ETIOLOGY.--Intense irritants will produce an irritation on mucous membranes. In the larynx the product is, according to the severity of the irritation, either a catarrhal or a phlegmonous or a croupous laryngitis. The irritating substances may be mechanical, chemical, or thermical. Heubner produced diphtheria of the bladder by cutting off, temporarily, the supply of circulation. Traumatic injury of the throat and larynx will soon show a croupous deposit. Caustic potassium, sulphuric acid, caustic ammonium, corrosive sublimate, arsenic, chlorine, or oxygen, applied to the trachea or larynx, produce croupous deposits.[1] Inhalations of heat, smoke, and chlorine have the same effect. These, however, are not the usual causes of croup. Cold and moist air is a more common cause, mainly during a prevailing epidemic of diphtheria. In former times, which are unknown to the younger generation of physicians, when no such epidemics existed, the only form of diphtheria occurring now and then was the local laryngeal diphtheria called pseudo-membranous croup. It was then a rare disease, while at the present time it is of but too frequent occurrence. In my _Treatise_ I have explained at some length the relations of the two (p. 128). [Footnote 1: A. Jacobi, _Treatise on Diphtheria_, p. 111.] Age has some influence in its development. The disease is not frequent in the first year of life; between the second and seventh years almost all the cases are met with. There are families with what appears to be a general tendency to croupous laryngitis. It may return. Even tracheotomy has been performed twice on the same individual.[2] It is contagious. In the same family, from a case of croup, either another case of laryngeal croup may originate or another form of diphtheria will develop in other members of the household. It is not so contagious, it is true, as generalized diphtheria must be, for the infecting surface is but small in uncomplicated membranous croup, and the membrane not so apt to macerate and be communicated. Boys appear to be affected more frequently than girls. But the previous constitution makes no difference. [Footnote 2: _Treatise_, p. 27.]