Dilatation of the trachea is either confined to the tube (when the synonym tracheaectasy is applied to it) or is diverticular. In the former case it may involve only a part or else the whole extent of the windpipe. Whenever free respiration, especially expiration, is chronically impeded, some portion of the air-tract below the obstruction is apt to become dilated; thus, a bottle-shaped dilatation is sometimes found immediately below an annular contraction. On the other hand, tracheaectasy may extend upward from bronchiectasy. It has been observed post-mortem to a slight extent in public criers, trumpeters, etc., and in old coughers from laryngeal disease, chronic bronchitis, pulmonary emphysema, etc., but without giving rise to distinct symptoms during life. Diverticular dilatation forms an air-containing tumor which either looks into the oesophagus or is discernible on the outside of the neck. Though rarely met with, it ought to be thought of in all appropriate cases, and when pointing externally ought always to be recognized by the careful practitioner. It is either hernial, glandular, or fistular--three pathological conditions which have hitherto been confounded. On account of the construction and position of the trachea there can be but little protrusion outward without previous {144} dilatation. Unless there be a deficiency of the cartilaginous rings, only the posterior wall, which is always unsupported, and to a slight extent also the intercartilaginous membranous portions, are liable to tracheal hernia. This is properly called tracheocele; but the various terms aërial goitre, aërial bronchocele, pneumatocele, tracheal air-cyst, tracheal retention-cyst, internal tracheal fistule, subcutaneous or incomplete fistule of the trachea, have been indiscriminately used as synonyms of tracheocele, and have added all the more to the confusion, as some of them originated, no doubt, as correct appellations of the particular cases to which they were applied. Aside from the occasional occurrence, both congenital and acquired, of tracheo-cutaneous fistule, complete and incomplete, and the still more rare occurrence of hernia of entire portions of the mucous membrane, the cases of diverticular dilatation of the trachea--or saccular tracheaectasy, as it may be called--are glandular, as found by Rokitansky more than fifty years ago. Virchow seems to regard all such glandular dilatations as retention-cysts (see Morbid Growths), but although retro-tracheal retention-cysts doubtless do occur (Gruber has reported two unquestionable instances), and although the tumors now under consideration do in fact sometimes contain a little mucus in addition to air, they do not constitute cysts or adenomatous new growths, but are simply distended portions of the tracheal mucous membrane, respiratory glands, whether the dilatation be caused, as Rokitansky thought, by traction (Zerrung) and hypertrophy of the mucous glands, or, as Eppinger suggests--and which is more