MOST text-books of elementary zoology contain an account of the circulation of the frog. These accounts, which are based largely on the views of Brucke1 and Sabatier2, differ in minor details, but agree that the frog's heart, though possessing only a single ventricle, functions, nevertheless, so that the so-called arterial blood, returning by the pulmonary veins, is mixed only slightly with the venous blood. The mixed blood is sent through the systemic arches to the hinder part of the body. Meanwhile the head region is supplied through the carotid vessels with pure arterial blood, while the greater part of the venous blood is sent through the pulmo-cutaneous vessels to the lungs and skin.