This form, which is rare, occurs when the air vesicles or bronchi are ruptured and the air escapes into the interstitial tissue. The air vesicles may be actually torn open by a broken rib coming against the lung, or by the lung being directly wounded. On the other hand, the air vesicles may rupture from acute over-distension. . Thus it may be the result of very violent expiratory' efforts, generally with, but sometimes without, obstruction of the air passages. It has been met with in whooping-cough, in diphtheria, and in violent coughing from the inhalation of irritating material. The violent efforts with closed glottis cause such compression of the air in the alveoli that at some place the vesicles rupture.
As the parts are seen after death the air appears in the form of minute rows of bead-like bubbles, visible through the pleura. These rows of beads demarcate the lobules. Occasionally there are larger bullae, which have been known to rupture externally, and so lead to pneumothorax.
The air sometimes travels along the connective tissue for some distance, just as in the case of Subcutaneous or Surgical emphysema. It may pass to the root of the lung, and from there up along the trachea and out to the subcutaneous tissue of the neck, and so lead to a surgical emphysema. This has in some cases induced a mistake in diagnosis, as Virchow has pointed out. Interlobular emphysema sometimes occurs in diphtheria, and may lead to subcutaneous emphysema in the way just mentioned. But if tracheotomy has been performed, it may be thought that the emphysema has taken origin in the wound.
An unusual cause of interstitial emphysema was that in a case observed by the author, in which a tuberculous gland burst into a bronchus. The rupture of the bronchus allowed of the passage of air to the parts outside the tube, and the air inflated the connective tissue of the lung on the one hand, and on thf. other hand extended up to the neck and chest, producing a subcutaneous emphysema.