114 VERTEBRATE RESPIRATION 



pulmonary contains deoxygenated blood derived from the right 

 auricle whereas the right systemic contains the most oxygenated 

 blood and both carotid arches arise from this systemic artery. 

 The left systemic would be expected to contain mixed blood, 

 being intermediate both in the position of its origin at the ven- 

 tricle and in the part of the body which it supplies. 



X-ray cinematography of hearts following the injection of 

 radio-opaque material into the veins near the heart has fully 

 confirmed this account. There appears to be little mixing of the 

 blood in the dorsal ventricular chamber and all blood entering 

 the pulmonary arch is derived from the right auricle while the 

 right systemic conveys blood entirely derived from the left 

 auricle. The left systemic carries a mixture of blood from both 

 sources. These studies also revealed that the path of the blood 

 in the ductus caroticus, which persists on both sides, is from the 

 systemic to the carotid arch. In this way the left internal carotid 

 (fig. 33a) receives some mixed blood from the left systemic. 



In the Chelonia (tortoises and turtles), both the pulmonary 

 and left systemic arches arise from the right side (= cavum ven- 

 trale of lizard) of the ventricle, the right systemic arising from 

 the left side of the ventricle as before. As in the lizard heart, both 

 auricles open into the same ventricular chamber which corres- 

 ponds to the left ventricle. When the auricles contract, blood 

 from the right auricle passes rapidly across the incomplete 

 septum and following ventricular systole it becomes distributed 

 to the pulmonary and left systemic arches alone. Oxygenated 

 blood from the left auricle is distributed in the right systemic 

 arch. Accounts of turtle and lizard hearts based on measure- 

 ments of O2 tensions confirm that blood passing to the pul- 

 monary arch contains much less oxygen than that in the 

 systemics. It appears, however, that the blood in both the right 

 and left systemic arteries has very nearly the same Oo tension. 



It is only in the crocodiles that one important condition for a 

 double circulation is found. This is that the auricles open separ- 

 ately into two separate ventricles (fig. 34a). The left auricle 

 opens into the left ventricle which communicates with the right 

 systemic from which arise both carotid arches. The right auricle 



