of the mucous membrane lining the air-tubes is a com mon cause of death in infancy and childhood. The disease may be danger ous not only in itself but through its tendency to be accompanied by collapse of the lung or to pass into broncho-pneumonia. In young infants death, when it occurs in bronchitis, is seldom due to the uncomplicated disease. It is usually to be ascribed to one of the consequences which have been referred to. In older children a simple bronchitis may prove fatal, but up to the age of five or six years the untoward result is commonly due to extension of the inflammation to the finest tubes and terminal alveoli.
Bronchitis may be a mild complaint or an affection of the utmost gravity. When the disease attacks only the large tubes, it is usually of little consequence and can be readily cured by judicious treatment, although even in these cases, if the patient be a weakly infant, fatal col lapse may occur very suddenly and unexpectedly. When the disease spreads to the smaller tubes (capillary bronchitis) the illness is a very serious one, and many of these cases prove fatal.
Causation.—Bronchitis may arise from exposure to weather and to changes of temperature like other forms of catarrhal derangement. It may also be set up by irritants inhaled into the air-passages. Thus an escape of gas in the nursery is sometimes a cause of bronchial catarrh. During the pyrexia attendant upon dentition children are especially sensi tive to the causes of pulmonary disorder, and very slight chills will give rise to bronchitis in such subjects. Some children are said always to " cut their teeth with a cough." In other words, their exceptional sensibility at this time to atmospheric influences makes them catch cold very readily.
Damp and cold combined, especially where great variations of tempera ture occur, are fruitful causes of catarrhal disorders ; and if in a climate where such conditions prevail the child is insufficiently clothed, he usually becomes a frequent sufferer from bronchial derangements. Some mothers: have a curious dislike to flannel worn next to the skin, and accustom their children in all seasons to depend solely upon the warmth of their frocks and wrappers for protection against the cold. The common result of such
a practice is to increase the natural susceptibility to chill ; and many a child's life has been sacrificed to this senseless prejudice.
Besides the primary form of bronchitis which is induced by the above causes, the disease is frequently met with as a secondary affection. There are many forms of illness which are habitually complicated by pulmonary catarrh. Whooping-cough, measles, typhoid fever, and acute pulmonary tuberculosis are amongst the number. In others an intercurrent bronchi tis is a frequent phenomenon. Thus in scarlatina, small-pox, diphtheria, certain special lung diseases, as croupous pneumonia and pleurisy, and in diseases of the heart and kidneys, bronchitis is a frequent complica tion.
Morbid Analomy.—The anatomical changes induced by the disease in volve primarily the mucous membrane, and may spread thence to deeper structures. The membrane is congested and consequently reddened and thickened. Sometimes it is softened. The secretion is at first diminished, but afterwards becomes copious and watery ; then thicker and more like pus. Under the microscope we find epithelial cells (many of them em bryonic), granular cells, and pus corpuscles.
When the bronchitis is capillary, the finer tubes are often found com pletely occluded by this viscid muco-pus. This is especially the case in the lower lobes, into which the secretion has probably penetrated by inhalation and gravitation. More or less collapse is then usually found in the tissue with which the obstructed tubes are in connection.
The inflammatory process is at first limited to the mucous membrane, but if the disease continues, may penetrate to the submucous tissue or even involve the whole thickness of the bronchial wall. In these cases dila tation of the channel may take place, and acute enlargement (emphysema) of the air-cells may be found. Often the two opposite conditions of lob ular collapse and lobular emphysema may be found side by side.