The changes which occur in this disease have already been referred to in considering its manifestations in the air passages (see p. 725). It usually begins in the fauces and has its centre there. There is in the fauces a fibrinous exudation usually associated with necrosis of the mucous membrane. There are other signs of inflammation, chiefly the presence of leucocytes infiltrating the mucous membrane and passing into the exudation. As a rule, there is necrosis of the mucous membrane, and this in all degrees till we come to the so-called gangrenous form. The resulting sloughs frequently look more serious than they are, but there may be considerable loss of substance, and ulcers of some depth and size may be left.
The position of the patches of exudation is very various. Sometimes they are mainly on the tonsils, sometimes on the soft palate and uvula. Extension to the posterior nares, on the one hand, and to the larynx, on the other, is very frequent, the latter, as we have seen, being particularly common.