This disease, as already seen, depends on the action of a specific microbe, the bacillus of diphtheria (p. 354). The local action of the microbe is not limited to the larynx and trachea, but usualty affects also the fauces and frequently the naso-pharynx. It very often extends downwards into the bronchi. Besides its local action the toxine absorbed into the blood produces the serious effects already referred to, especially on the peripheral nerves (see p. 355).

The local action consists in a violent inflammation of the mucous membrane, the results of which differ somewhat in the fauces on the one hand and the air passages on the other. The conditions in the former situation are described further on under the diseases of the Alimentary canal. In the larynx and trachea as in the fauces a catarrh is the first sign of inflammation. The mucous membrane is hyperaemic and there is increased mucous secretion. This is succeeded by the formation of a fibrinous exudation which covers the mucous membrane with a whitish false membrane. The false membrane consists of fibrine and leucocytes. The mucous membrane at the same time shows evidence, microscopically, of the most intense inflammation. The vessels are dilated and the tissue is permeated with leucocytes in immense numbers. As already mentioned, the bacillus is found in the false membrane, usually in association with other microbes. It is readily separated by cultivation.

Croup is a name generally given to membranous sore throats in which the exudation is mainly or entirely in the larynx and trachea. The name was first applied before the eminently contagious diphtheria was distinguished, and it was commonly understood to designate a peculiarly violent inflammation of the air passages. There seems to be no doubt that, now-a-days at least, the great majority of cases of croup are really cases of diphtheria in which the disease is mainly or entirely in the larynx and trachea. The name is also given to throat affections in general, in which respiration is accompanied by a crowing sound, this being indeed the original meaning of the term Croup, which is a Scotch word signifying the sound made in such cases.

We have seen that in the larynx diphtheria produces an inflammation in which, after the shedding of the epithelium, a fibrinous exudation occurs. It is asserted by some that the diphtheritic poison is the only agent capable of producing this form of inflammation. Looking at the matter from a purely pathological point of view apart from clinical experience, it certainly seems possible that other irritants may produce similar results. Croup has been produced artificially in rabbits by the injection of ammonia into the trachea (Weigert), and in cases of poisoning by gaseous ammonia a definite membrane has been found lining the trachea and bronchi (Monro and Workman). In these cases the irritant first kills the epithelium,, and then fibrine is deposited. If croup occurs in man apart from diphtheria, the irritant must be strong enough to desti'oy the surface epithelium. Apart from the action of microbes, such an irritant must be of rare occurrence. We shall see afterwards that in rare cases we meet with a bronchial croup where there can be no question of diphtheria, and so we may have laryngeal and tracheal croup of a simple inflammatory kind. In such cases there will be no signs of general disease, but all the symptoms will be referable to the local inflammation and obstruction of the larynx. It should be added that laryngeal croup has been met with in smallpox, measles, pyremia, etc., and in that case it is to be ascribed to the action of specific morbid poisons, as in diphtheria.