PATHOLOGY.--In cases dependent on central disease the pathological changes are to be sought for outside of the larynx. In rickets and other morbid conditions which by reflection produce spasm of the glottis the pathology proper is distant and not in the organ; there is only an excess of motility in the nerves and muscular apparatus. Efforts have been made to differentiate spasm and false croup, but the confusion is only equalled by the disagreement as to the relation of diphtheria to true croup. It is probably true that the cramp is generally due to some excess of motility in the system at large, and that the larynx is the seat of pathological changes that determine the spasm in that organ. This is especially true in those cases associated with rickets, derangement of the alimentary canal, etc. It seems to be a fact, nevertheless, that in a majority of cases the mucous membranes are, as already stated, the seat of a very mild inflammation. Or perhaps we should say they are slightly hyperæmic. So far as we can judge from {72} examination in cases which have terminated fatally, as well as from ante-mortem observation, there is no structural change of tissue to be recognized by the naked eye, unless it be, during life, a slight fulness of the vessels. There is a change, however, in the form of the organ, at least at the entrance to the larynx. The constrictors are in a state of action, so as to partly close the superior opening to the larynx, and the epiglottis is rolled so as, in some instances, to become almost a tube. I have repeatedly recognized this in the image seen in the laryngeal mirror. Cohn reports a case of impaction even of the epiglottis in the vestibule of the larynx (p. 627). This fact is also suggested by the difficult inspiration and the altered voice and cough. In young children the yielding character of the cartilages probably adds largely to the obstruction produced by spasm of the muscles about the vestibule. DIAGNOSIS.--The diseases with which spasm of the larynx is most likely to be confounded are true croup, simple inflammation of the larynx, foreign bodies in the larynx, and possibly, in the absence of the history of the case, tumor situated in the glottis or along the vocal cords.