Persistence of a functioning bulbus cordis homologue in the turtle heart

The functional and structural characteristics of a discrete myocardial segment at the base of the turtle ventricle were studied. The study indicates that this segment is a homologue of the primitive bulbus cordis. Cinematographic techniques were utilized to observe the function of this segment. It contracts later than the main ventricular chamber and acts as a sphincter at the base of the pulmonary artery in late systole. It is suggested that this mechanism serves to regulate the normal left-to-right shunt into the lungs through the incomplete ventricular septum and to maintain adequate systemic blood flow. A resemblance between the arrangement in the turtle and that seen in certain cases of human congenital heart disease is briefly discussed.