The heart of Lacerta viridis is described. It has two auricles and a ventricle, the latter partially subdivided into three parts. The roles of the three parts of the ventricle have been the matter of some discussion. We have attempted to analyse the functions of the various parts of the heart by radiography.

The radiological anatomy of the heart and great vessels of Lacerta viridis has been described and cine-radiographs showing the passage of the radioopaque medium, thorotrast, through the heart have been made by both “direct” and “indirect” methods.

The interpretation of the radiographs shows that there is a marked difference in treatment of the blood entering the heart by way of right and left auricles. The blood from the lungs and left auricle passes into the left portion of the ventricle (cavum arteriosum) and is sent to the right systemic arch and to the carotid vessels of both sides.

The blood from the general body circulation entering the heart through the sinus venosus and right auricle is divided into two portions. One passes into the right dorsal portion of the ventricle (cavum venosum) and is sent out mainly through the left systemic arch. The other portion passes from the dorsal portion of the ventricle into the ventral portion (cavum pulmonale) and so into the pulmonary arteries.

As the ventricle is only partially divided there is some mixture of blood from the two auricles but our results show that the degree of mixture is small.