As the primary bronchus passes through the goose lung it gives rise to two sets of secondary bronchi. One set is located on the dorsal surface of the lung and the other on the ventral surface. These two sets of bronchi are connected by numerous parabronchi which run parallel to one another through the lung. Each parabronchus is the center of and supplies air to a cylindrical lung lobule. Numerous outpocketings of the parabronchial wall, which are called atria, connect with the finest air spaces of the lung parenchyma.
The epithelium lining the parabronchi is continuous with flattened epithelial cells which line the atria and the finest air spaces. The air-blood barrier is composed of endothelium, a basal lamina, an attenuated pulmonary epithelium, and an 80–120 Å thick osmiophilic layer which covers the attenuated epithelium. Multiple layers of a unit-membrane-like material similar to that lining each fine air space is found covering and invaginating the epithelial cells lining the atria and parabronchi. Within many of these atrial and parabronchial cells, laminated osmiophilic material is found within cisternae of rough-surfaced endoplasmic reticulum, and inclusion bodies which may contain both laminations and a granular matrix similar to that within adjacent Golgi vesicles. It is suggested that the endoplasmic reticulum may be involved in the synthesis of the surface material found lining the goose lung.