These are not infrequent, but are mostly of minor importance. Apart from absence and exceeding smallness of one or both lungs, which occur as parts of general malformations, there are cases where single lobes have been wanting, and their place taken by cicatricial tissue. These have probably their origin in obliteration of a bronchus in early fœtal life. Again, the lungs may be normal in form, but very small in size. In such cases the whole body, and especialty the circulatory system, will remain ill-developed.

It is quite common to meet with abnormal lobulation of the lungs, the regular lobes, being divided by the formation of deep fissures. Rokitansky has described a case in which an accessory lobe existed between the base of the left lung and the diaphragm, and quite separate from the lung. It had, however, no bronchus.