THE VASCULAR SYSTEM 27 1 



hindlegs may thus return to the heart either through the mesonephroi, or 

 by the abdominal vein. The median ventral abdominal vein, which made 

 its first appearance in Dipnoi, takes the place of the lateral abdominal 

 veins of elasmobranchs. The increased flexure of the heart brings the 

 atria anterior as well as dorsal to the ventricle. (Fig. 246) 



Reptiles. The reptilian vascular system strikingly resembles that 

 of amphibians. The main arteries and veins are homologous in the two 

 groups. The chief differences appear in the heart and truncus arteriosus. 

 The ventricle is partly divided by a septum in lower reptiles and more 

 or less completely divided in the crocodiles and alligators. Conse- 

 quently pure and impure blood are separated in the two sides of the heart 

 as in mammals. A peculiarity of the reptiUan circulation, however, is 

 manifested in the triple splitting of the truncus arteriosus. Three arteries, 

 instead of the two characteristic of mammals, leave the heart. One of 

 these is the pulmonary artery carrying venous blood from the right ven- 

 tricle to the lungs. The remaining two vessels are the systemic arteries, 

 one of which comes from the right, and the other from the left, ventricle. 

 Soon after leaving the heart each artery crosses to the opposite side of the 

 body. Thus the right aortic arch comes from the left ventricle and con- 

 veys pure blood to the dorsal aorta and the head. The left aortic arch 

 comes from the right ventricle and carries mixed blood into the dorsal 

 aorta. Consequently, the dorsal aorta of reptiles contains mixed, and not 

 pure, blood. Since the celiac and mesenteric arteries are given off from 

 the left aortic arch which carries mixed blood, they carry mixed blood to 

 the stomach and intestine. In some reptiles a foramen Panizzae con- 

 nects the blood streams in the two ventral aortae so that some mixing of 

 the blood in the two vessels may take place. In the lower reptiles systemic 

 and carotid arches are connected with one another, as in urodeles, by the 

 dorsal aortae. In the crocodiles this connexion is lost, as in mammals. 

 The connexion between the postcava and the postcardinals is lost in reptiles 

 and the blood from the kidneys returns to the heart by the postcava as in 

 mammals. Both right and left common cardinals (ductus Cuvieri) 

 persist and bring blood from the head and anterior limbs into the sinus 

 venosus. Thence it passes to the right atrium. Blood from the hind legs, 

 as in amphibians, may return to the heart either by the renal portal veins 

 or by the abdominal vein. 



Mammals. The complete division of the heart into a right venous 

 half and a left arterial half, which was attained by reptiles, is retained by 

 mammals. In mammals, however, the sinus venosus merges into the 

 right atrium. In this region is located the sinu-auricular node, a bundle 

 of muscular and connective tissue richly supplied with nerve fibers, which 

 is said to be the "pace-maker" of the heart-beat. Mammals have a 

 single ventral aorta. Of the paired systemic arches of amphibians and 



