in the head, but there may be a more intense and specific inflammation in diphtheria and scarlet fever.

In the case of the specific fevers, the special irritant concerned in their production may attack the nares and produce acute inflammation there. This is nearly always the case in measles, but sometimes also in small-pox, diphtheria, and scarlet fever. For ordinary catarrhs Cold is usually assigned as the cause. The mucous membranes of the air passages are indeed much more liable to inflammations than those of the alimentary canal, and an exposure to cold air which would not produce any catarrh of the mouth might possibly do so in the nares. Moreover, the fact that the nares are the nearest part of the air passages to the air, and are therefore most exposed to the action of cold, is an indication that such exposure has probably something to do with it.

Mere inhalation of cold air is not enough to produce catarrh, as every one who is liable to cold in the head will admit. A person is able at one time to brave any amount of exposure without the risk of catarrh, while at another time a slight draught, or no perceptible exposure at all, will bring it on. Such circumstances as these, taken along with the fact that the catarrh has a definite course, usually of a week's duration, have induced some to suppose that the disease is really due to a specific morbid poison. In this view it is necessary to suppose that the microbes are usually present in the air or in the mucous membrane, and that they take, as it were, advantage of the mucous membrane, when, at any time, it is in a specially predisposed state. This predisposition may be induced by cold applied either directly to the mucous membrane or to some other part of the body, but other predisposing causes may exist. For instance, when a person is overheated the arteries of the skin and of the respiratory mucous membranes are relaxed, and there is an active hyperaemia. At such a time a catarrh is readily induced, apparently because the mucous membrane is less able than usual to resist the action of irritants. There is apparent confirmation of such a view as this in the fact that nasal catarrh undoubtedly occurs as the result of the action of specific irritants. In measles the catarrh of the nares and conjunctivae is referable to the specific virus of that disease. In hay asthma the irritation of the pollen, etc., is the cause of the catarrh. Again, it is commonly stated that nasal catarrh is communicable from person to person, and it is hardly conceivable that this could occur unless the disease had, as a part of its cause, some specific materies morbi.

The catarrh begins with an inflammatory hyperaemia of the mucous membrane. This may, of itself, lead to swelling so great as partially to obstruct the passages, giving the feeling of a "stuffed nose." The inflammation soon passes on to exudation. This finds its way for the most part to the surface, but in its passage it increases the swelling of the mucous membrane. The exudation is the usual one of inflammation, namely the blood-plasma with leucocytes. At first this is mixed with mucus, but as it increases in amount it rapidly assumes a serous character, and we know that a great abundance of serous fluid may be discharged from the nostrils. As time goes on the leucocytes increase in the exudation, and it may assume a semi-purulent character. Sometimes red corpuscles are present, and the discharge has a greenish yellow colour.

It is to be remembered there are various cavities in direct communication with the nares, of which the principal are the Frontal sinuses and the Antrum of Highmore, and that these frequently take part in the acute catarrh. It will be observed that in all stages of the catarrh there is more or less swelling of the mucous membrane, and that this leads to the obstruction of the passages which is such a marked symptom. This swelling is temporary in the acute disease, but in chronic catarrh it is apt to give way to a more permanent thickening.

Acute inflammation sometimes extends to the nares in Diphtheria. The anatomical changes will be considered in next section.