Functional anatomy of the lungs of the green lizard, Lacerta viridis

The gas-exchange area in the lung of Lacerta viridis has been studied by light microscopy and electron microscopy. The interior of the lung in this species is partitioned into air sacs by radially disposed septa. The surfaces of each septum are covered by a continuous epithelium, the cells of which are termed 'pneumonocytes'. Deep to the epithelium there is a close-meshed plexus of capillaries. The middle layer of the septum contains smooth muscle and fibrous tissue. Two varieties of pneumonocytes can be identified. The type I cells are squamous and give off attenuated sheets of cytoplasm which spread widely over the septal surface; these sheets contain few organelles. The type II cells are more compact and possess many organelles; their osmiophilic inclusion bodies are especially conspicuous. The pulmonary capillaries of Lacerta are evaginated into the air sacs and often display marked attenuation of their endothelium. The possible functional significance of these features is discussed.