ANATOMY OF THE FROG 57 



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 venous 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 synangium. 

 Now, the arteries are already distended with blood under 

 considerable pressure, which was only prevented from re- 

 turning 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 

 from the middle of the ventricular cavity, finds the least 

 resistance in the wide openings of the right and left systemic 

 arches, which are accordingly filled with mixed blood until 

 their resistance becomes as great as, and greater than, that of 

 the carotid arches, the resistance in which has been consider- 

 able, both because of their small diameter and because the 

 flow of blood through the carotid arteries is impeded by the 

 carotid glands. Thus the last portion of the blood, consisting 

 of nearly pure arterial blood from the left end of the ventricular 

 cavity, passes into the carotids, and is distributed to the head. 

 It seems probable, from the relation of the carotid opening 



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