A scanning electron microscope study of the microvasculature of the avian lung.

1. The microvasculature of the lung of the duck and pigeon was studied by scanning electron microscopy of vascular casts and critical point dried preparations of the gas exchange tissue. 2. The gas-exchange cappilaries are discrete tubular vessels intimately associated with air capillaries in a three dimensional network. 3. The capillaries originate from arteries at the periphery of the parabronchus, and are collected by veins which run close to its luminal surface. 4. The capillary bed of 3-5 atria is drained by a single vein. It is suggested that the vein and its associated capillaries may form a controllable subunit of pulmonary perfusion.