200 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



heart itself the freshly oxygenated or arterial, and the 

 impure or venous blood never commingle ; in the 

 Crocodile, where an aortic arch is in communication 

 with each half of the ventricle, the arterial and 

 venous blood commingle outside the heart at the 

 point of union of their two vessels (the so-called 

 foramen Panizzse) ; in birds and mammals there is but 



Ji.AO. 



Fig. 86. Diagram of the Ventricle and connected parts in the Turtle ; 

 showirg the transversely elongated ventricle, with the right and 

 left auricles (RA, LA) lying towards the left, the auriculo- 

 veutricular valves (uu) formed by the inter-auricular and septum. 



RA, Right auricle; LA, left auricle; , the right; t/, the left median auriculo- 

 ventricular valves; SL., arrow showing the course of the blood in the left 

 aorta; t, arrow showing the course of the blood in the right aorta ; x, arrow 

 showing the course taken by the blood from the left ; and to, from the right 

 auricle into the ventricle ; y. showing the course of the blood from the cavuiu 

 venosum into the cavum pulmonale ; z, from the latter into the pulmonary 

 artery ; a, the incomplete septum marking off the cavum pulmonale (cp) ; PA, 

 pulmonary artery ; RAO, LAO, right and left aortoe ; t>, cavum venosum. (After 

 Huxley.) 



a single aortic arch, which arises from the left ven- 

 tricle, and the blood from the right never, therefore, 

 passes into the aorta. 



The differences between the arrangement of the 

 auriculo-ventricularvalves have been already described, 

 and we now need only point out that there must be a 

 difference in the way in which these valves perform 

 their office; in the Sauroid they are muscular, and 

 therefore actively close the entrance to the auricles by 

 contracting when the ventricles contract, while in the 

 majority of Mammals the membranous flaps are 



