ANATOMY OF THE FROG 57 



taken up oxygen from the air contained in the lungs ; it is of 

 a bright scarlet colour, and is known as arterial blood. 



The muscular contraction which started in the sinus 

 venosus spreads over the auricles, which contract simul- 

 taneously, and drive the blood onward through the wide 

 auriculo-ventricular aperture into the single ventricle. The 

 return of blood from ventricle to auricles is prevented by the 

 membranous valve described above. As the right auricle 

 contains venous and the left auricle arterial blood, the 

 transversely-elongated cavity of the ventricle is filled with both 

 kinds, the venous blood being on the right side nearest the 

 opening of the truncus arteriosus, the arterial blood farthest 

 away from it on the left. In the middle of the ventricular 

 cavity the blood is, of course, mixed. The contraction of the 

 ventricle follows immediately upon that of the auricles. The 

 auriculo-ventricular valves close, and the blood is forced into 

 the truncus arteriosus, the venus blood on the right side 

 passing first. The wave of contraction passes forwards over 

 the truncus from the base of the ventricle to the roots of the 

 aortic arches, and any reflux of blood into the ventricle is 

 prevented by the closing of the semi-lunar valves which guard 

 the two ends of the pylangium. 



On entering the pylangium the blood is necessarily directed 

 by the spiral valve. It therefore passes from the dorsal side 

 round to the right and thence to the ventral side of the 

 truncus, and the venous blood passing over the free border 

 of the upper end of the spiral valve enters the cavum pulmo 

 cutaneum. Now, the arteries are already distended with 

 blood under considerable pressure, which was only prevented 

 from returning into the truncus by the semi-lunar valves at 

 the distal end of the pylangium. This pressure is overcome 

 by the blood forced forward at greater pressure by the con- 

 tractions of the ventricle and pylangium, and the semi- 

 lunar valves are forced open. In the synangium the blood 

 newly arrived follows the path of least resistance, which is 

 found in the wide aperture of the pulmonary artery. The 

 first gush of nearly entirely venous blood therefore passes 

 to the lungs, but as it fills the vessels of the lungs the 

 resistance in them becomes as great as or greater than that in 

 the synangium and other aortic arches. The next succeeding 

 portion of the blood, which is now mixed arterial and venous 



