The internal structure of heart of frog is studied with the help of vertical section. It is a conical hollow, muscular structure with different chambers. The different chambers remain separated by valves which help to keep the one way flow of blood during rhythmic contraction and relaxation of heart.
The two auricles are thin walled and remain separated from each other by a thin vertical inter-auricular septum. Right auricle is larger than left one as it receives blood from different parts of the body. The sinus venosus opens into dorsal wall of right auricle by a large oval aperture, sinu-auricular aperture. It lies at middle close to inter-auricular septum and remains guarded by a pair of flap like values. The common pulmonary veins open into left auricle, near inter-auricular septum, by a small opening without valves. Both auricles open into ventricle by a common large auriculo-ventricular aperture guarded by two pairs of flap like auriculo-ventricular values.
The ventricle is thick walled, muscular and spongy in nature. Its inner surface has irregular ridges or folds called the columnae carneae with depressions which is also known as fissures. These folds reduce cavity of ventricle. The flaps of auriculo-ventricular values remain connected with wall of ventricle by thread like chordae tendineae.
The opening of ventricle into truncus anteriosus is guarded by three semilunar values, which prevent the back flow of blood from truncus arterious into ventricle. The spirally twisted cavity of truncus anteriosus remains divided by three semilunar valves into a long thick walled proximal conus arteriosus or pylangium and a short distal, thin walled bulbous arteriosus or synangium. A longitudinal spiral valve is present on pylangium which remain attached dorsally while free ventrally. It divides cavity of pylangium into left dorsal cavum pulmoutaneum and right ventral cavum aorticum. The common opening of two pulmocutaneous arches lies in cavum pulmocutaneum while separate openings of carotid and systemic arches lie in synangium.

Working of heart: Heart constantly beats during life time under nervous control to pump blood into blood vessels. Contraction of heart is called systole, while relaxation is called as diastole. When sinus venosus contracts, its non-oxygenated blood passes into right auricle through sinu-auricular aperture the blood from lungs come into left auricle through pulmonary vein. The two auricles now contract almost simultaneously forcing their blood into ventricle through auriculo-ventricular aperture.

A) OLD VIEW: This view is purposed by Brucke (1858). According to the old view, two kinds of blood do not mix to any great extent on being forced into single ventricle. The two lines of blood could not mix to any great extent because of their viscous nature and also because of the network of columnae carneae.
When ventricle starts contraction, first the deoxygenated blood from the right side being nearer flows into the truncus arteriosus and directed by the spiral valve into the common opening of pulmo-cutaneous arches and carried to lungs and skin for oxygenation. Spiral valve now closes the opening of pulmo-cutaneous arches. Next follows the mixed blood which is pushed through cavum aorticum into the systemic arches and is sent to the body and limbs. Finally enters the oxygenated blood of left side and directed through carotid arches to the head. Thus, according to old view the spiral value in truncus arteriosus plays an important role in directing blood into different arches.

B) MODERN VIEW: This view was put-forward by Vandervale (1933) and Foxon (1953), show that it is actually completely mixed blood in ventricle and truncus which flows simultaneously through the three pairs of arches to all parts of the body. The blood received from skin and buccal cavity into sinus venosus and right auricle is equally oxygenated to the blood received from lungs to left auricle.