This is sometimes Congenital. There may be a congenital deficiency in the middle part of the tube, a fibrous cord representing the occluded tube. In some cases the oesophagus below the occlusion opens into the trachea. Obstruction also arises as a consequence of pressure from without, by tumours, aneurysms, abscesses, but is still more frequent from disease in the tube itself. Ulcers of various kinds, by the cicatricial contraction incident to the process of healing, may induce obstruction, as, for example, ulcers from swallowing strong acids, syphilitic ulcers, etc. Still more frequent are obstructions from tumours of the oesophagus, and especially Cancers (see further on). As mentioned above, obstructions of the oesophagus frequently give rise to dilatations with hypertrophy of the muscular coat above the seat of constriction. Sometimes the dilatation is greater in one locality so that with a general dilatation there is a partial diverticulum.