These vascular membranes are called the lateral septa.
Inner and outer membranes may exist on the bronchi.
The majority of birds possess a pair of internal tympaniform membranes forming the inner or median walls of the bronchi, which are there furnished with semi-rings only.
External tympaniform membranes exist, with great variations, between the specialized one or two last tracheal and some of the first bronchial rings.
Elastin occurs either as thick strands or as membranes; it constitutes the " elastic tissue " of the anatomist.