COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 



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

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

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

 interventricular septum. On bringing the two flaps of the specimen together 

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

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

 right and left ventricles communicate with each other. The right ventricle t( 

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

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

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

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

 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 aort; 

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

 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 valve: 

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

 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 tun 

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

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

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

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

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

 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 somi 

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

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

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

 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 ou: 

 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 ventriclt 

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

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

 into the left aorta, as the diminished pressure again opens up the gap in the' septum. I1 

 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 

 junction of right and left aortae, the dorsal aorta also carries mixed blood. It is universal!) 

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

 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. 



