A uniform dilatation of this canal is not unfrequently seen in marasmus or senile atrophy. Its existence in advanced age is interesting, since it always occurs with senile marasmus of the lungs (emphysema senile), and is more or less proportional to it. They are both dependent on the wasting of the tissues entering into the formation of the larynx and trachea.

There is another form of dilatation, which proceeds from hypertrophy and relaxation of the posterior wall of the trachea, with or without sac-cular or hernial protrusion of the mucous membrane. This form is extremely rare in the larynx, as, indeed, might have been naturally expected from the protected state of the interstices between its different cartilages; in fact, we scarcely ever see even a tendency towards it: while in the trachea it is of frequent occurrence, and is sometimes developed to an astonishing degree. Although bronchial dilatations have been well understood since the time of Laennec, little has been observed in reference to dilatation of the trachea, and especially in regard to this form.1

In the first place, there is a relaxation of the posterior wall of the trachea, giving rise to a great augmentation of surface, especially in the lateral directions. Moreover, its mucous membrane, transverse muscular fibres, and mucous glands increase in bulk, and the excretory ducts of these glands become dilated; while, on the other hand, the elastic, longitudinal fibres become attenuated and disappear. If protrusion of the mucous membrane should now occur, it gradually makes its way between the thickened transverse fibres in the form of a cleft or funnel, and finally of a transversely-placed, saccular expansion, usually deepest at the posterior part of the tracheal rings, where we find a distorted, cleftlike orifice of the excretory duct of a mucous gland. The larger this hernia or false diverticulum becomes, so much the more prominently do the muscular bands, which limit it, project on the inner surface of the trachea; and here, if the herniae be numerous and close upon one another, the muscular fibres form a lattice-work, in which the cross-bars are usually single, but occasionally bifurcated at one of the extremities.

This condition is dependent on repeated and chronic catarrhs of the trachea, and forms one of a number of analogous cases occurring in other parts of the body. In saccular dilatation, the hypertrophied mucous glands on the posterior wall of the trachea, by the traction through the medium of their ducts, draw the tracheal mucous membrane between the bundles of transverse fibres. These dilatations sometimes extend along the whole trachea, and even into the bronchi.

These dilatations of the trachea closely correspond with the similar dilatations of the bronchi, proceeding from hypertrophy and paralysis.