This may occur in any part of the respiratory apparatus; but the nature and degree of the affection may be extremely various; in fact, in the latter point of view, the change may proceed to closure and perfect obliteration.

1. It may be dependent on external pressure. There may be contraction of the larynx and of the trachea from an enlarged thyroid gland; of the trachea and bronchi from enlarged lymphatic glands, aneurisms, large cancerous deposits in the neck and mediastinum, enlarged thymus glands, and effusions into the cavity of the chest; and of the left bronchus by a dilated left auricle (King). In this manner the air-tubes become forced in various directions from their normal position, and their calibre, as may be seen in contraction of the trachea, may be so encroached on as to represent a mere fissure, having a transverse, an antero-posterior, a straight, or a crescentic form.

2. Contraction may be the result of disease of the mucous membrane, or of the subjacent mucous tissue of the air-passages, as of hypertrophy, inflammatory swelling, or oedema of the mucous membrane or of the submucous areolar tissue, of various excrescences, cancerous deposits, or cicatrices after loss of substance; the most frequent cause, however, is bronchitis, especially when it has given rise to obliteration of the finer bronchial tubes.

3. The calibre of the air-passages may be diminished by foreign bodies of various kinds which have penetrated into them either from without or from the intestinal canal through the pharynx or morbid openings; and by products of morbid processes in the mucous membrane and in the deeper tissues, as in adjacent organs, which abnormally communicate with the air-passages; amongst such products we may enumerate coagula of blood, clots of mucus, frothy bronchial secretion, croup-membranes, pus, masses of tubercle, fragments of cancerous matter acephalocysts, pieces of necrosed cartilage and bone, etc.