COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 239 



septum and forms a fold or valve on each side, which partially occludes the 

 auriculo-ventricular opening. The right valve continues ventrally into a ridge 

 which is on the ventral flap of the specimen. This ridge is the incomplete 

 intervenlricular septum. On bringing the two flaps of the specimen together, 

 it will be seen that the interventricular septum was connected with the muscular 

 wall of the dorsal flap and that a space is left dorsal to the septum by which the 

 right and left ventricles communicate with each other. The right ventricle to 

 the right of the septum is very small, while the left ventricle is much larger and 

 communicates with the cavity of both auricles owing to the incomplete character 

 of the interventricular septum. Spread the flaps of the specimen again and 

 pass a probe ventral to the interventricular septum. The probe emerges in 

 the pulmonary artery. Probe into the other arterial trunks and find their open- 

 ings into the ventricle. The opening of the left aorta is to the right of the inter- 

 ventricular septum, into the small right ventricle, while that of the right aorta 

 is to the left of the septum, into the left ventricle; however, owing to the gap 

 dorsal to the septum, the left aorta can also obtain blood from the left ventricle. 

 By slitting open the arterial trunks find the little pocket-like semilunar valves 

 which guard their exits from the ventricle. They represent the remains of the 

 conus arteriosus. 



We may now attempt to explain the course of the circulation through the turtle's heart. 

 The matter is somewhat complicated and further details will be found in P and H, page 359. 

 We have noted that all of the venous blood returns to the sinus venosus and that this in turn 

 connects with the right auricle which passes it on into the right side of the ventricle. Although 

 this is imperfectly separated from the left side of the ventricle, the venous blood is well retained 

 in the right side owing to the spongy nature of the ventricular walls. Meantime, the two 

 pulmonary veins have returned the blood from the lungs to the left auricle. Since the function 

 of the lungs is to aerate the blood, this blood is arterial. From the structure and relations of 

 the heart, the right auricle always contains venous blood, and the left auricle arterial blood. 

 The left auricle passes the arterial blood into the left side of the ventricle. There is some 

 slight mixture of venous and arterial blood in the ventricle. As the ventricle contracts both 

 kinds of blood are moved toward the arterial trunks. We have noted that the pulmonary 

 artery springs from the small right ventricle and that the opening into this artery is to the 

 right of and somewhat concealed by the interventricular septum. When the ventricle con- 

 tracts, the pressure practically closes the septum so that most of the venous blood passes out 

 into the pulmonary artery. Simultaneously, the nearly pure arterial blood in the left ven- 

 tricle passes into the base of the right aorta, since that is connected with the left ventricle 

 and since the communication with the left aorta is closed temporarily by the interventricular 

 septum. Toward the end of the contraction the remaining blood in both ventricles passes 

 into the left aorta, as the diminished pressure again opens up the gap in the septum. It 

 thus happens that the brachiocephalic artery passing to the anterior part of the body con- 

 tains nearly pure arterial blood, while the left aorta contains mixed blood. On account of the 

 function of right and left aortae, the dorsal aorta also carries mixed blood. It is universally 

 true among vertebrates that the arrangement of the circulatory system is such that the 

 purest blood is received by the head; and this is no doubt due to the greater oxygen require- 

 ments of the nervous parts of the head. 



