The anatomy of the pulmonary vascular bed in the toad Bufo marinus.

1. The pulmonary vascular anatomy of the toad, Bufo marinus was studied by scanning electron microscopy of vascular corrosion casts and of tissues prepared for light microscopy. 2. The lung is divided by septa into three orders of alveoli. Arterial branches run in the septa and cannot be observed directly through the lung surface. 3. The capillary bed forms a meshwork over the lumenal surface of septa and lung walls. It is divided into longitudinal zones of arterial distribution; it is broken along primary septal margins into primary alveolar units; within alveoli, it is functionally divided by distortion over circum-alveolar smooth muscle bundles. 4. The capillaries form a network of tubes rather than a perforated sheet of vascular space. Capillary beds on either face of septa are interconnected through the septum. 5. There is no evidence of collateral ("bronchial") vessels or of anatomically definable shunt vessels. 6. Intrinsic pulmonary arterial branches have a muscular coat.