Pressure and volume relationships in the ventricle, conus and arterial arches of the frog heart.

In the amphibian heart the pattern of blood flow from the auricles through to the arterial arches is not fully understood (de Graaf, 1957; Simons, 1959; Johansen, 1963; Foxon, 1964a). The action of the single-chambered ventricle and the imperfectly divided conus arteriosus, together with the details of their relationships to one another and to the arterial system, still constitute major problems. The conus is developed from the bulbus cordis (following Goodrich, 1930) seen during embryonic development in all vertebrates but persisting as a functional and contractile chamber of the heart only in amphibia and some fish. The amphibian conus, situated as it is between the divided truncus arteriosus and the undivided ventricle, must have considerable effect on blood distribution and flow pattern from the heart into the major arteries. In a recent monograph on the cardiovascular dynamics of Amphiuma, Johansen (1963) expressed views on the second of these two functions of the conus which were later criticized by Foxon (1964 ft). Both workers accepted that the blood pressure and flow of blood in the major arteries during different parts of the cardiac cycle were greatly modified because of conus action. Johansen suggested that the conus had a depulsating and distributing effect on the ventricular output which was so effective that outflow was maintained from the heart throughout the whole cardiac cycle. This opinion was not shared by Foxon who cited direct observations of intermittent flow in anurans made by himself (1947) and Vandervael (1933). The aortic pressure measurements made by Simons (1957) in which an incisura early in the falling phase of the pulse was interpreted to indicate closure of the synangial valves also supported Foxon's view. However, de Graaf (1957) and Shelton & Jones (1965) placed a different interpretation on the inflexion which Simons called the incisura, suggesting that it separated ventricle and conus pressure components as recorded in the aortic arches. Some features of the pattern of flow through the heart can be decided if the blood pressures within the ventricle, conus, and aortic arches are known. Since inertia may cause continued flow in the face of small but reversed pressure gradients (Spencer & Greiss, 1962) it is also desirable to know whether the valves at the proximal (pylangium) or distal (synangium) ends of the conus are opened or closed. Sanchez-Casco & Foxon (1963) recorded pressures in the conus of frogs but without determining them elsewhere. Johansen (1963) recorded pressures simultaneously in the ventricle, conus and aorta of the urodele Amphiuma, He showed that the pressure due to ventricular contraction was very much reduced in the conus and was followed by a