164 The Vertebrate Organ Systems 



gen and rich in carbon dioxide. From the right anricle, it passes into 

 the right ventricle through the opening which is guarded by the tri- 

 cuspid valve. The contraction of the right ventricle forces the blood 

 out through the pidmotiayy artery and to the lungs. Close to the 

 heart, the pulmonary artery branches into two smaller arteries, one of 

 which goes to each lung. In the lungs these arteries break into the pul- 

 monary capillaries and the gaseous exchange takes place. These capil- 

 laries gradually join to form the pulmonary veins which return the 

 oxygenated blood to the left auricle. This is the only vein in the body 

 that carries oxygenated blood ; the ])ulmonary artery is the only artery 

 that carries unoxygenated blood. 



The pulmonary veins empty the blood into the left auricle, and the 

 blood then passes through the opening guarded by the bicuspid valve 

 into the left ventricle. The contraction of this ventricle forces the blood 

 into the aorta, and into the systemic circulation. 



In the adult mammal, there are two completely separate circuits 

 for the blood to follow in its covu^se through the body. Before birth, 

 the pulmonary circuit is of little value, for oxygenation of blood occurs 

 in the fetal connection to the mother, the placenta. Therefore it is of 

 some advantage in the fetus for the blood to short-circuit this system. 

 One by-pass arrangement, pointed out above, is the foramen ovale ; an- 

 other is the ductus arteriosus, which connects the pulmonary artery 

 with the aorta. Blood passes from the pulmonary artery into the aorta 

 instead of to the lungs. Both of these by-passes cease functioning shortly 

 after birth. If either or both fail to close, the child has an impaired 

 oxygen supply. 



The Aortic Arches.- — Soon after the aorta leaves the heart it 

 bends sharply to the left in human beings to form the dorsal aorta. 

 In birds, the arch is to the right. This variation in direction of aortic 

 arching is easily understood when the evolutionary history of the aorta 

 is studied. (Fig. 47.) 



In the fish, the ventral aorta rises from the conns arteriosus and 

 carries the blood forward. Branches or arches from the aorta carry 

 the blood to each of the gills. Within the gills, the arches break into 

 numerous capillaries which then anastomose to form the remainder of 

 the arches, and finally the dorsal aorta. In the primitive condition, it 

 is believed that there were six pairs of these arches ; however, there are 

 no modern animals with this condition. The shark, for example, has 

 but five pairs of arches. 



